A Transcriptomic Atlas of Adult Tissues of the Sea Urchin Lytechinus variegatus Identifies Early Transitions in Gonadogenesis.

Abstract

Sea urchins have been a well-used model of fertilization and of the molecular biology of embryonic development. Our understanding of the molecular identity of adult tissues, on the other hand, has lagged significantly, a deficiency all too apparent following recent successes in mutational analyses in transgenerational studies. Here we present molecular analyses of gonadogenesis, with particular reference to developmental features emerging when a juvenile first begins to make eggs and sperm. We observed great variation in developmental time to reach fertility in Lytechinus variegatus, with the earliest gamete-producing adult at 4.5 months post-fertilization. Instead of age, or just size, we find the mass to test size ratio of 120 mg/mm test diameter is far more predictive of sexual maturity. Body size is also correlated with external developmental metrics of secondary spine formation, and opening of the gonadopores. In young juveniles, we identify an internal sac-like structure that by both morphological and transcriptomic analysis appears to be an indeterminant gonad that transitions into an ovary or testis. We follow this development into adulthood and compare the transcriptomes of these developing sacs and gonads to transcriptomes of all 16 major tissues of the adult. This result reveals a broad definition of the tissue types in a sea urchin, and displays genes that are differentially enriched, and/or specifically expressed by each tissue. Ovaries of developing juveniles and of adults show the greatest number of differentially expressed genes compared to all other tissues. These datasets serve as a guide for gene selection in traditional CRISPR/Cas9 mutagenesis, for design of integrated transposase reporters, and for parsing out unique functions in tissues that in combination give this animal its unique attributes.