Sex-Specific Genes Identified in Sea Urchin Gonads are Expressed Prior to Metamorphosis

A great collaboration between the germline and somatic cells of an organism is the creation of a functional gonad. Here we begin to test this mechanism in a sea urchin by use of RNA-seq, and quantitative mRNA measurements throughout the life cycle of the sea urchin, Lytechinus variegatus (Lv) . We found through de novo analyses that the ovary and testis of this echinoderm contains the many transcripts predicted for gamete morphology and function, but we also discovered many uncharacterized gene products unique to each gonad. We found that transcripts involved in glycolysis are highly enriched in the ovary compared to the testis, and that Lv has an expansion of nanos genes not observed in other sequenced echinoderms. We developed a pipeline integrating timepoint RNA-seq data throughout development to identify hallmark gene expression in gonads. We found activation of meiotic genes surprisingly early in development, and observed that candidate genes involved in sex determination are first expressed during larval growth, well before metamorphosis. We further discovered that individual larvae express varying levels of male- or female-hallmark genes before metamorphosis, including factors for the germ line, oocyte, sperm, and meiosis related genes. Together these data support the hypothesis that sex determination in the sea urchin is initiated prior to metamorphosis, from a shared profile of male and female factors, and that the many uncharacterized genes unique to the gonads may reveal unique pathways and mechanisms in echinoderm reproduction. Highlight Testis and Ovary of the sea urchin, Lytechinus variegatus , have distinct transcriptome profiles L. variegatus gonad RNA-seq reveals the expression of 9 nanos genes The ovary in the sea urchin uniquely expresses genes of the glycolytic pathway, including an ovary-specific transcript of GAPDH Genes involved in meiosis and in sex determination are expressed early in larval development From gene expression profiles we conclude here that sex determination is initiated prior to metamorphosis in this echinoderm