ИСТИНА

The model organisms of biology are selected for their ability to answer questions cutting across the boundaries of traditional disciplines and incorporating molecular, organismal and zoological studies. The growing interest in out-groups to the Bilateria reflects the strategic shift. Marine invertebrates, such as Cnidaria and Protochordata (ascidia) represent good out-groups. Up to now molecular studies of marine invertebrates have emphasized a candidate gene approach, characterizing invertebrate homologs of genes with conserved functions in the Bilateria. These studies revealed an unexpected complexity in the genomic regulatory systems and the developmental patterning mechanisms, blurring the boundary between the supposedly ‘‘simple’’ Cnidaria and ascidia and the ‘‘complex’’ Bilateria. The comparative “gene approach” studies make obvious a similar gene pattern in the same differentiation pathways in distant species to such an extent that the very term “tissue” becomes uncertain. For instance, if Aurelia aurita medusa possesses striated muscles with gene expression characteristic of the myogenic pathway, could they be designated a special tissue in spite of their origin? The “candidate gene approach” makes it possible to check whether there are any of the known vertebrate genes, but such a way leads nowhere if a special gene, absent in other animals, makes inbertebrate tissues so different from the rest of the internal medium tissues. In an attempt to find genes specific for precise invertebrate proteins we used quite a different way than the candidate gene approach. We began with the question – what is the difference between the medusa and the polyp body plan? In molecular terms it led to the new protein mesoglein identification and its’ gene cloning. Mesoglein possession of ZP domain attracted our attention to the oocyte contact plate, which could be the first sign of the future Zona Pellucida. Mesoglein happens to be expressed only by mesogleal cells (Mc) of the adult medusa. It seems that mesoglein expression may help to distinguish the tissue identify of mesoglea as a distinct layer from ectoderm. But the overview of all our results rather confirms that Mc and, probably, the whole mesoglea originate from ectoderm. The origin of the ascidian morula cells, which participate in the adult tunic formation is also not clear. The test cells with similar to morula cells antigens surround the ascidian oocyte and involved in the formation of the larval tunic. We found two major proteins of Stiela rustica morula cells – p26 and p48 kDa. MS-MS analysis determined the probability of several amino acid sequences of the tryptic fragments of proteins, which allow to produce degenerate primers for p26. We used RACE-PCR on cDNA (prepare from poly(A)RNA ascidian blood cells), and obtained fragment of open reading frame of 567 nucleotides. This sequence is unique and not described previously. The full-length gene of the p26 will help to answer the questions about its’ differential expression. The identification of a single gene and the demonstration of its presence in a tissue layer of interest do not provide a positive solution of its’ origin. It only shows that the candidate gene approach is insufficient to fully characterize tissues and that an unbiased approach will be necessary: complete systems should be compared and next generation sequencing techniques give such a possibility. The results of current work will come in molecular biology terms – with precise figures, gene patterns and sequences. This type of information does not agree happily with the way of thinking in classical terms such as “tissues” or “germ layers”. There is an urgent necessity of a new biological language, one combining the classical point of view with the formal and exact computer and database-based language of modern biology.