Conserved genes with variable expression and function: Vasa, Piwi, and Dnmt1 in Oncopeltus fasciatus gametogenesis

The production of gametes is one of the universal processes of life and given millennia of evolution many of the core genes involved are expected to be highly refined and resistant to change. Here we examine the pattern of expression and functionally characterize three genes widely involved in gametogenesis: Vasa, Piwi, and Dnmt1. We examine their cellular location during both oogenesis and spermatogenesis in Oncopeltus fasciatus, a hemipteran insect, using fluorescent in situ hybridization chain reaction. In females, expression of Piwi was restricted to the trophocytes, but Vasa and Dnmt1 were expressed in developing ooctyes as well as the trophoctyes. In males, Piwi was expressed in the testis germline stem cells (GSCs), but Vasa and Dnmt1 expression was absent from these cells. Additionally, Piwi and Vasa were expressed in secondary spermatogonia and spermatocytes while Dnmt1 was predominantly expressed in the primary spermatogonia. We also functionally characterized their necessity for gametogenesis after RNAi-mediated gene expression knockdown. Vasa was not required for oogenesis but was required during spermatogenesis. Piwi and Dnmt1 were required for both oogenesis and spermatogenesis. These results were in contrast to their requirement for these processes from other organisms, which highlight the unexpected variation found in the processes and suggest that conservation depends on the level at which genes are examined: sequence, expression, or phenotypic and biochemical function. This suggests there is a more nuanced evolutionary story of conserved, yet plastic, gametogenic gene set in Metazoa that deserves further study across more organisms.