Comparative Phylotranscriptomics Reveals Putative Sex Determining Genes in Bivalvia

Mollusks, especially bivalves, exhibit a great diversity of sex determining mechanisms, including both genetic and environmental sex determination. Some bivalve species can be gonochoristic (separate sexes), while others are hermaphroditic (sequential or simultaneous). Several models have been proposed for specific bivalve species, utilizing vital information gained from gene expression data, as well as limited RAD-seq data (e.g. from Crassostrea gigas). However, these processes are not as well studied as those in model organisms (e.g., Mus musculus, Drosophila, Caenorhabditis elegans). We used phylotranscriptomics to better understand which possible sex determining genes are in bivalves and how these genes relate to similar genes in diverse phyla. We collected RNAseq data from eight phylogenetically diverse bivalve species: Argopecten irradians, Ensis directus, Geukensia demissa, Macoma tenta, Mercenaria mercenaria, Mya arenaria, Mytilus edulis, and Solemya velum. Using these data, we assembled representative transcriptomes for each species. We then searched for candidate sex determining genes using BLAST and confirmed the identity of nine genes using phylogenetics analyses. From the analysis of DMRT genes, we confirmed the identify of a Mollusk-specific sex determining DMRT gene: DMRT1L. Using a phylogenetic tree made up DMRT genes from ten phyla, we uncovered that the known DMRT sex determining genes (mab-3, dsx, DMRT1, DMRT1L) likely evolved from the same DMRT gene, rather than evolving independently. Based on gene expression data from M. edulis and previously proposed models, we developed a putative sex determination pathway representative of all bivalves.