Aphelidea

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Aphelidea
Systematics
Domain : Eukaryotes (Eucaryota)
without rank: Amorphea
without rank: Opisthokonta
without rank: Nucletmycea
Trunk : Cryptomycota
Class : Aphelidea
Scientific name
Aphelidea
Gromov , 2000

The Aphelidea are a species-poor group of intracellular parasitoids of marine or freshwater living algal species of phytoplankton . Approximately 10 species in three genera are currently known; further evidence is available from gene samples obtained in waters without the associated organisms having already been found and described. The group has attracted interest as unicellular relatives of the multicellular animals and fungi that are grouped together in the opisthokonta group. You are shedding light on the origins of these groups.

Ultrastructural

The flagellated round to oval zoospore of Aphelidium has a flagellum with 9 + 2 axonemes . The structure of the kinetosome has not yet been elucidated in detail. The second centriole can be parallel to perpendicular to the kinetosome and is connected to it by a fibrous bridge. At the front end of the cell there are small filopodia that are used to attach to the host cell. The nucleus sits near the front end, followed by a dictyosome . The amoeboid spores of Amoeboaphelidium protococcarum are characteristically divided into two parts, with all cell organelles in the rear part.

Of the Aphelidea are mitochondria describes both flat as well as tubular cristae form; this characteristic is obviously not constant within the group.

Life cycle

Aphelidea have a complex life cycle. In the genus Aphelidium , a zoospore swimming freely in the water with a flagella looks for suitable host algae cells. When a potential host is reached, it encysts , throwing off the scourge. It penetrates the host's cell wall with an infection tube. First a pseudopodium is stretched out looking for a pore or opening in the wall; if none are found, the infection will fail. In successful cases, a vacuole forms within the cyst , which rapidly increases in volume until its pressure is sufficient to push the cell contents through the tube into the algae cell. Here the parasite forms an intracellular amoeba-like stage that devours the host's cell content with pseudopodia and transports it into a central food vacuole, where it is digested until its content is completely consumed. The strongly swollen cell forms a plasmodium with a central vacuole and residual body (excreted and indigestible parts of the host cell) through nuclear divisions . The mature Plasmodium divides into mononuclear cells, each of which differentiates into zoospores. No separate sporangium is formed, but the cell wall of the former host cell is used. The mature zoospores eventually leave the cell through the hole made by the infection tube. The zoospores then swarm out in search of new host cells. This completes the cycle.

The other genera have a comparable cycle. In Amoeboaphelidium , the amoeba are initially separated and only finally fuse to form a Plasmodium. In this genus, the zoospores are also amoeboid, they have a functionless pseudocilium at the rear end as a relic. In Pseudaphelidium , amoeboid cells are released, which encyst and only then release one to four zoospores from the cysts.

Some types of aphelidea can also develop thick-walled permanent spores. Although not proven in the group itself, it is assumed, in analogy to other species with a similar life cycle, that they are more resistant to adverse environmental conditions as well as being related to sexual reproduction. The permanent spore with a thick, yellowish cell wall forms inside the cell wall of the host cell that has been eaten empty.

Ecology and hosts

Aphelidea is found preferentially in nutrient-rich ( eutrophic ) waters. They parasitize unicellular algae or cell threads forming algae, mostly phytoplankton, but also soil-living (benthic) species or those of the growth (epiphytic). They even attack soil-dwelling algae species near water. They are widespread in the waters investigated as a result, but by no means ubiquitous. More precise data on density or role in aquatic ecosystems are not yet available.

Algae species of the Chlorophyta (green algae), Xanthophyceae (yellow-green algae) and Bacillariophyta (diatoms) are used as hosts . Most species parasitize the green algae genus Chlorococcum and similar species or the yellow-green alga Tribonema gayanum , which is often used for laboratory cultures of the aphelidea. Both species and individual strains within species can be very host-specific and specialized in just one or a few types of algae. Substantial genetic differences between strains of the same species from different geographical areas of origin were sometimes reported.

Letcher and colleagues found the species Amoeboaphelidium protococcarum as a parasite in a culture of the green alga Scenedesmus dimorphus , which was cultivated experimentally to produce biofuel from algae biomass. Such mass cultures of one species are known to be susceptible to parasites. Since it reduces yield, it may also be of economic importance as a pest.

Phylogeny

1885 described Wilhelm Zopf the type Aphelidium deformans as the first representative of the group; he assigned them to the "Monadinea" or "Eumycetozoa". They were later classified under the " Rhizopoda ", a group that is now also recognized as artificially assembled. While the life form and life cycle resemble simple mushrooms like the Chytridiomycetes , the phagocytosis diet was better suited to amoebas , which were classified as animals.

Thomas Cavalier-Smith recognized in 1998 that he belonged to the group of opisthokonta he had just described and thus initiated the modern re-evaluation. In 2013 he classified them as the order "Aphelidida" in the class Rozellida among the collar- shaped flagellates (called Choanozoa by him). He had overlooked the fact that Boris V. Gromov had already described the "aphelidea" as an order in 2000. In the Cavalier-Smith classification, the aphelidea belong to the Holozoa , the branch of the Opisthokonta to which the animals also belong. This also corresponds to the view of Sina Adl and colleagues in their comprehensive systematics of eukaryotes.

More recent phylogenomic investigations, i.e. investigations of the relationships based on the comparison of homologous DNA sequences, however, revealed doubts about this classification. A study by Sergey A. Karpov and colleagues in 2013 revealed a grouping with the genus Rozella ( Rozella allomycis ) and the Microsporidia . Peter M. Letcher and colleagues had already made a similar observation on Amoeboaphelidium protococcarum . The group did not consider another current study by Jordi Paps and colleagues. In a study by Meredith DM Jones and colleagues , Rozella allomycis , which was shown to be closely related in the investigations , was found to belong to a group of organisms that had previously been overlooked and formed a sister group to the fungi, called Cryptomycota by them. Inclusion in this group is therefore currently more likely than a closer relationship with the other Holozoa. Karpov and colleagues even go so far as to set up a new tribe for them in their cladogram , which they call "Aphelida" , due to the basal diversion of the group .

Systematics

The aphelidea comprise the following three genera:

Individual evidence

  1. ↑ a b c d e Sergey A. Karpov, Maria A. Mamkaeva, Vladimir V. Aleoshin, Elena Nassonova, Osu Lilje, Frank H. Gleason (2014): Morphology, phylogeny, and ecology of the aphelids (Aphelidea, Opisthokonta) and proposal for the new superphylum Opisthosporidia. Frontiers in Microbiology Volume 5, Article 112. doi : 10.3389 / fmicb.2014.00112
  2. ↑ a b Peter M. Letcher, Salvador Lopez, Robert Schmieder, Philip A. Lee, Craig Behnke, Martha J. Powell, Robert C. McBride (2013): Characterization of Amoeboaphelidium protococcarum, an Algal Parasite New to the Cryptomycota Isolated from an Outdoor Algal Pond Used for the Production of Biofuel. PLoS ONE 8 (2): e56232. doi : 10.1371 / journal.pone.0056232
  3. ↑ scan of the original description at archive.org
  4. ↑ T. Cavalier-Smith (2013): Early evolution of eukaryote feeding modes, cellstructural diversity, and classification of the protozoan phyla Loukozoa, Sulcozoa, and Choanozoa. European Journal of Protistology 49: 115-178. doi : 10.1016 / j.ejop.2012.06.001
  5. ↑ Adl, SM, Simpson, AGB, Lane, CE, Lukeš, J., Bass, D., Bowser, SS, Brown, MW, Burki, F., Dunthorn, M., Hampl, V., Heiss, A. , Hoppenrath, M., Lara, E., le Gall, L., Lynn, DH, McManus, H., Mitchell, EAD, Mozley-Stanridge, SE, Parfrey, LW, Pawlowski, J., Rueckert, S., Shadwick, L., Schoch, CL, Smirnov, A. and Spiegel, FW (2012): The Revised Classification of Eukaryotes. Journal of Eukaryotic Microbiology 59: 429-514. PDF online
  6. ↑ Sergey A. Karpov, Kirill V. Mikhailov, Gulnara S. Mirzaeva, Iskandar M. Mirabdullaev, Kira A. Mamkaeva, Nina N. Titova, Vladimir V. Aleoshin (2013): Obligately Phagotrophic Aphelids Turned out to Branch with the Earliest- diverging fungi. Protist Volume 164, Issue 2: 195-205. doi : 10.1016 / j.protis.2012.08.001
  7. ↑ Jordi Paps, Luis A. Medina-Chacón, Wyth Marshall, Hiroshi Suga, Iñaki Ruiz-Trillo (2013): Molecular Phylogeny of Unikonts: New Insights into the Position of Apusomonads and Ancyromonads and the Internal Relationships of Opisthokonts. Protist Volume 164, Issue 1: 2-12. doi : 10.1016 / j.protis.2012.09.002
  8. ↑ Meredith DM Jones, Irene Forn, Catarina Gadelha, Martin J. Egan, David Bass, Ramon Massana, Thomas A. Richards (2011): Discovery of novel intermediate forms redefines the fungal tree of life. Nature 474: 200-203 doi : 10.1038 / nature09984