Genotype-dependent differences in gonadal transcriptomes for spontaneous and temperature-induced neomales in wild zebrafish populations

Sex determination in fish is a labile trait easily influenced by environmental factors, often temperature. A mismatch between genotypic and phenotypic sex usually tends towards genotypic females developing as males referred to as neomales. Neomales are being discovered in wild populations of several fish species. Thus, understanding the genetic basis leading toward neomales is of great interest in a global climate change scenario. Wild zebrafish strains possess a ZZ/ZW chromosomal sex determination system, in contrast to laboratory strains, which lack a Z chromosome and assume a polygenic sex determination mechanism. Whether temperature alters the sex ratio in wild strains resulting in neomales and whether the gonadal transcriptome differs between genetic males (ZZm) and neomales (ZWm) is unknown. We assessed the rate of sex reversal in Nadia and EkkWill wild strains using different families exposed to 28 °C (control) and 34¿36 °C (masculinizing) temperatures during sex. In adults, histomorphometrics and transcriptomic analyses by RNA sequencing provided insights into gonadal morphology and function. Results showed, surprisingly, that both wild strains were at least as susceptible to the masculinizing effects of elevated temperature as the laboratory strain, with abundant spontaneous and temperature-induced neomales, although the two natural strains had different genotype-by-environment interactions. Histologically, Nadia ZWm but not ZZm showed a higher number of spermatozoa after exposure to elevated temperature. Transcriptomic results had a strong family influence. Further, ZWm testes had a transcriptome profile indistinguishable from ZZm testes. Taken together, these results suggest a genetic basis underlying both the production of neomales and their response to temperature with potential functional consequences in reproductive capacity. Identifying the genes involved will improve our understanding of the mechanisms of zebrafish sex determination