Comparative genomics of sex-determination-related genes reveals shared evolutionary patterns between bivalves and mammals, but not fruit flies

The molecular basis of sex determination (SD), while being extensively
studied in model organisms, remains poorly understood in many animal
groups. Bivalves, a diverse class of molluscs with a variety of
reproductive modes, represent an ideal yet challenging clade for
investigating SD and the evolution of sexual systems. However, the absence
of a comprehensive framework has limited progress in this field,
particularly regarding the study of sex-determination related genes
(SRGs). In this study, we performed a genome-wide sequence evolutionary
analysis of the Dmrt, Sox, and Fox gene families in more than 40 bivalve
species. For the first time, we provide an extensive and
phylogenetic-aware dataset of these SRGs, and we find support to the
hypothesis that Dmrt-1L and Sox-H may act as primary sex-determining
genes, by showing their high levels of sequence diversity within the
bivalve genomic context. To validate our findings, we studied the same
gene families in two well-characterized systems, mammals and fruit flies
(genus Drosophila). In the former, we found that the male sex-determining
gene Sry exhibits a pattern of amino acid sequence diversity similar to
that of Dmrt-1L and Sox-H in bivalves, consistent with its role of master
SD regulator. In contrast, no such pattern was observed among genes of the
fruit fly SD cascade, which is controlled by a chromosomic mechanism.
Overall, our findings highlight similarities in the sequence evolution of
some mammal and bivalve SRGs, possibly driven by a comparable architecture
of SD cascades. This work underscores once agaithe importance of employing
a comparative approach when investigating understudied and non-model
systems.