Large-scale transcriptome sequencing reveals novel expression patterns for key sex-related genes in a sex-changing fish

Abstract

BackgroundTeleost fishes exhibit remarkably diverse and plastic sexual developmental patterns. One of the most astonishing is the rapid socially controlled female-to-male (protogynous) sex change observed in bluehead wrasses (Thalassoma bifasciatum). Such functional sex change is widespread in marine fishes, including species of commercial importance, yet its underlying molecular basis remains poorly explored.MethodsRNA sequencing was performed to characterize the transcriptomic profiles and identify genes exhibiting sex-biased expression in the brain (forebrain and midbrain) and gonads of bluehead wrasses. Functional annotation and enrichment analysis were carried out for the sex-biased genes in the gonad to detect global differences in gene products and genetic pathways between males and females.ResultsHere we report the first transcriptomic analysis for a protogynous fish. Expression comparison between males and females reveals a large set of genes with sex-biased expression in the gonad, but relatively few such sex-biased genes in the brain. Functional annotation and enrichment analysis suggested that ovaries are mainly enriched for metabolic processes and testes for signal transduction, particularly receptors of neurotransmitters and steroid hormones. When compared to other species, many genes previously implicated in male sex determination and differentiation pathways showed conservation in their gonadal expression patterns in bluehead wrasses. However, some critical female-pathway genes (e.g., rspo1 and wnt4b) exhibited unanticipated expression patterns. In the brain, gene expression patterns suggest that local neurosteroid production and signaling likely contribute to the sex differences observed.ConclusionsExpression patterns of key sex-related genes suggest that sex-changing fish predominantly use an evolutionarily conserved genetic toolkit, but that subtle variability in the standard sex-determination regulatory network likely contributes to sexual plasticity in these fish. This study not only provides the first molecular data on a system ideally suited to explore the molecular basis of sexual plasticity and tissue re-engineering, but also sheds some light on the evolution of diverse sex determination and differentiation systems.Electronic supplementary materialThe online version of this article (doi:10.1186/s13293-015-0044-8) contains supplementary material, which is available to authorized users.

Both Male-Biased and Female-Biased Genes Evolve Faster in Fish Genomes

“…Although there were also some studies focusing on sex-biased genes in gonad or brain in zebrafish and other fishes (Small et al 2009; Wong et al 2014; Liu et al 2015), few of them have investigated the evolutionary dynamics of sex-biased genes and their driving forces. Our results in zebrafish are consistent with this from other model organisms.…”

“…Although there have been several studies of sex-biased genes expression in zebrafish and other fish species (Small et al 2009; Wong et al 2014; Liu et al 2015), few of them detailed the evolutionary dynamics of sex-biased genes. Hence, a comprehensive analysis of the expression and evolution of sex-biased genes in fishes is needed.…”

The power and promise of RNA‐seq in ecology and evolution

“…However, a much broader distribution of effect sizes describes the much more varied expression landscape between male and female gonad, compared to the shallow expression landscape of the brain, where very few genes show sex-specific expression differences. In bluehead wrasse gonad, tens of thousands of transcripts show significant sex-biased expression, while fewer than 10 transcripts are significantly differentially expressed between male and female forebrain (Liu et al 2015). Where expression landscapes are shallow and most meaningful expression differences are subtle, DE tests will have lower power and study designs will require more biological replicates.…”

“…Unfortunately, often only a small proportion of DE contigs can be matched to known proteins [e.g. <20% in the bluehead wrasse (Liu et al 2015), 31% in Acropora coral (Meyer et al 2011)]. There will be many (e.g.…”

Sexual plasticity: A fishy tale

“…More surprisingly, no sex‐bias was observed in gonadal expression of foxl2 in protogynous three‐spot wrasses; indeed, foxl2 expression actually increased during aromatase inhibitor‐induced sex change (Kobayashi et al, ). In bluehead wrasses, foxl2 expression was much higher in ovaries than testes (Liu et al, ). The delayed pattern of expression of foxl2 during mid‐transition does not provide a strong case for Foxl2 initiating the sex change (Liu, ); elevated foxl2 expression may instead reflect a feedback response to E2 depletion (Liu et al, ; Guiguen et al, ) or may be stimulated by other factors, such as long non‐coding RNAs or microRNAs (Piferrer, ; Rosario et al, ; Boulanger et al, ).…”