Not-so-simple Trichoplax

Trichoplax is the only species in Placozoa group and one of the most mysterious and amusing invertebrate animals. It looks like a thin plate about 4 mm in size and consists of two layers of primitive epithelial cells without basal membrane. Between these layers there are a lot of motile cells that can have different functions… but we don’t quite know which ones. It seem like Trichoplax is a very simple creature: it has no color, no constant body shape and, as some suggest, no symmetry. But it’s biology is not as easy to understand as it seems to be.

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Since Tricholax had been discovered and described by F. Schulze in 1883 it has been posing a great deal of questions to scientists. In first years of observations people didn’t know where it was from because it would just suddenly appear in sea aquaria and escape them . Some scientists were sure that Trichoplax is just unknown Cnidaria or Spongia larvae. Nowadays Trichpoplax is found in Pacific Ocean, but we still don’t exactly understand it’s ecology, food and sex behavior, and especially anatomy and morphology features. However, of main interest for biologists is the systematic position of this not-so-simple animal. Is it a first multicallular animal? Or maybe it’s an ancestor of Spongies, or Cnadaria, or bilaterians?

Mansi Srivastava from University of California  with colleagues from different countries made a whole genome sequencing trying to answer these questions. The size of Trichoplax genome is about 98  million  bp  and it contains 11,514 protein coding genes.  Most of them have a detectable similarity to other animals’ genes and 7800 of these genes are common with bilaterian and cnidarians. Analysis of the exon–intron structure of orthologous genes demonstrated a high degree of conservation in Trichoplax relative to other eumetazoans. Using nine whole-genome sequences of other Metazoan, scientists showed that Placozoan is a sister group to all of them. So demosponge sequences diverge before the Trichoplax–cnidarian–bilaterian clade. But there is no evidence to prove that Trichoplax is either a derived or basal cnidarian or bilaterian.

Moreover, it has been shown that Trichoplax with its simple body and without normal epithelial layer in histological meaning has a lot of genes that encode complex processes appropriate to eumetazoans. For example, there are some transcription factors and signal pathways involving proteins associated with development and cell-type specification.

This work has just been started, but it discovers different possibilities to understand life of Trichoplax and some evolutionary and developmental questions.

M. Srivastava. The Trichoplax genome and the nature of Placozoans. Nature, 2008, vol. 07191. P. 955-961. doi:10.1038

Report by Lera Khabibulina

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