Drosophila melanogaster

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Drosophila melanogaster
Drosophila melanogaster (male)

Drosophila melanogaster (male)

Systematics
Superordinate : New winged wing (Neoptera)
Order : Fly (Diptera)
Subordination : Flies (Brachycera)
Family : Fruit flies (Drosophilidae)
Genre : Drosophila
Type : Drosophila melanogaster
Scientific name
Drosophila melanogaster
Meigen , 1830

Drosophila melanogaster (from ancient Greek δρόσος drosos "dew", φίλος philos "loving", μέλας melas "black" and γαστήρ gaster "belly") is one of over 3000 species from the family of fruit flies (Drosophilidae). It is one of the best studied organisms in the world. The rather uncommon German terms black-bellied fruit fly or black-bellied fruit fly for this animal are relatively new and did not appear in German-language literature until after 1960. In technical German usage, “fruit flies” were originally not the representatives of the Drosophilidae family, but only the Tephritidae . "Black bellied" is the translation of the scientific species name into German.

Drosophila melanogaster (synonymous with Drosophila ampelophila Loew, among others ) was first described in 1830 by Johann Wilhelm Meigen . As a suitable test organism, it was first used in 1901 by the zoologist and geneticist William Ernest Castle . He examined in D. melanogaster strains the effect of inbreeding over many generations and the effects occurring after crossing inbred lines. In 1910, Thomas Hunt Morgan also began to breed the flies in the laboratory and to examine them systematically. Since then, many other geneticists have obtained essential knowledge about the arrangement of genes in the chromosomes of the genome of this fly using this model organism .

description

View from above
Front view

Drosophila melanogaster was originally a tropical and subtropical species. However, it has spread with humans all over the world and overwinters in houses. The females are about 2.5 millimeters long, the males are slightly smaller. The latter are easily distinguishable from the females by their more rounded abdomen, almost uniformly dark in color due to melanins , which, when viewed from above, have a more pointed abdomen and the black melanins are embedded more in the form of a horizontal stripe pattern in the body wall ( cuticle ) of their rear end. The eyes of the small flies are typically red in color due to the inclusion of brown ommochromes and red pterins .

Phylogeny: Drosophila or Sophophora?

The genus Drosophila in the classical sense comprises 1450 valid species and is the most species-rich taxon of the Drosophilidae. More recent work based on phylogenomics (investigation of relationships by comparing homologous DNA sequences), but also on morphology, for example of the male genital fittings, have shown that the conventional genus Drosophila is paraphyletic . This means that some species, which are currently listed in at least eight, but probably more likely fifteen other genera, are more closely related to certain species groups within Drosophila than they are to each other. The subgenus Sophophora Sturtevant , 1939, is relatively basal, i.e. it splits off from the remaining species complex at an early stage (however, it is also paraphyletic itself).

The normal procedure in such a case would be to split up the large genus Drosophila and raise the (Old World clade of the) subgenus Sophophora to the genus rank, which would lead to the recombination Sophophora melanogaster for our species. This would be more or less commonplace for fly taxonomists. However, it would have serious effects on the extremely important applied research on the species, where Drosophila is often only referred to in abbreviated form . Simply letting the nested genera merge into Drosophila as a supergenus would also have undesirable consequences: Four different species were then called Drosophila serrata and four other Drosophila carinata . Kim van der Linde tried to have Drosophila melanogaster declared as a type species of the genus, which the ICZN rejected. Others suggested deviating from the rules of cladistics and allowing paraphyletic genres again. The formal revision of the genus Drosophila has not been carried out so far, and exclusively for this reason, so that Drosophila melanogaster continues to be the taxonomically valid name of the species, because no taxonomist has been prepared to answer for the consequences of the renaming.

development

The females lay a total of about 400 whitish-yellowish eggs , which are covered by a chorion and a vitelline membrane and are about half a millimeter in size, on fruit and rotting, fermenting organic material. Their preference for the citrus scent protects fruit flies from parasites. The length of the development time depends on the ambient temperature. At a temperature of 25 ° C hatches from each egg after about 22 hours as a larva , a maggot , which is immediately in search of food. The food consists primarily of the microorganisms that decompose the fruit, such as yeast and bacteria , and only secondarily of the sugary fruit itself. After about 24 hours, the larva, which is constantly growing, molt for the first time and reaches the second larval stage. After going through three larval stages and a four-day pupal stage, the flightable insect hatches at 25 ° C after a total of nine days of development, which is then sexually mature within about 12 hours.

Embryonic development

Copulating fruit flies

After the fertilization of the D. melanogaster egg and the fusion of the cell nuclei , several synchronous nuclear divisions ( mitoses ) occur in rapid succession , in which there is no delimitation by cell membranes . The result is an embryo that consists of a cell with many cell nuclei that are not delimited by membranes. This condition is called syncytial blastoderm or polyenergid . After the seventh division of the nucleus, most of the nuclei migrate to the periphery of the embryo, i.e. under the outer cell membrane. Between the eighth and ninth nucleus division, eight to ten cell nuclei are enclosed in the posterior pole plasma and then begin to divide independently of the other nuclei. The germ cells develop from these so-called pole cells .

The “cellular blastoderm” arises from the syncytial blastoderm about 2.5 hours after oviposition, namely through the invagination and growth of the outer cell membrane in the regions between the individual nuclei. In this way, the first single-layer epithelium of the resulting fly is formed and the cell nuclei are thus denied access to asymmetrically distributed, morphogenic gene products ( see for example bicoid ). Correspondingly, the development potential of the cells is already largely determined at this point in time, depending on their position.

Cuticle of a larva

A ventral furrow along the longitudinal axis (ventral furrow) initiates gastrulation , through which the blastoderm epithelium is divided into three germ layers: The mesoderma appendage is created through the ventral furrow, which occurs on the "ventral side" along the embryo . An invagination (invagination) anterior to the ventral furrow, which forms the stomodeum , and an invagination which takes place at the posterior pole of the embryo, which forms the proctodeum , delimit the later endoderm . The cells remaining on the outside of the embryo and the end areas of the stomodeal and proctodeal intussusception form the ectoderm . With the lengthening of the germ line, the pole cells migrate from posterior to the interior of the embryo. The organogenesis sets in and the first time an embryonic is metamerism recognizable. The shortening of the germ line begins around 7.5 hours after fertilization and ends with dorsal closure . After further differentiation steps, the fully developed larva hatches about 21 hours after fertilization .

Larval development

The footless, segmented maggots have a dark chitin pen at their slightly more pointed front end , which can be stretched and retracted and which contains the rather meager mouthparts. The larvae crawl around in the food pulp or in the vicinity of the food source, eat and grow within a few days from the size of the egg (0.5 mm) to the size of the fly (2.5 mm). They molt twice during this time. Accordingly, a distinction is made between three larval stages.

Pupation

In the last larval stage, the insect soon stops crawling and pupates. The pupa gradually turns brown at first, but does not resemble a typical insect pupa in the case of D. melanogaster , but looks more like a shriveled and dried-up maggot. A barrel doll develops inside the maggot skin, the shell of which consists of hardened larval skin. After a few days, a lid at the end of the barrel bursts open, and a fully developed fruit fly crawls out, which subsequently discolors and hardens its body cover and aligns its wings.

Determination of gender

Chromosomes from D. melanogaster

The sex of the fruit fly is - as is usual with many animals - genetic. D. melanogaster only has four different chromosomes ; they occur in pairs in the cells. This double set of chromosomes contains a pair of sex chromosomes , also known as the first chromosome or X or Y chromosome, and three pairs of autosomes , known as the second, third and fourth chromosomes. Just like humans, D. melanogaster has two sex chromosomes : females have two X chromosomes and are homogametic ; Males have an X and a Y chromosome and are heterogametic . In contrast to humans, however, the Y chromosome does not have a sex-determining component, rather the ratio of the X chromosomes to the autosomes is sex-determining.

Female (left) and male D. melanogaster

If the ratio of the X chromosome to the autosome set is greater than or equal to 1 (e.g. two Xs in the diploid set), the result is a female; if it is less than or equal to 0.5 (e.g. an X in the diploid sentence), the result is a male. Mutants with ratios in between, such as XX and a triploid autosome set (ratio: 0.67), form intersexes with male and female traits distributed like a mosaic (so-called “salt-and-pepper pattern”). The gender is determined by each cell itself; it can be different if the effect is not clear (between 0.5 and 1).

The compensation of the different gene doses of non-sex-determining genes of the X chromosome is achieved through a greatly increased transcription rate in males . This is made possible by acetylation of lysine residues of histone H4, which decreases the electrostatic interaction between the histone complex and the sugar-phosphate backbone of the DNA; the less strongly bound DNA to the nucleosomes is now easier to read. In this way, hyperactivation of the male's singular X chromosome can compensate for the lower gene dose .

The decision as to which gender-specific genes are transcribed and how is controlled by the sex lethal ( Sxl ) gene . In females is Sxl active, inactive in males. The gene product Sxl is an RNA- splicing enzyme that splices the so-called transformer mRNA . The resulting protein “Transformer” (tra) is also a splicing factor that splices the mRNA of the double sex ( dsx ) gene . The dsx produced then causes the actual sex determination at the molecular level, also as a transcription factor. The protein dsx is available in male and female variants.

Females: sxl aktiv, tra aktiv, dsxF (female) emerges. The male realizer genes are repressed. Males: sxl inactive, tra inactive, dsxM (male) arises. The female realizer genes are repressed.

The connection between the activity of “sex lethal” and the X chromosome dose is explained as follows: 3 genes for transcription factors in the syncytial blastoderm, which are also called “numerator genes”, are activated on the X chromosome. These factors (example: sisterless) bind to the so-called early promoter , a regulatory region in front of the Sxl gene, and activate it. On the other hand, genes called "denominator genes" can be found on the autosomes. They code factors (example: deadpan) that counteract this.

The ratio of X chromosomes to autosomes is therefore to be understood as a ratio of numerator genes to denominator genes. If there is a female chromosome set (XX), the numerator genes predominate and activate Sxl transcription. In the case of a male sentence (XY), on the other hand, the denominators are in the majority, the transcription of Sxl is repressed. In this case Sxl is inactive during development.

The Sxl gene also has a late promoter . This is constitutively activated in both sexes in later development. However, by autoregulating Sxl, the level of Sxl protein remains high in female cells and low in males. In female cells, Sxl early protein binds to poly (U) sequences in introns later Sxl pre-mRNA. Those introns flank exon 3, which contains a stop codon . When Sxl protein binds to these introns, exon3 will not be recognized as such and will be spliced ​​out. The translation of the Sxl mRNA generated in this way yields another effective Sxl protein. In male cells, the concentration of early Sxl protein is almost zero, so that the stop codon of the late Sxl pre-mRNA becomes effective. The translation of the mRNA generated from that is therefore incomplete and does not result in an effective isoform of Sxl.

In D. melanogaster , the determination of gender is therefore "cell-autonomous", i. H. explainable by internal control mechanisms of the individual cells. Each cell “counts” its X / Y ratio, so to speak, and develops accordingly.

General anatomy of the central nervous system

Larval stage

Immunostaining chaGAL4. The two brain lobes and the ventral ganglion of the developing D. melanogaster larva can be seen.

The central nervous system of the D. melanogaster larva is made up of the two brain lobes and the ventral ganglion , which is the abdominal marrow . The two brain lobes are connected ventrally. The fusion point of the two is pierced by the esophagus , which runs dorsally over the ventral ganglion. The window through which the esophagus passes is called the foramen.

Central nervesystem

Overlay of an immunostaining of the genotype repoGAL4x10xUAS-myr-GFP (green) with mouse anti-Brp (magenta). Anti-Brp stains the central neuropil in the entire nervous system, but not the somata in the cortex. The demarcation between the cortex of the neuronal somata and the neuropil becomes visible.
Overlay of an immunostaining of the genotype chaGAL4x10xUAS-myr-GFP (green) with mouse anti-FasII (red). You can see the central nervous system with the ventral ganglion, the mushroom bodies and the brain lobes.

The antenna nerve and the Bolwig nerve emanate from each cerebral lobe. A cross-section of the brain lobes shows that the brain lobes are composed of a cortex made up of neuronal somata and a central neuropil . The neuropil is characterized by a large density of dendrites and synaptic endings, which communicate with one another via synaptic contacts.

The ventral ganglion is also divided into the cortex and neuropil. Each cerebral lobe has a mushroom body, an optical neuropil and a larval antenna lobe. A central complex has not yet been found in the larva. In theory, however, it should be there, since it is responsible for the visual coordination of movement. Neurons, which do not build up a central body in the typical way, may take over these tasks. In the larval stage, the brain and ventral ganglion increase in size. This is based on the fact that neuroblasts already begin to divide during the larval phase, and in large parts of the brain they generate neuronal precursor cells for the later neurons. In contrast to vertebrates , the most common excitatory neurotransmitter in the CNS is acetylcholine . Glutamate and others also occur. The main inhibitory transmitter is γ-amino-butyric acid (GABA).

Larval antenna lobe

The projections of the olfactory receptor neurons end in the larval antenna lobe. Output neurons (so-called projection neurons) move from the larval antenna lobe via the antenna cerebral tract to the mushroom body. Here, 21 projection neurons project onto 28 calyx glomeruli of the mushroom body.

Mushroom bodies

The structure of the mushroom body in the larval stage is much simpler than that of the adult fly. After hatching, the L1 larva has approx. 250 Kenyon cells, the number of which increases to approx. 2000 cells within the 3 larval stages. The mushroom body integrates various sensory information and has an important function in olfactory learning. The mushroom body consists of a calyx ("calyx"), to which a stem (pedunculus) is attached ventrally. The pedunculus divides into different praises. The mushroom body also receives olfactory inputs from the antenna lobe.

Ventral ganglion

The ventral ganglion is located in the third thoracic segment and extends to the first abdominal segment of the larva. The ventral ganglion consists of three suboesophageal neuromers, three thoracic neuromers (pro, meso- and metathoracic neuromers) and eight abdominal neuromers, which are fused together to form a ganglion. The structural design of the nervous system in the early embryonic development of D. melanogaster is similar to that of a rope ladder. In the late embryonic development there is a fusion of the abdominal and thoracic neuromers. Individual ganglia can no longer be seen after the fusion. A paired segmental nerve emerges from each of the eight abdominal neuromers and innervates the corresponding segments. The segmental nerve carries sensory information along the afferent pathways from the periphery to the central nervous system. In addition, the segmental nerve conducts motor information on efferent pathways from the central nervous system to the periphery.

Adult stage

The adult central nervous system of Drosophila melanogaster is composed of a fused upper and lower pharyngeal ganglion (brain), as well as thoracic and abdominal ganglia that are fused to form a ventral ganglion.

Central nervesystem

The symmetrical upper pharyngeal ganglion contains approx. 100,000 neurons, the volume is approx. 0.2 mm³ and the weight approx. 0.25 mg. It consists of three fused parts that evolve from the three original head segments: a large protocerebrum, a smaller deutocerebrum and a very small tritocerebrum. On the protocerebrum are the two optical lobes, brain lobes, which are responsible for visual processing. The Deutocerebrum receives olfactory information via olfactory receptor neurons, which gets into the antennae. Mechanoreceptors for detecting mechanical stimuli are also located on the antennas. This information is sent to the antennomechanical center in the Deutocerebrum.

Central complex , optical praises, antenna alloves and mushroom bodies represent important functional units of the adult brain. The central complex consists of four clearly delimited neuropil regions. Of this, the protocerebral bridge is the furthest posterior ("back"), anterior in front of it is the central body with a larger upper unit (fan body) and a smaller lower unit (ellipsoid body), as well as the two posterior nodules. The central complex plays a role in motor control and visual orientation. For example, flies with mutations in the central complex have a reduced ability to visualize.

The optical praises are responsible for processing optical stimuli. They contain four interconnection levels: lamina, medulla, lobula and lobular plate. The olfactory inputs are processed in the two antenna albums, which consist of so-called glomeruli. These spherical structures represent condensed neuropil. Via olfactory receptors on the antennas, olfactory stimuli are detected and converted into electrical signals. The excitation is conducted via receptor neurons into the glomeruli and from there via projection neurons into the mushroom body and lateral horn, where the information is processed. Local neurons that innervate the glomeruli are used for modulation.

The mushroom bodies are composed of calyx and pedunculus and are the seat of higher integrative services, such as olfactory learning and memory. Various working groups could B. show by transgenic techniques in rutabaga mutants.

Sub-canal ganglion and abdominal marrow

The sub- canal ganglion has a clear segmental structure. It lies below the esophagus and consists of the three fused neuromers of the mandibular, maxillary and labial segments. Afferent pathways from the periphery that carry sensory information, e.g. B. from the mouthparts, lead, terminate in the subterranean ganglion. Efferent pathways that innervate the motor system in the periphery arise from the sub-canal ganglion. The sub-canal ganglion is connected to the abdominal marrow via the pharyngeal connector.

Peripheral nervous system

In D. melanogaster , as in other insects, the visceral nervous system , which innervates the digestive tract and the genitals, is part of the peripheral nervous system and is in turn subdivided into the ventral visceral, the caudal visceral and the stomatogastric system.

The stomatogastric nervous system innervates the anterior pharyngeal muscles and the foregut. Although the frontal nerve and the recurrent nerve are present, the stomatogastric nervous system lacks a typical frontal ganglion, which is only formed as a nerve junction . However, the stomatogastric nervous system contains a proventricular ganglion and a hypocerebral ganglion, which are connected to one another via the proventricular nerve. The ventral caudal system refers to the branches belonging to the unpaired median nerve and is connected to the thoracic and abdominal neuromers of the abdominal cord. The ventral caudal system, for example, innervates the trachea .

Nervous system during metamorphosis

The adult nervous system does not develop completely anew during the metamorphosis , but is mainly formed from a framework of larval sensory neurons, inter- and motor neurons. Most sensory neurons from the larval stage degenerate during metamorphosis and are replaced by adult neurons that develop from the imaginal discs . This creates part of the peripheral nervous system. The adult interneurons consist to a small extent of remodeled larval interneurons, but the majority is only formed from neuroblasts during metamorphosis . These neurons are mainly used for the optical system, the antennas, the mushroom body and the thoracic nervous system in order to process the information of the adult-specific structures (complex eyes, legs, wings). The motor neurons remain predominantly and converted during metamorphosis into adult specific neurons. These motor neurons are mainly required for the new leg and flight muscles, as well as for the body wall muscles. The postembryonic regeneration of neurons, the death of larval-typical neurons, as well as the modification of existing larval neurons are regulated by gene cascades that are mainly triggered by the steroid hormone ecdysone .

12-14 hours after pupation, larval elements degenerate, especially in the abdominal region, while the remaining neurons shorten their axons and dendrites. There is also a constriction between the suboesophageal and thoracic areas of the CNS, which thus loses its larval appearance.

Adult neurons begin to differentiate fully 24 hours after pupation, as their branches spread out into larger areas. In addition to the formation of new neurons, this contributes to the enlargement of the brain. In the larval stage, for example, the olfactory system consists of only 21 sensory neurons, whereas in the adult antennae it consists of around 1200 afferent fibers.

After the metamorphosis is complete, motor neurons and peptidergic neurons, which are only needed for hatching and have no function in the adult animal, die off.

Sexual dimorphism in the central nervous system

The adult brain of the D. melanogaster shows gender-specific differences in morphology. Males have certain regions in the brain, so-called MERs (male enlarged regions), which are significantly larger than females. On average, these are about 41.6% larger. Females also have enlarged structures, here FERs (female enlarged regions), which, however, are on average only around 17.9% larger than their male counterparts. By calculating the volume of the MERs, it is possible to make a statement about the gender of the fly using only the brain. Most of the MERs are in the olfactory area of ​​the brain. This explains the different behavior of males and females to smells. For example, if both sexes are exposed to the male pheromone cVA, this has a repulsive effect on males, but aphrodisiac on females.

Similar to gender determination, the two genes sex lethal ( sxl ) and transformer ( tra ) are responsible for the gender-specific, morphological differences in the brain. If both are active, the doublesex ( dsx ) gene produces the female variant of the DsxF protein, which enlarges the regions characteristic of females (FER). However, if the genes sxl and tra are inactive, the gene dsx produces the male variant DsxM, which is responsible for the differential formation of the MERs. In order to synthesize two different proteins from a single gene, alternative splicing is necessary. In this case, this is done through the regulator genes sxl and tra .

In addition, the fruitless ( fru ) gene is involved in the sexual dimorphism of the central nervous system. In the female wild type it produces the nonfunctional protein FruF. Accordingly, the protein FruM is produced in the male. This is crucial for the normal courtship behavior of the males. In experiments in which female mutants were produced which were able to synthesize the FruM protein, it was found that the regions that are normally enlarged in males were also present in these females, although not to the same extent.

The optical system

Development of the optical system from the embryo to the imago

From embryogenesis to larva

The larval optical system. Brain of a L3 D. melanogaster larva of the genotype gmrGAL4xUAS-10xmyr-GFP.

During embryonic development , a plate-like thickening occurs in the anterior dorsal blastoderm , which invagines, sinks into the depths and forms so-called placodes , which are paired and attach laterally to the surface of the developing brain as optical systems. During the larval stages, the optical systems increase in size and transform to differentiate into the adult optical praise in the pupa. The optical systems split into the inner and outer optical lobes and the adult retina . Two outer optical neuropiles , the lamina and medulla, develop from the systems of the external optical praise . The predispositions for the internal optical praise develop into the lobula and the lobular plate. In the second larval stage, the developing lamina and medulla have already taken up most of the volume of the larval brain. During the third larval stage, the lamina and medulla differentiate further. The connections between the inner optical praises and the central brain are also formed. Adjacent to the systems of the optical lobes are the eye-antennae- imaginal discs , which develop from undifferentiated stem cells during embryogenesis . During the third larval stage, differentiation begins, which in metamorphosis progresses to fully differentiated complex eyes . The functional organ of vision of the larva is the Bolwig organ. It arises during embryonic development from precursor cells that split off from optical intussusception. The Bolwig organ consists of 12 photoreceptor cells with the rhodopsins Rh5 and Rh6. Rh5 absorbs light in the short-wave blue range and ensures light sensitivity. Rh6, on the other hand, absorbs long-wave light and also plays an important role in the larval internal clock. The axons of the photoreceptors bundle and form the Bolwig nerve. This projects through the eye-antennae imaginal discs into the larval brain in the larval optical neuropil . From there, three different interconnections follow: to serotonergic dendritic branched neurons (SDA), to dendritic branched ventral lateral neurons (LNvs) and to visual interneurons (CPLd).

From larva to imago (metamorphosis)

Immunofluorescent labeling against fasciclin II; Genotype: gmrGAL4-UAS-10xmyr-GFP

The ommatidia of the complex eye develop from the eye-antennae- imaginal discs . The photoreceptor axons of the ommatidia move into the brain via the optic nerve ( Nervus opticus ). With the 24-hour old doll, the eye is a relatively thick-walled, flat cup in which the individual ommatidia are clearly visible. As the eye cup flattens even more, the ommatidia become thinner and shorter. Later the ommatidia are round. At the end of the second day of pupal development, the formation of the corneal lenses begins and the first pigmentation occurs. After two and a half hours, the pigmentation in the corneal lenses progresses, giving the eye a brownish color. At the end of the pupal stage, the ommatidia increase in length and finally differentiate.

The Hofbauer-Buchner eye emerges from the Bolwig organ and, like the Bolwig organ, plays an important role in the circadian rhythm . At the end of metamorphosis the neural located superposition eye of Imago ago.

Location and structure of the adult optical system

The complex eye

The complex eye of an adult D. melanogaster consists of approx. 800 ommatidia, each of which represents a functional unit of the retina. The ommatidia are directed hexagonally to each other. Each ommatidium has a dioptric apparatus , which is composed of a corneal lens and a crystal cone. In addition to the dioptric apparatus, an ommatidium has 8 photoreceptors, each of which has a microvilli edge directed towards the center . These microvilli extensions are called rhabdomers . Since D. melanogaster has a neural superposition eye, unlike the apposition eye and the optical superposition eye, the rhabdomers are not fused together, but are isolated from one another. When light falls, the corneal lens first absorbs the light and transmits it to the crystal cone. From there, the light is detected by the color pigments, the rhodopsins, in the rhabdomers. The eight rhabdomers are arranged differently in the ommatidium: There are six rhabdomers (R1-R6) in a circle around the 7th and 8th rhabdomer, the 7th rhabdomer is above the 8th. The particular thing about the neural superposition eye is that the rhabdomer R1 -R6 and R7 + R8 of an ommatidium perceive different points of view because the photoreceptors are at different angles to one another, with R7 and R8 aiming at the same point of view. When light comes in through the 7th rhabdomer, the unabsorbed light is passed on to the 8th rhabdomer below. Although each photoreceptor of an ommatidium fixes a different point, each point of view is captured by six photoreceptors. This point is detected by six different photoreceptors in six neighboring ommatidia. In total, an ommatidium can perceive seven different points, i. H. one through photoreceptors R7 + R8 and the remaining six through six photoreceptors R1-R6. The retinotopic organization of the stimulus processing of the photoreceptors R1-R6 ensures that the information recorded by the six photoreceptors is collected together in a functional unit in the lamina. This functional unit is called a cartridge. Since a lamina cartridge contains the same information six times, the light sensitivity is improved by a factor of 6. With the same spatial resolution, this enables improved adaptation to poor lighting conditions. The information from the photoreceptors R7-8, which is essential for color vision, is not transmitted to the lamina, but directly to the medulla.

The optical praises of the adult brain

The optical lobes, consisting of the lamina, medulla and the lobular complex, represent interconnection regions of the adult optical system. They are made up of repetitive subunits and are responsible for interpreting the information from the light-sensing cells of the complex eye.

Lamina

The lamina of the complex eye contains five different interneurons L1-L5 per cartridge, which differ in their functions. In the middle of each cartridge are the interneurons L1 and L2. Your job is to perceive movement. Interneuron L3 links the external photoreceptors with the interneurons of the medulla, which are also connected to photoreceptors R7 and R8. The individual cartridges are connected to one another by L4 neurons.

Glial cells ensure chemical and electrical insulation of the cartridges and divide the lamina into six layers, each of which has a characteristic glial cell type.

The first layer is the fenestrating layer, in which the glial cells envelop bundles of photoreceptors that emerge from the retina.

The second layer is the pseudocartridge layer, since axon bundles here form a shape similar to that of cartridges. The glial cells have a long, horizontally extended structure.

The third and fourth layers contain the satellite glia. These layers mark the beginning of the lamina cortex with the somata of the monopolar neurons L1-L5.

The fifth layer is the lamina neuropil , in which bundles of receptor terminals and interneurons are directly enveloped by glial cells. In addition, the glial cells form protuberances in the axons of R1-R6, which on the one hand provides structural support and on the other hand causes a lively metabolic exchange between glia and neuron.

The sixth layer is the proximal boundary layer. Marginal glial cells form the end of the lamina neuropil and thus mark the growth limit for the axons of R1-R6. The last layer is only traversed by the axons of the photoreceptors R7 and R8, which reach directly into the medulla.

Medulla

The medulla, like the lamina, consists of sub-units that are known as “pillars” due to their structure. The medulla is horizontally divided into 10 layers (M1-M10), the thickest layer being called the serpentine layer. The serpentine layer divides the medulla into a distal and proximal part. Tangential neurons run within the serpentine layer, connecting the vertical columns with each other, interconnecting their information and partially forwarding it to the central brain. The axons of the L1-L5 cells of the lamina end in the corresponding column in the medulla, as do the photoreceptor cells R7 and R8. The axons form an optic chiasm between the lamina and the medulla . The bundled information arrives in each medulla column from one point in the field of vision, indirectly via the monopolar cells of the lamina (L1-L5) and directly via the receptor cells R7 and R8. Two types of projection neurons exit the medulla from the different layers. These are transmedulla cells of the type Tm and TmY, which connect different columns of the medulla with the lobula (Tm-type) or with the lobula and lobular plate (TmY-type) and thus form a second optic chiasm.

Lobular complex

The lobular complex, consisting of the anterior lobula and the posterior lobular plate, is positioned proximal to the medulla and connected to it by an internal optic chiasm. The lobular complex represents a connection between the medulla and the visual centers of the central brain, i.e. it links visual perception with flight behavior. The lobula forwards the image information received via the anterior optical tract to the central brain, while the lobular plate forwards the respective movement information via horizontal and vertical cells. The lobular complex has a direct neural connection to the flight apparatus and codes the movement of stimulus patterns depending on the direction.

Function of the optical system

The function of the visual system at D. melanogaster is the perception and processing of visual information, as well as the differentiation of light conditions during day and night. D. melanogaster can fly very quickly. Therefore the visual system has to provide a very high temporal resolution as well as a well organized forwarding of the information. In addition, the fly can react to possible sources of danger in good time and thus ensure its survival. The temporal resolution is 265 frames per second.

The fly can differentiate between different objects based on different light spectra and light intensities. The eye's spectral perception is between 300 and 650 nm. The 8 different photoreceptors differ in the absorption maxima of their photopigments, the rhodopsins . The photoreceptors 1-6 (R1-6) located in the periphery of the ommatidium express blue-green rhodopsin 1 (absorption maximum at 478 nm) and also contain short-wave ultraviolet-sensitive pigments. The photoreceptors 1-6 are activated by weak light intensities and contrasts. In the photoreceptor R7 there is either Rh3 (345 nm) or Rh4 (374 nm). Photoreceptor R8 expresses blue-light-sensitive (Rh5, 437 nm) or green-light-sensitive rhodopsins (Rh6, 508 nm).

At the dorsal edge of the eye, R7 and R8 express rhodopsin 3, which absorbs ultraviolet light. This area of ​​the retina is used to detect the e-vector of polarized light. With the help of the e-vector, the flies can orient themselves to the sun. In the rest of the retina there are two types of ommatidia, "pale (p)" and "yellow (y)". In the p-type ommatide, R7 expresses Rh3 and R8 blue-sensitive Rh5. In the y-type, R7 expresses Rh4, which absorbs long-wave UV light, and R8 the green-sensitive Rh6.

For a long time it was assumed that photoreceptors 1-6 are exclusively responsible for motion vision and receptors 7 and 8 for color vision. Flies that have photoreceptors 1-6 turned off show little response to movement. However, all photoreceptors 1-8 are involved in motion vision. In the early larval stage, the larva’s main goal is to eat. For this reason the feeding larvae stay inside the food and show negative phototaxis . Only shortly before metamorphosis do they show positive phototaxis, the wandering larva leaves the food source to look for a place outside to pupate.

Circadian system

Endogenous clocks help living organisms adapt to the daily cycles of the environment. Like many other living beings, D. melanogaster has such an "internal clock". This so-called circadian system regulates metabolic processes, development and behavior, among other things.

Location and structure

Larval brain; left: PDF expressed in the hemispheres of D. melanogaster . Right: PDF expressed in the LN

In D. melanogaster , the central clock is located in the brain and consists of two lateral and one dorsal neuron groups per hemisphere. These groups of neurons are part of the protocerebrum. The first lateral neuron group (LN) consists of 5-8 dorsally lying neurons (LNd), the second group lies ventrally (LNv) and is further subdivided into 4-6 large LNv (l-LNv) and five small LNv (s-LNv ). The third group consists of the neurons (DN) lying dorsally in the brain. The dorsal group is further subdivided according to the morphology and location of the individual neurons into about 15 DN1 and 2 DN2, which are medium-sized and located posteriorly in the dorsal superior brain. About 40 small DN3 cells are located laterally in the dorsal brain. In the larva there are four PDF-expressing (see external regulation / circadian control of behavior) lateral neurons in each hemisphere (see Fig. 1), which correspond to the s-LNv in the adult animal. The l-LNv, LNd and DN arise during metamorphosis. Except for the l-LNv, all groups of neurons project into the dorsal protocerebrum. In addition, s-LNv, l-LNv, DN1 and DN3 send projections to the accessory medulla. The l-LNv connect the two accessory medullae with each other via the posterior optic tract. One target of the output pathways of the internal clock could be the mushroom body and central complex. The mushroom body is presumably under rhythmic control of the s-LNv cells, which could have a circadian influence on learning and memory. The LNd cells suspect innervation of the central complex, which may be a switching station for circadian signals. Movement activity is also controlled by circadian signals. s-LNv cells route the signals to DN1 and DN2, where they are switched and forwarded to the movement centers. In contrast, the signals of the LNd are switched in the central complex. These signals are also processed further in the movement centers.

Circadian control of behavior

Among other things, the circadian system controls movement behavior, which has two activity peaks during the day. Under light-dark conditions (12 hours of light and 12 hours of darkness), she recorded two locomotor activity peaks in the morning (ZT = 0) and in the evening (ZT = 12). These activity peaks can also be observed under constant conditions (e.g. dark-dark situation). The daily rhythm in null mutants, however, has no rhythm under constant conditions. If they are exposed to light-dark cycles, however, they have a diurnal rhythm. From this it can be concluded that the rhythm in locomotor activity can be traced back to the internal clock and daylight. The circadian synchronization takes place via two coupled oscillators, which consist of a network of LNv and LNd. The LNv regulate the activity shortly before dawn, while the LNd regulate the activity before dusk. The neuropeptide PDF, which is expressed in the s-LNv and l-LNv of the two hemispheres, plays an important role here. PDF is an output signal from the internal clock that is necessary for rhythmic activity in a 12-12h light-dark cycle. In the absence of PDF, D. melanogaster become arrhythmic in permanent darkness.

The light synchronization takes place via the internal photoreceptor cryptochrome (CRY), which occurs in almost all pacemaker cells. In addition, the light perception also takes place through the compound eyes, the Hofbauer-Buchner-Äuglein and the Ocellen. In addition to the light, other factors can act as external timers, e.g. B. Temperature and pheromones.

Molecular Mechanism

Above: Overview of the molecular circadian oscillator of D. melanogaster , below: TIM expressed in the hemispheres of D. melanogaster (here colored green with GFP)

The mechanism briefly outlined here represents a central aspect of the circadian system in D. melanogaster : In order to maintain the internal clock or the day-night rhythm, D. melanogaster has a number of genes ('clock genes') , the expression of which fluctuates cyclically over the course of the day. The clock genes expressed in the clock neurons in the D. melanogaster brain include Cycle ( CYC ), Clock ( CLK ), Period ( PER ), and Timeless ( TIM ).

The two regulatory proteins Clock (CLK) and Cycle (CYC) can together activate the transcription of the genes period (per) and timeless (tim). Since the protein TIM is very light-sensitive and stabilizes TIM PER, these two proteins can only be accumulated in the evening or at night. The proteins PER and TIM then form a dimer that migrates into the nucleus and can then inhibit the transcription of the Cycle and Clock genes in the cell nucleus (see picture). This is a positive (clk, cyc) and negative (per, tim) feedback loop, which guarantees a cyclic expression of the clock genes. Since this mechanism takes place at the genetic level, it is also referred to as an endogenous molecular oscillator.

behavior

In the larval stage, D. melanogaster exhibits negative phototaxis regulated by the circadian system , which is characterized by minimal light sensitivity at the end of the subjective day (CT = 12) and maximum light sensitivity towards subjective morning (CT = 0). In evolutionary terms, this behavior of the larvae probably serves to avoid predators. The adult fly also exhibits several circadian clock-dependent behavior patterns, such as: B. the adult hatching from the doll, which happens at the time of the subjective morning (CT = 0) in order to prevent rapid water loss. The feeding rhythm is influenced by both light and the circadian clock. Under light-dark conditions, there is a food intake peak in the morning (ZT = 0-2) and then a long phase of greatly reduced food intake (ZT = 8-22). Under dark-dark conditions there is a food intake peak from morning to noon (CT = 0-6) and a greatly reduced food intake from late day to early evening (CT = 8-14). The ability to develop an olfactory associative short-term memory has a peak at the time of the subjective early night (CT = 13) and a further peak shortly before midnight (CT = 17). The best perception of chemical odorous substances, which can be represented by electroantennograms , takes place during the subjective night (CT = 17). However, a connection to certain behavioral patterns is unclear. The immune system is more susceptible to bacterial infection by Pseudomonas aeruginosa and Staphylococcus aureus during the subjective day (time of infection: CT = 5) than during the subjective night (time of infection: CT = 17), during which an increased expression of antimicrobial peptides (AMP ) in comparison to an infection occurring during the day. Finally, courtship and mating behavior is also subject to rhythmic fluctuations. These fluctuations are mainly determined by the behavior of the male. The courtship and mating behavior has a peak at the time of the subjective morning (CT = 0) and around midnight (CT = 18), as well as a low point at the time of the subjective evening (CT = 12).

The neuroendocrine system

The neuroendocrine system is used for cell-cell communication. It sends signals from cells of the nervous system via hormone-like messenger substances to target cells in the tissue of various organs. The neuroendocrine system consists of neurosecretory cells that project into neurohemal organs or neurohemal zones and from there release messenger substances (typically peptides ) into the circulation in order to act on the target tissue. It is this property that distinguishes neurosecretory cells from ordinary neurons. The pars intercerebralis and the pars lateralis are important centers in the dorso-medial protocerebrum that contain such neurosecretory cells.

Neuroendocrine system of the larva

In the larva, the axons of the secretory neurons of the pars intercerebralis and the pars lateralis project into the ring gland via the nervi corporis cardiaci. In the larval stage, the ring gland is a complex of two endocrine glands, the prothorax gland and the corpus allatum, and a neurohemal area, the paired corpora cardiaca, which are associated with the aorta. The structure of the ring gland changes during metamorphosis into an adult insect (see metamorphosis). In the larval stage, the ring gland can be recognized by its conspicuous structure, anterior to the two cerebral hemispheres. The ring gland is connected to the larval brain by the Nervi Corporis Cardiaci (NCC). The prothorax gland takes up the largest volume in the ring gland. The cells divide and enlarge as the larvae develop. The corpus cardiacum is unpaired ventrally within the ring gland and has a U-shaped structure. The adipokinetic hormone (AKH) is produced in its glandular area, which stimulates the breakdown of fats and carbohydrates in the fat body. In the prothorax gland, the synthesis of the steroid hormone ecdysone is activated by PTTH (prothoracotropic hormone). Ecdysone is responsible, among other things, for adult moulting and, in combination with the juvenile hormone, for larval moulting. The corpus allatum synthesizes the juvenile hormone. In each hemisphere of the brain there are five neurons of the lateral protocerebrum that innervate the two endocrine glands. These are in close proximity to axons of the circadian pacemaker neurons. This connection is possibly responsible for the circadian rhythm of molting and metamorphosis. In addition, a ventromedial neuron was found that innervates the ring gland and is responsible for the production of the eclosion hormone.

Perisympathetic Organs

The perisympathetic organs (PSO) are neurohemal organs that appear as thickenings on the median and transverse nerves. In the D. melanogaster larva, they are found associated with the three thoracic neuromers and the abdominal neuromers A2 - A4. The thoracic PSO are each innervated by a cell pair of Tv neurons, the abdominal PSO each by a cell pair of Va neurons in the same neuromer. During metamorphosis, the PSO disappear and the innervating peptidergic neurites are incorporated into the ventral ganglion. After metamorphosis, their terminals lie between the cell body cortex and the glia surrounding the nervous system, where they form a neurohemal zone.

Neurochemistry

source

Larval and adult neuropeptide hormones Release site
Adipokinetic Hormones (AKH) Corpus cardiacum (CC)
Bursicon (BURS) neurohemal zone
CAPA-Periviscerokinin (CAPA-PVK) abdominal PSO
CAPA pyrokinin abdominal PSO, CC
Corazonin (CRZ) CC
Crustacean cardioactive peptides (CCAP) neurohemal zone
Diuretic hormone 31 (DH31) CC
Ecdysis-triggering hormone (ETH) epitracheal cells
FMRFamide thoracic PSO
Hugin pyrokinin CC
Insulin-like peptides (DILP) CC
Leukokinin (LK) neurohemal zone
Myoinhibitory Peptide (MIP, AstB) neurohemal zone
Myosuppressin (DMS) CC
Partner of Bursicon (PBURS) neurohemal zone
Prothoracotropic hormone (PTTH) CC
Slip hormone (eclosion hormone, EH) CC
short neuropeptide F (sNPF) CC

At least 42 differently coding genes for precursors of neuropeptides , peptide hormones and protein hormones were found in D. melanogaster . Most peptide hormones activate G-protein coupled receptors (GPCRs). At least 45 neuropeptides, peptide and protein hormones GPCRs were identified. Each neuropeptide gene has specific expression patterns in the larval and adult nervous systems of D. melanogaster . Neuropeptides can be produced by different types of neurons. These include in D. melanogaster , the olfactory receptor neurons different types of neurons , neurosecretory cells , motor neurons and secretory neurons.

DIMMED

The basic helix-loop-helix (bHLH) transcription factor DIMMED is a crucial regulator in neuroendocrine cell differentiation. It is selectively expressed in neuroendocrine cells and is apparently responsible for coordinating their molecular and cellular properties. A transcriptional control occurs, which leads to the regulated secretory pathway being taken. The transcription factor enables the cell to develop and accumulate LDCVs (large dense-core vesicles). These vesicles can store neuropeptides and secrete them after increasing the free intracellular calcium concentration. On the other hand, DIMM activates the complete post-translational processing of neuropeptides. This enables biologically active peptides to be produced from the prepropeptides. DIMM can transfer properties of neuroendocrine cells to neurons that do not otherwise belong to this type: Non-peptidergic neurons usually do not accumulate ectopic neuropeptides. However, after ectopic expression of DIMM, they can. There are peptidergic neurons that do not express DIMM, these are interneurons. With overexpression in the wild type, both the level of secretory peptides in neuroendocrine cells and the number of cells showing a neuroendocrine phenotype increase.

Function of the neuroendocrine system

Functions of neuropeptides and peptide hormones Neuropeptides and peptide hormones
Development and growth DILP
Eating behavior Hugin-PK, NPF, sNPF
Water and ion balance DH44, DH31, LK, CAPA-PVK
Courtship behavior SIFamid, SP, NPF
Aggressive behavior NPF
Move AKH, DTK
Circadian output factor PDF
metabolism DILP, AKH

metamorphosis

The metamorphosis in D. melanogaster is controlled by the interplay between ecdysone and juvenile hormone. If a high concentration of juvenile hormone is present in the larva, ecdysteroids induce larval molt. The juvenile hormone promotes larval growth and inhibits metamorphosis. If the concentration of this hormone is low, the larvae ecdysone-induced moults. If the juvenile hormone is no longer present, but there is a high concentration of ecdysone, imaginal molting is initiated. Changes in the ring gland during metamorphosis

The endocrine glands of the ring gland undergo drastic changes during metamorphosis. After pupation begins, the ring gland migrates from its position above the cerebral hemispheres to the esophagus to just before the forestomach (proventriculus). A new basal lamina wraps itself around the individual parts of the ring gland. The prothoracic gland separates from the corpus allatum and from the corpus cardiacum. The corpus cardiacum fuses with the hypocerebral ganglion to form a complex. At the end of the metamorphosis, the ecdysone biosynthesis decreases as the prothoracic gland degenerates. 24 hours after pupation, the port thoracic gland cells begin to shrink, move away from each other and eventually induce cell death. Only the cells of the prothoracic gland degenerate. Corpus allatum and corpus cardiacum, on the other hand, are also found in the adult animal.

Aging process

Depending on the living conditions, the fruit fly lives 2-8 weeks. However, the lifespan in males is only about 10 days. Aging in D. melanogaster is controlled by hormones. These include ecdysone and juvenile hormone in particular, which affect senescence . Mutations in the insulin signaling pathway extend the lifespan in D. melanogaster and influence the hormone level of other hormones, including juvenile hormone and ecdysone in particular. If the endocrine tissue of the JH-producing corpus allatum is removed, the survival of the flies is extended and mortality is reduced. Adult flies treated with juvenile hormone show an increased mortality. It follows that age is regulated, at least in part, by the neuroendocrine control of juvenile hormone. Diapause in adult flies delays senescence and can increase survival. In experiments, induced diapause slowed aging. Age and mortality depend on the neuronal regulation of the juvenile hormone. The neuroendocrine response depends, among other things, on the environment, which in turn influences aging.

Drosophila melanogaster as a research object in genetics

Culture vessels in the laboratory

The fly as an object of investigation in classical genetics

D. melanogaster became an experimental animal of classical genetics in the first half of the 20th century through the research of the American zoologist and geneticist Thomas Hunt Morgan and his school . This species has only four different chromosomes , which are found in pairs in the D. melanogaster cells: a pair of sex chromosomes , also known as the first chromosome or the X or Y chromosome, and three pairs of autosomes , the second, third and fourth chromosome. However, the fourth chromosome is very small and contains only a few genes. Also ideal for research is that it is easy to breed large numbers of flies in bottles and that the succession of generations is short. "Half a carton of milk with a piece of rotting banana was enough to keep two hundred fruit flies happy for a fortnight," writes Martin Brookes in his 2002 book on Drosophila . A very large number of cross-breeding experiments with the fruit flies have thus been carried out. In this coupling groups of genes that on the same chromosome sit, found the phenomenon of crossing over discovered and also some mutants described and analyzed in detail, such as birds with white instead of red eyes or copies with stubby wings that are unable to fly. Hermann Muller was the first to recognize the mutation-inducing effect of x-rays on the genetic material of the fruit fly. Since then, the hard rays have been used to induce a variety of different mutations in flies.

D. melanogaster's popularity as a model organism initially lasted until the 1940s.

With Drosophila synthetica there is a genetically modified variant that has been modified so much in the laboratory that it can be viewed as a separate species.

Sequence analysis results

Sequencing of the genome was completed in 2000. A total of 139,731,881 base pairs and around 13,600 different genes were identified. This first estimate has to be revised after ten years, since 19,806 genes are now known. Many of these genes have astonishing resemblance to human genes . Researchers have found that around 70 percent of the human genes that have been described in connection with cancer and that are suspected to be involved in the development of cancer in a mutated state also occur in the genome of the fruit fly.

Developmental research

In the context of developmental studies, numerous findings have been made on the embryonic stages of fruit flies. As early as 1900, Harvard professor William Ernest Castle was the first to come across the fruit fly in search of an organism that was suitable as an object for embryological studies. A lot has happened in this area since then. In the 1970s, Christiane Nüsslein-Volhard began to study the developmental genes of D. melanogaster . The development of the fly from egg to imago is controlled by a gene cascade of different gene groups. The gene groups that appear earlier in this gene cascade influence the subsequent ones, but not vice versa. In the first place are the maternal coordinate genes already expressed during oogenesis in egg cells , nutrient cells and follicle cells. This is followed first by the gap genes , then the pair rule genes and finally the segment polarity genes during larval development . The homeotic genes ultimately ensure the development of the organs in the corresponding segments. In 1980 she published her groundbreaking study of the "mutations, the number and polarity of the segments in D. melanogaster influence" for 1995, along with Eric Wieschaus and Edward Lewis the Nobel Prize in Physiology or Medicine was awarded.

Advantages of Drosophila melanogaster as a model organism

D. melanogaster is a species of fly that is very easy and cheap to grow. In genetic research, D. melanogaster is the preferred research object because it has a short generation sequence (about 9-14 days), up to 400 offspring can arise from one generation, each individual has only four pairs of chromosomes and because the species shows many easily recognizable gene mutations . With the Gal4 / UAS system , a genetic tool is available which allows the expression of any genes in specifically selected cells.

Use as live food

In addition to its use in genetics, D. melanogaster is also popular as a food animal , for example for feeding fish or small reptiles and amphibians. Flightless mutants are mainly used because they are easier to handle.

Drosophila melanogaster research community

In the USA, the largest international Drosophila conference takes place every year in different cities . It has about 2000 participants. The European Drosophila Conference has an average of 400 to 500 participants and takes place every two years in different European countries. There is a small German regional conference every year. Furthermore, D. melanogaster is represented as a research object at many international life science, developmental biology, neurobiology and other conferences.

Mutations

The cultivations in the scientific laboratories have produced a myriad of mutations. Most of the 13400 genes have now been mutated in systematic screens .

literature

  • Karl-Friedrich Fischbach : Functional differentiation and interactions of the receptor systems in the complex eye of Drosophila melanogaster . Freiburg 1976, DNB 770769349 .
  • David B. Roberts: Drosophila: A practical Approach. IRL Press, Oxford, Washington DC, 1986, ISBN 0-947946-45-4 .
  • Peter A. Lawrence : The making of a fly. The genetics of animal design . Blackwell Science, 1992, ISBN 0-632-03048-8 .
  • Robert E. Kohler: Lords of the fly. Drosophila genetics and the experimental life . University of Chicago Press, 1994, ISBN 0-226-45062-7 .
  • Gary Rubin , Edward B. Lewis : A brief history of Drosophila's contributions to genome research. In: Science . Volume 287, 2000, pp. 2216-2218.
  • Martin Brookes: Drosophila - The Success Story of the Fruit Fly. Rowohlt Verlag, Hamburg 2002, ISBN 3-498-00622-3 .
  • Christian Dahmann (Ed.): Drosophila: methods and protocols. Humana Press / Springer, Berlin 2008.

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