Simple animal with complex genome : Trichoplax adhaerens
Trichoplax adherens is the only named (other cryptic species may exists) member of the phylum placozoa and arguably the simplest free-living animals on this planet.Trichoplax are soft-bodied, about 0.5 mm across,made up of four kind of cells (resembling somewhat a large amoeba). The name T. adhaerens was given because it tends to stick to its substrate.
Many people ,including me expected things to become lot simpler regarding nature of placozoans ,once the genome is sequenced.But Trichoplax seems to be in no mood to shed the enigmatic tag attached to it at the moment. Mansi Srivastava et al, reported the sequencing and analysis of the ,98 million base pair nuclear genome of the placozoan, Trichoplax adhaerens in recent issue of Nature, which provided some valuable insights pertaining to phylogenetic puzzles but opened doors to new developmental questions.
The phylogenetic relationship of placozoans to other metazoans always remained controversial. Analyses of the mitochondrial genome place the phylum Placozoa at the root of Metazoa ,Whereas early studies on the basis of a small number of genes suggested that placozoans could be secondarily simplified cnidarians.phylogenetic analysis performed by the authors supports the identification of placozoans as a basal eumetazoan lineage that diverged before the separation of cnidarians and bilaterians but after the divergence of demosponges from other animals.Data obtained from genome sequence of trichoplax shows that it retain many ancestral features for its last common ancestor with cnidarians and bilaterians.Unfortunately,the genome sequence alone cannot answer to all evolutionary questions, but it does provide a platform for further studies.
The Trichoplax body plan is simple, but still when one look at its genome content which encodes a rich array of transcription factors and signalling pathways , typically associated with eumetazoan developmental patterning and cell-type specification.
Important features of Trichoplax genome :
1) Trichoplax genome might contains appox.11,514 protein coding genes.
2) Nearly 87% of these predicted genes have detectable similarity to proteins known from other animals like Sea anemone and bilaterians.
3) comparable intron density :Trichoplax genes have an intron density (7.6 per kb) , vertebrates (8.5 perkb) and the starlet sea anemone (6.7 per kb).
4) placozoans are found to be a sister group to the other eumetazoans (or just diverging before cnidarian–bilaterian clade).
5) No support for Trichoplax as a derived or basal cnidarian or bilaterian.
6) Disagrees with results from mitochondrial tree,which put Placozoans at base of metazoans.
7) Shows Conserved synteny with other eumetazoans.
8.) Rich array of Transcription factors :Homeobox-containing genes from the ANTP,Paired (PRD), POU and SIX subfamilies. Also have putattive members of SOx and T-Box families.Sequences for multiple LIM-homeobox genes typically associated with subtype specification in neurons, multiple basic helix–loop–helix family genes associated with neural and muscle cell fates are found.
9) Complete set of Wnt and TGF-b signalling pathway are observed but hedgehog pathway seems to be missing.Notch and JAK/STAT pathways are present, but these pathways seem to be incomplete . No true homolog for Notch receptor and Janus kinase in the JAK/STAT pathway could be traced.
10) Elements of the animal stress response NF-kB pathway are found.
A big question comes to mind : What role these signalling molecules and Transcription factors are playing in simple Trichoplax that are typically associated with diverse cell types and developmental processes in eumetazoans.Authors are of opinion that Cellular morphology may be deceptive, and complex gene expression patterns may define functionally distinct but morphologically cryptic cellular subtypes.These signalling and transcription factors might play role in known (fission,growth) and unknown ( sexual reproduction and embryonic development ).
It has been sugested that Trichoplax is a living fossil and now with genome sequence it might well be close to hypothetical Ureumetazoa, common ancestor to all Cnidarians and bilaterians.
Reference :
Mansi Srivastava et . al,
The Trichoplax genome and the nature of placozoans.
Nature 454, 955-960 (21 August 2008) | doi:10.1038/nature07191
Suggested Links :
Video of Trichoplax
Bernd Schierwater lab website
Daniel S. Rokhsar lab website
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