Revealing Mytilus galloprovincialis transcriptomic profiles during ontogeny.

Mediterranean mussels are a worldwide spread bivalve species with extraordinary biological success. One of the reasons of this success could be the reproduction strategy of bivalves, characterized by the presence of trochophore larvae. Larval development in bivalves has been a topic of raising interest in the scientific community but it deserves much more attention. The principal objective of this work was to study the transcriptomic profile of the ontogeny of Mytilus galloprovincialis analyzing the gene expression in different developmental stages, from oocytes to juveniles. For this purpose, after conducting a 454 sequencing of the transcriptomes of mussel hemocytes, adult tissues and larvae, a new DNA microarray was designed and developed. The studied developmental stages: unfertilized oocytes, veliger, pediveliger, settled larvae and juveniles, showed very different transcriptomic profiles and clustered in groups defining their characteristic gene expression along ontogeny. Our results show that oocytes present a distinct and characteristic transcriptome. After metamorphosis, both settled larvae and juveniles showed a very similar transcriptome, with no enriched GO terms found between these two stages. This suggests: 1.- the progressive loss of RNA of maternal origin through larval development and 2.- the stabilization of the gene expression after settlement. On the other hand during metamorphosis a specific profile of differentially expressed genes was found. These genes were related to processes such as differentiation and biosynthesis. Processes related to the immune response were strongly down regulated. These suggest a development commitment at the expense of other non-essential functions, which are temporary set aside. Immune genes such as antimicrobial peptides suffer a decreased expression during metamorphosis. In fact, we found that the oocytes which express a higher quantity of genes such as myticins are more likely to reach success of the offspring, compared to oocytes poor in such mRNAs, whose progeny died before reaching metamorphosis.