A classification system for gonad development in triploid Crassostrea virginica

Abstract

Triploidy is a common form of genetic improvement that confers benefits associated with reduced fecundity. Triploid oysters produce fewer gametes yet undergo gametogenesis and develop gonads to variable extents. The variable and typically abnormal gonad development in triploid oysters has often been summarized using criteria developed for the more uniform development of diploid oysters, which can lead to misleading characterizations. Classification systems designed for triploid oysters, such as that for triploid Crassostrea gigas, have allowed for less subjectivity, more repeatability, and have engendered hypotheses of developmental pathways specific to triploids. Despite recent interest in gametogenesis of triploid Crassostrea virginica in connection with mortality events, gonad development in triploid C. virginica has primarily been summarized using criteria developed for diploid oysters. In this work, a novel classification system was developed for gonad development in triploid C. virginica while examining triploids and diploids sampled regularly from a site with and a site without a “triploid mortality” event. Triploids were classified based on the type of gonia present, presence of spermatogenic cells or oocytes, and relative abundance of gametes. The system developed for triploid C. virginica was in stark contrast to the previously characterized system for triploid C. gigas. A relationship between the nature of the gonia and fecundity, as described for triploid C. gigas, was absent, and instead, we hypothesize that the nature of the gonia indicates sex. Regular, “normal” gonia were associated with male triploids, whereas irregular gonia indicated female lineages. Pathways for gonad development in triploid C. virginica are proposed based on anatomical observations and time series data. Gonads were mostly similar for triploids at the affected (exhibiting triploid mortality) site and control site as well as between moribund and live triploids sampled during the mortality event. The specific anatomy of gonad development appears unlinked to triploid mortality; however, underlying metabolic processes during gametogenesis remain the leading culprit.