Dictyostelium discoideum
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Fruit bodies of Dictyostelium discoideum |
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| Raper |
Dictyostelium discoideum is a species of slime mold that occurs worldwideand lives as a single cell in the ground. Triggered by a lack of food, many of the unicellular amoebas form a multicellular association (“social amoeba”) and form a fruiting body on a long stalk from which spores are released.
Since its discovery in 1935, the species has become an important model organism in biology.
Features and life cycle
The life cycle of Dictyostelium is divided into a growth phase and a development phase. In the change from growth to development phase, a real multicellular organism emerges from a population of individual amoeba made up of two tissues with differentiated cells: dead stem cells and germinable spores.
Growth phase
In the vegetative phase, Dictyostelium discoideum is a haploid , unicellular and mononuclear amoeba of irregular shape and a diameter of around 10 to 20 micrometers. A cell wall is missing, the cell is only closed off by a cell membrane . It has numerous pseudopods and food vacuoles .
With sufficient availability of food, Dictyostelium discoideum lives as a single cell and multiplies through cell division. It feeds on soil bacteria , which it ingests by flowing around them and thus enclosing them in a vacuole ( phagocytosis ).
After that, vacuoles combine with digestive enzymes called lysosomes , and the bacterium is digested. Between two cell divisions, D. discoideum phagocytoses around 1000 bacteria. Dictyostelium moves by forming cell processes called pseudopods , which it stretches forward and then pulls the cell body along. As with the movements of human muscle cells, this also works here through the cooperation of actin and myosin filaments. With these movements, they react to chemical stimuli ( chemotaxis ), so to speak, to the smell of the bacteria on which they feed or, later, when the slime mold forms, to the messenger substances of their own kind.
Hunger phase
If the ratio between the amount of food available and the population density of the amoeba falls below a critical value, D. discoideum changes from the vegetative growth phase to a development phase that is characterized by fundamental morphological changes and different gene expression .
The amoeba are able to use a glycoprotein, the pre-starvation factor (PSF), to recognize the relationship between population density and bacterial food source. During the G2 phase of the cell cycle, this factor is synthesized, secreted and accumulated in the close vicinity of the amoeba. If a defined concentration ratio is exceeded, the hunger phase is initiated, which represents the transit to the development phase. In the hunger phase, only a few amoebas begin to excrete (secrete) cAMP as a hunger signal. The development phase is initiated immediately via a signal cascade. The immediate further course of the signal cascade has not yet been precisely clarified.
phase of development
Streaming
CAMP secreted by some amoeba attracts other amoeba, which in turn produce and secrete cAMP. The secreted cAMP initiates chemotaxis, which causes the amoeba to begin moving in the direction of the increasing cAMP concentration. This leads to the formation of the typical branched paths. Due to the release of cAMP on the side of the cell facing away from the direction of movement, a “head to tail” formation is formed.
Pseudoplasmodium
As a result, 50,000 to 100,000 amoebas aggregate and form a pseudoplasmodium , also known as a “mound”. However, the cells do not fuse, but form a multicellular association with a population density of at least 400 cells / mm². Already at this point there is a change in the activity of the development-relevant genes and thus a differentiation of the cells into two different cell types: pre-spore cells and pre-stem cells.
Slug
In the further course of development, the “mound” becomes a “slug” (English for “slug”). This slug, surrounded by a layer of mucus, is able to react to phototactic, chemotactic or thermotactic stimuli with movement. In the slug stage, the equilibrium of approximately 20% pre-stem cells and 80% pre-spore cells is established and sorted. The sorting is controlled by the different sensitivity of the two cell types to cAMP. The pre-stem cells collect due to an increased cAMP sensitivity in the front area of the slug, while the pre-spore cells form the remaining slug.
Finger stage
In the further course the slug can go straight to the culmination or wander around. This serves to find a more suitable place for the culmination. The slug stretches upwards (finger stage) and then wanders around like a snail (wandering "slug").
culmination
In the course of further development, a cAMP gradient is triggered at the tip of the slug , as the pre-stem cells increasingly express extracellular cAMP phosphodiesterases. The decrease in cAMP concentration at the slug tip triggers culmination.
Mexican Hat
A stem tube forms at the base, at the top of which the pre-stem cells are located ("mexican hat"). The pre-stem cells swell due to vacuolization and then die; at the same time the pre-spore cells are lifted up. They condense when they release water, surround themselves with a mucopolysaccharide shell to protect themselves from heat and drought and then go into a state of rest. The spore head is fixed to the stem of the fruiting body by the "upper" and "lower cups".
Fruiting bodies
The sporocarp ("fruiting body") forms the end of the development cycle . This consists of dead stem cells, germinable spore cells and a basal disk for anchoring on the substrate. The oval spores are insensitive to heat and dehydration and, under suitable environmental conditions, are able to germinate again as single-cell organisms and initiate a new development cycle.
The genome
The haploid genome in the nucleus of D. discoideum is around 34 Mb and codes for 12,000 to 13,000 genes. The genes are distributed over 6 chromosomes, which are between 4 and 7 Mb in size. Another component of the genome are the approximately 90 copies of an 88 kb extrachromosomal palindrome , which code for the ribonucleic acids 5S, 5.8S, 17S and 26S rRNA . They are also located in the nucleus and make up 23% of the DNA in the nucleus. In addition, there are around 200 mitochondria in each cell, each with a copy of an approximately 55 kb mitochondrial DNA, which mainly codes for genes of energy metabolism.
The Dictyostelium Genome Project, which is being carried out by an international consortium, set itself the goal of sequencing the entire genome of D. discoideum, strain AX4, with the sequencing of the six chromosomes being carried out by different working groups (Jena, Cologne, Houston , Paris and Hinxton). In 2005 the sequencing of the chromosomes was completed and published on the Internet. The Dictyostelium genome will enable the systematic elucidation of the functions of many genes and, by comparing them with gene sets from other organisms, will be able to make a clear statement about the phylogenetic classification.
Systematics and research history
Dictyostelium discoideum was first described in 1935 by Kenneth Bryan Raper , who subsequently devoted himself to researching the species for almost 50 years. Due to its life cycle, it serves as a model organism for the transition from unicellular to multicellular organism (especially John Tyler Bonner ). Particular interest lies in researching the transition from cells of the same type to the differentiation of a macrostructure with cells of different specialization. The genome of Dictyostelium discoideum was deciphered in 2005 by an international team of researchers.
distribution
Dictyostelium discoideum is distributed worldwide in all climatic zones from the cold-temperate climate zone to the tropics, but shows a distribution gradient: it is most common in the cool-temperate zones, then it becomes increasingly rare up to the tropics.
proof
Footnotes directly behind a statement cover the individual statement, footnotes directly behind a punctuation mark the entire preceding sentence. Footnotes after a space refer to the entire preceding paragraph.
- ^ A b Pascale Gaudet, Jeffery G. Williams, Petra Fey, Rex L. Chisholm: An anatomy ontology to represent biological knowledge in Dictyostelium discoideum. In: BMC Genomics. 9: 130, 2008, doi : 10.1186 / 1471-2164-9-130
- ↑ [1]
- ^ Richard H. Kessin: Dictyostelium: Evolution, Cell Biology, and the Development of Multicellularity . Cambridge University Press, Cambridge 2001, ISBN 0-521-58364-0 , pp. 9-18 .
- ↑ James C. Cavender: Geographical Distribution of Acrasieae. In: Mycologia. 65: 5, 1973, pp. 1044-1054.
swell
- Monika Unha Baik: Influence of CbfA on growth and development in Dictyostelium discoideum . Dissertation. Frankfurt am Main 2004, DNB 974551120 .
Web links
- Dictyostelium Discoideum Genome Project Center Biochemistry University of Cologne
- dictyBase - Online Informatics Resource for Dictyostelium