Heterodera glycines with an egg, a parasite of the soybean
|Rudolphi , 1808|
The roundworms (Nematoda), also known as nematodes ( ancient Greek νῆμα nema , German 'thread' ) or Älchen , are a very species-rich strain of the animal kingdom. To date, more than 20,000 different species have been described. They are probably the most individual group among multicellular animals : According to one estimate, they make up around 80% of all multicellular animals . They are mostly relatively small, white to colorless, filamentous worms that live in moist media. The majority of species feed on microbes, but there are also predatory and numerous parasitic species that attack plants, animals and humans.
Roundworms have successfully adapted to almost every terrestrial and aquatic ecosystem, including extreme habitats such as deeper areas of the top crust and the polar regions. In many habitats they often represent the largest group in the metazoan fauna both in terms of the number of individuals and the diversity of species .
Nematodes are triploblastic primordial mouths (protostomia). They have a typical worm-shaped shape, are long and round in cross-section. There is no segmentation. The body cavity is a narrow pseudocoel , as in many other smaller animal phyla.
The front end of a roundworm contains a large muscular mouth opening with a subsequent pharynx (throat). The anatomy of the mouth and pharynx is adapted in many ways to the type of food consumed. The tubular body contains the digestive and reproductive organs. The anus is just before the rear end.
The largest nematode is in the sperm whale - placenta living gigantissima Placentonema . Females reach a length of up to 8.40 m and a diameter of 2.5 cm, the males are only 4 m long with a diameter of 0.9 cm. This species belongs to the class of Secernentea (subclass Spiruria, family Tetrameridae).
Epidermis and cuticle
The epidermis (skin cell layer ) of a roundworm is remarkable because it does not consist of individual cells , as in other animals , but of a mass of cellular material that is not divided into individual cells by membranes and has several cell nuclei. Such formations are called syncytium .
The epidermis secretes a much thicker, multi-layered cuticle that protects the nematodes from drying out or other unfavorable environmental conditions, and in the case of parasitic species also from the digestive juices of the host. In addition to the species living in hot springs, species have also been found whose cuticula enables them to withstand pH values of 2.5 (e.g. lemon juice , vinegar ) to 11.5 (e.g. household ammonia ) (see also the following pH value scale ), or those that remain alive for several hours in liquid helium (approx. −272 ° C to −268 ° C). The presence of a "stiff" cuticle in connection with the longitudinal muscles (nematodes have almost no circular muscles) only allows them to move in a meandering manner.
In nematodes, the cuticle consists of up to four layers:
- The inner fiber layer consists of fibers running diagonally with one another in opposite directions. This layer is the main contributor to the strength and elasticity of the cuticle.
- The matrix layer has a less defined structure.
- The cortical layer is made up of collagen.
- The outer epicuticle is lipidhaltig and is additionally covered in some species by a lipid layer.
Like roundworms, roundworms only have longitudinal muscles that extend from head to tail. The muscle cells consist of three parts:
- The monocyton: a non-contractible part that contains the cell nuclei, the mitochondria and the Golgi apparatus .
- A contractible part that contains the actin and myosin fibers.
- The process, a non-contractile part of muscle cells that can form connections with other muscle cells or nerves.
The threadworm muscles lie like a tube beneath the skin. This unit of different tissues is called the skin muscle tube . The strong cuticle and the high internal pressure of the pseudocoelic fluid , which is between 70 and 210 mmHg , represent a so-called hydroskeleton . Together with the longitudinal muscles as antagonists, the nematode can meander or stretch a part upwards.
There are also circular muscles, but only on the mouth and anus.
The nematode nervous system has a very simple structure. It consists of a circumpharyngeal or circumoesophageal ring, of which a dorsal and a ventral main cord pull backwards. It is able to perceive and process simple and diverse stimuli. The longitudinal nerves are in direct contact with the muscle cells and the cytoplasm and extend throughout the body. What is remarkable is the fact that, unlike in other animals, in which the nerve cells spread to the muscles, the muscle cells of the nematode spread to the nerve pathways themselves.
In the male, a cloaca is formed at the end of the body, which includes the vas deferens , rectum and the spicular apparatus. The latter represents the taxon-typical copulation organ and consists of paired, hardened, hook-shaped spicula (singular: spiculum ), which lie in a pocket of the cuticle and are guided in movement by a guide structure (gubernaculum). Only one spiculum is rarely present due to reduction or fusion. The spicula are not used to transport semen, but rather hook the male in the female's vulva during the act of copulation so that the sperm can be transferred directly from the seminal duct.
The food is varied and ranges from free-living species of bacteria and algae to fungi, carrion and faeces to predatory animals. There are often small appendages on the mouth that are used to ingest food or to feel. There the food is drawn in and crushed by strong muscles. The food then passes from there into a simple long intestinal space, where it is processed and digested. Nematodes do not have a vascular system with which they could distribute the food components in the body. Instead, the nutrients are processed in the intestinal cavity and from there passed directly through the walls to the body cells where they are needed.
Endosymbiotic microorganisms ( bacteria and fungi ) that are required for the breakdown of certain food components can also occur in the intestine , e.g. B. for the breakdown of cellulose . In addition, endogenous cellulases were found in a few species such as Bursaphelenchus xylophilus and the beetle-living Pristionchus pacificus . For the origin of their cellulase genes, a horizontal gene transfer based on their endosymbionts has been announced.
Oxygen uptake works in a similar way to digestion. Since the nematodes have no respiratory organs and no vascular system, the oxygen is absorbed through the skin and diffuses directly to the body cells.
The propagation takes place sexually, usually with two separate sexes. The males are typically smaller than the females and often have a characteristically curved tail. However, self-fertilizing hermaphrodites, such as Caenorhabditis elegans , are not uncommon either. Parasitic species often have a rather complicated life and reproductive cycle with alternation of generations , which can be accompanied by a change of host or organ change in the host.
The nematodes shed their skin and are therefore counted among the molting animals (Ecdysozoa) , as well as on the basis of RNA investigations within the primordial mouths (protostomia) . In wild species, development usually takes place directly with four moults in the course of growth.
Nematodes are found almost everywhere, in the ocean , in freshwater and in terrestrial ecosystems. They are generally considered to be "frequent and omnipresent" and are often represented by more species and individuals in an ecosystem than any other group of multicellular animals (Metazoa). They are very important in the soils , where they occupy several of the lower trophic levels . Furthermore, nematodes have also colonized extreme habitats. The species Halicephalobus mephisto was found in South Africa in fractured waters at depths of up to 3.6 kilometers, making it the multicellular animal that lives deepest in the earth's crust . Others populate the soils of the McMurdo Dry Valleys in Antarctica , where they can survive extremely adverse conditions, under which virtually no soil water is available due to the interplay of extreme cold, salinity and drought , by entering a state of rest (anhydrobiosis). There are also a significant number of parasitic species, both in plants (see for example beet elbows ) and in animals, including humans. The nematodes that parasitize humans and live in their intestines include, for example, the roundworm ( Ascaris lumbricoides ), the whipworm ( Trichuris trichiura ), the medinaworm ( Dracunculus medinensis ), the pinworm ( Enterobius vermicularis ) and the dwarf threadworm ( Strongyloides stercoralis ) , whereas the filariae Wuchereria bancrofti , Brugia malayi and Loa loa live in the lymph vessels or in the subcutaneous fatty tissue.
Most free-living nematodes are microscopic and belong to the meiofauna . Only some animal parasites can become significantly larger.
Pathways and strategies of infection of parasitic species
The infection or infestation of serving as final host mammals, including humans, happens z. B. by eating raw meat that already contains larvae (e.g. trichinae ), or as a result of the ingestion of worm eggs by eating feces ( e.g. dogs). Food contaminated with faeces (with worm eggs) due to poor hygiene (fertilizing food plants with liquid manure , no hand washing after defecation) can play a role in the transmission. In several species, however, infection can also occur through active penetration of (filariform) larvae through the skin (hookworms, e.g. Ancylostoma duodenale or Necator americanus ). Representatives of the filariae such as Onchocerca volvulus , the causative agent of river blindness , are usually spread through insect bites.
With regard to their life and reproductive cycle, some parasitic nematode species, as well as parasitic representatives of other large groups of invertebrates or unicellular organisms (cf. → Changes in behavior due to toxoplasmosis ), are able to manipulate the behavior and, in some cases, the external appearance of their hosts. Myrmeconema neotropicum is a particularly complex example . This representative causes the filled with his balls Gaster of as intermediate host serving ant Cephalotus atratus of discolored black to bright red. The ants affected, which in comparison to their non-infected conspecifics show virtually no aggressive defense behavior when harassed and do not produce any alarm pheromones, constantly stretch their red abdomen upwards. This is likely to attract fruit-eating birds, which serve as final hosts, and to encourage them to ingest the gaster which can easily be detached from the rest of the body.
The pine wood nematode Bursaphelenchus xylophilus , a quarantine pathogen that is notifiable in Germany and Switzerland , uses longhorn beetles of the genus Monochamus ("artisan goats") as a transport host to spread .
The nematodes were originally introduced by Nathan Cobb in 1919 as the strain Nemata, later classified as the class Nematoda in a no longer valid strain Aschelminthes. Here the roundworms are managed as a separate strain.
- Class adenophorea
- Class Secernentea
Nematodes do not have any body parts that can be easily preserved over geological periods and are relatively easily identifiable in sedimentary rock . Therefore, their fossil record is very sketchy and limited in relation to their wide recent distribution and diversity and is limited to very specific forms of conservation or sedimentary facies .
The most and best preserved fossils were nematode in amber of the Cretaceous and Tertiary found the oldest of these specimens from the Lower Cretaceous of Lebanon come. The oldest direct fossil records ( body fossils ) come from the famous Rhynie Chert of the Lower Devonian of Scotland and are also the oldest direct records of parasitic nematodes. They consist of eggs, juvenile stages and adult individuals, which were discovered in the substomatic cavity ("respiratory cavity") of the early terrestrial plant Aglaophyton major and described in 2008 under the name Palaeonema phyticum . Sedimentary structures, interpreted as trace fossils of free-living marine, infaunal nematodes, have been described from claystones of the Lower Ordovician of the Hubei Province of China.
In the Kolyma lowlands in Siberia, nematodes were found that had been preserved in the permafrost soil for about 42,000 and 32,000 years, respectively. Despite the fact that they had been frozen since the New Pleistocene , two specimens of these worms, known as Panagrolaimus aff. detritophagus and plectus aff. parvus have been successfully resuscitated.
Many nematode species are pests in agriculture and horticulture, as they can severely impair plant metabolism by penetrating the root systems. Various chemical substances, the so-called nematicides , and, alternatively, biological control methods, such as planting the infested arable land with special crops ( e.g. resistant oil radish , marigold and mustard ), as well as thermal processes such as steaming (soil disinfection) with superheated steam, are used to combat nematode infestation used for soil disinfection .
Use and research
The species Caenorhabditis elegans has become a popular test animal for geneticists due to its simple keeping and cell constancy (Eutelia) and functions as a model organism . The nematode Pristionchus pacificus was established as a satellite organism for Caenorhabditis elegans . By comparing these two species, it is possible to research how development processes - the transition from egg to adult organism - change in the course of evolution. In addition, nematodes are increasingly used as beneficials against snails , weevils and other plant pests.
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- In Steinernema longicaudum , the male only develops a mature sexual apparatus and sperm when a female is nearby, cf. L. Ebssa, I. Dix, C. Griffin in Current Biology, Vol. 18, Issue 21, pages R997-R998
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- Ute Schönfeld: The pine wood nematode ( Bursaphelenchus xylophilus ). Plant protection information: Plant health control 54/2015. Plant Protection Service of the State of Brandenburg, State Office for Rural Development, Agriculture and Land Management, Frankfurt (Oder) 2015 ( PDF 452 kB)
- Therese Plüss, Simone Prospero, Thomas Röthlisberger, Bea Schwarzwälder, Christiane Lellig (ed.): Guide to dealing with the pine wood nematode ( Bursaphelenchus xylophilus ). Federal Plant Protection Service, Federal Office for the Environment, Federal Office for Agriculture, Bern 2015 ( PDF 1.44 MB)
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- For an overview of the finds especially in the Eocene Baltic amber see Wolfgang Weitschat, Wilfried Wichard: Atlas of plants and animals in Baltic amber. Pfeil, Munich 1998, ISBN 978-3-931516-45-1 , p. 54 ff.
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- Nematode.net - nematode systematics
- Plant and Insect Parasitic Nematodes - University of Nebraska Page (Retrieved October 20, 2009)
- Pristionchus pacificus the Wikionchus page of the Max Planck Institute for Developmental Biology
- ProGemüse - German-Dutch EU project on the problem of nematodes in vegetable growing
- Max Planck Institute for Developmental Biology