Microbiota associated with echinoid eggs and the implications for maternal provisioning

Mothers impact the survival and performance of their offspring through the resources they provision, and the degree of maternal investment in an individual offspring can be broadly estimated by egg size for organisms that lack parental care. Animals may also actively maintain symbiotic partnerships with microorganisms through the germ line, but whether microbes are a fundamental component of maternal provisioning is an untested hypothesis in evolutionary symbiosis. We present a preliminary test of this by comparing the egg-associated microbiota of 10 sea urchin species with ecological factors known to influence egg size. We found that the microbiota associated with sea urchin eggs had a phylogenetic signal in both composition and richness, which varied between years but not between individuals or within a clutch. Moreover, we found a negative correlation between microbiome richness and taxonomic dominance, and that community diversity covaried with egg size and energetic content but not with pelagic larval duration or latitude. These data suggest that there are multiple parallels between the ecological factors that govern changes in egg size and microbiome composition and diversity, implying that microbial symbionts may be another constituent potentially provided by the mother.