Genetic, endocrine and epigenetic mechanisms underlying sexual size dimorphism in fish

Background Sexual size dimorphism (SSD), i.e., size difference between sexes, is common in fishes and spans from negligible to extreme body size differences, with both female‑ and male‑biased directions. While evolutionary drivers behind SSD such as sexual selection, fecundity selection and natural selection are increasingly well understood, our understanding of the underlying mechanisms of how sex‑specific growth trajectories develop is less clear. Summary Here we review recent findings of such mechanisms in fishes and reveal that SSD arises from an interplay of sex-linked and autosomal genetic factors. In teleosts, master sex-determining genes and growth regulators such as dmrt1/dmY, sdY, amhr2by, gdf6Y and gsdfY play key roles, while quantitative trait loci (QTL) influence growth and maturation, further contributing to SSD. Essential sex-specific regulation of hormones across brain, pituitary, liver and gonad determines SSD directionality. Epigenetic mechanisms, such as DNA methylation and non-coding RNA further modulate gene expression in growth and reproductive pathways. We identify the basic mechanisms, highlight knowledge gaps, and propose that multi-omics approaches can disentangle sex effects from dimorphism-specific regulation, linking together endocrine, genetic and epigenetic drivers. Key Messages 1. At the mechanistic level, SSD results from an interplay of genetic, endocrine and environmental influences. 2. Sex chromosomes and autosomal loci form the genetic architecture that shapes growth differences between males and females. 3. The somatotropic axis, involving GH/IGF signaling, together with the actions of sex steroids, serves as a central effector system underlying SSD in fish. 4. Epigenetic mechanisms help establish and maintain sex-specific gene expression programs, but integrative multi‑omic approaches are needed to uncover causal relationships and phenotypic plasticity in SSD.