The role of gene duplication and neofunctionalization in the sexual plasticity of teleost fish

Abstract

Sex determination and differentiation varies widely across vertebrates. In teleost fish these processes are remarkably plastic. In contrast to a gonochoristic system where the sexes are separate, fish that begin life as one sex and change sex as part of their lifecycle are sequentially hermaphroditic. Such a process requires drastic anatomical and physiological changes, requiring a switch between the bistable, antagonistic genetic networks that establish and maintain separate sexes. The trigger of such a change is often a social one and the genetic underpinnings of the process of sex change is still to be fully elucidated. However, recent transcriptomic work exploring sex change in bluehead wrasse has shown sharp up- and downregulation of sex-associated genes. Interestingly, certain genes that were thought to play a role in female sex determination and differentiation were found in multiple copies across the genome, with one copy playing the known role in female sex determination and differentiation and the other copy/copies, on the contrary, being upregulated during transition to the male sex. Multiple copies of a gene typically arise due to gene or whole-genome duplication, with potential subsequent sub/neofunctionalization. Duplicated genes are a well-established substrate for divergent gene evolution. A duplicate gene leads to redundancy, with no evolutionary restriction on the additional copy/copies and therefore facilitated mutation and evolution of a new role. Based on the recently published insights on sex determination in bluehead wrasse, I hypothesise that gene duplication and subsequent neofunctionalization may underpin the sexual plasticity observed in teleost fish – an area that has not yet been extensively researched. This hypothesis can now be investigated thanks to recent advances in genomics research. To gain insight into whether gene duplication and subsequent neofunctionalization is implicated in the evolution of sequential hermaphroditism as an example of a sex-changing reproductive mode, I applied a comparative candidate gene approach, investigating and comparing the levels of gene duplication in key sex-related genes across and between sex changing and non sex changing species. I here present my analyses based on both, gene annotations and genomic sequence alignments. Overall, I found no statistically significant evidence that gene duplication and subsequent neo- or subfunctionalization is causal to the sexual plasticity observed in teleost fish. Both the genome annotation and sequence alignment approaches did not return any significant results. However, while completing my project, I managed to retrieve valuable preliminary data and identified potential caveats associated with the study of gene duplication. This work is of significance, having implications for our understanding of the evolution of sex determination and differentiation - not only for teleost fish, but for the entirety of the Chordata phylum.