Intergenerational microbial transmission in the little skate (Leucoraja erinacea)

Microbial transmission from parent to offspring is hypothesized to be universal in vertebrates. However, evidence for this is limited as many clades remain unexamined. Chondrichthyes, as one of the earliest-branching vertebrate lineages, provide an opportunity to investigate the phylogenetic breadth of this hypothesis. To assess the potential for bacterial transmission in an oviparous chondrichthyan, we used 16S rRNA amplicon sequencing to characterize the microbial communities associated with the skin, gill, and egg capsule of the little skate, Leucoraja erinacea, at six points during ontogeny. We identify site-specific microbiomes dominated by the bacterial phyla Proteobacteria and Bacteroidetes, a composition similar to, but distinct from, that of other chondrichthyans. Our data reveal that the skate egg capsule harbors a highly diverse bacterial community- particularly on the internal surface of the egg capsule- and facilitates intergenerational microbial transfer to the offspring. Embryonic skin and external gill issues host similar bacterial communities; the skin and gill communities later diverge as the internal gills and skin denticles develop. Our study in the first exploration of the chondrichthyan microbiome throughout ontogeny and provides the first clear evidence of vertical transmission in this group, which may be the primary mechanism for the signature of phylosymbiosis previously observed in elasmobranchs.