Characterization of a MuSashI-like transcript in a colonial chordate, phylogenetic analysis of the protein group and differential expression patterns in sexual versus asexual development.

Tunicates are the unique chordates to possess species reproducing sexually and asexually. Between them, Botryllus schlosseri forms similar organisms (oozooids and blastozooids) trough embryogenetic and blastogenetic pathways respectively. We here illustrate the characterization and expression pattern, during both pathways, of a transcript for a gene that we named BsMSI.We demonstrate that BsMSI falls into MuSashI-like (MSI-like) group, formed by MSI1, MSI2 and DAZAP1 genes. They encode for RNA-binding proteins phylogenetically related to other protein groups: the TARDBP and several hnRNPs. These proteins share same modular domain structure: a 2xRRM motif. Analyses show that MSIs-like and 2xRRM-hnRNPs had a common origin from a TARDBP type, and that 2xRRM-hnRNPs form a monophyletic group. A unique MSI-like, orthologous to DAZAP1, was recognized in tunicates. We hypothesize that, as MSIs are widely present in metazoans, tunicates have lost them during their evolution.In vertebrates MSI1 is considered a stem cells marker, but also MSI2 and DAZAP1 are involved in differentiative processes. BsMSI is expressed in all the tissues of developing buds. This ubiquitous blastogenetic expression differs from the embryogenesis one, which varies spatio-temporally according to the differentiation of embryonic, larval or oozooid tissues. Interestingly, BsMSI expression ends in differentiated or regressing structures (adult blastozooids, differentiated larval structures).Because the BsMSI expression patterndoesn’t overlap between blastogenesis and embryogenesis, we speculate it operates substitutive functions of the vertebrates orthologues DAZAP, MSI1 and MSI2, and that the function, related to differentiation or tissue maintaining, varies during the B. schlosseri life cycle.