ИСТИНА

Choanoflagellates and mesomycetozoeans are two groups of unicellular organisms that are the closest relatives of animals [1]. The ongoing genome sequencing effort aimed at their members is an attempt to understand the origin of animals and multicellularity in the context of evolution of genes and genomes [2]. These studies have brought about a notion of “Metazoa-specific” genes, genes found exclusively in metazoans, which are thus considered likely to be novelties specifically associated with the development multicellularity. The “Metazoa-specific” genes code a large number of cell signalling and adhesion proteins such as cadherins and TGFb pathway components, to name a few. However the list of “Metazoa-specific” genes is rapidly contracting as the number of sequenced genomes of unicellular relatives of metazoans increases. The genomes of choanoflagellates were found to contain a multitude of tyrosine kinases – proteins involved in the regulation of cell proliferation and motility that were originally considered to be a metazoan novelty [3]. Another example is a mesomycetozoean that possesses components involved in cell-matrix adhesion, such as focal adhesion kinase and integrin beta [4]. Here we present evidence for the exclusion of yet another set of genes from the “Metazoa-specific” list by demonstrating their presence in another mesomycetozoean and showing that they are actively expressed. The premetazoan ancestry of metazoan transcription factor families and signal transduction pathways is poorly accommodated by the traditional view of the metazoan ancestors as blastula-like colonies, which had subsequently undergone cell differentiation. The new data suggests that the elements of the genetic toolkit for the development of multicellular animals were possibly already in use by their unicellular relatives. Mapping of major gene families and ecological traits onto the phylogeny indicates that presence of different cell types at different stages of life cycle and appearance of multicellular aggregates is not an intrinsic property of metazoans, but of a much wider group of organisms – Opisthokonta [5]. The emerging scenario regards the last common ancestor of multicellular animals as an integration of different stages of the unicellular ancestor’s life cycle.