Dynamics of sexual development in teleosts with a note on Mugilcephalus

Abstract

The process of sex determination (SD) and sex differentiation (Sd) with associated sex-specific behaviour in teleosts is brought about by genetic factors and environmental factors under the influence of chemical messengers. SD is initiated by inherited genes, which in turn influence Sd by the production of hormones. To understand the plasticity of SD in fishes, the functional role of genes must be clearly elucidated. During early development, the dimorphic expression of male and female are mediated by the differentially sexualized brain. In mammals, SD region (SDR) on the Y chromosome (SRY) can be considered as male-specific copy of SOX3 and in the absence of SRY in teleosts suggests SD might be regulated by alternate genes. DM (doublesex/mab)-related genes, AMH (anti-mullerian hormone), TGF-b (transforming growth factor beta), GSDF (gonadal soma derived growth factor) and other genes (around 18) located in chromosomes of teleosts are found to be responsible for SD. The SD gene, sdY (sexually dimorphic on the Y chromosome) has been detected in more than 15 salmonid species, and in medaka and tilapia SD system is governed by heterogametic mechanism. In mullet, Mugil cephalus Dor et al. identified 27 SDR genes and suggested to be potential candidate genes for SD. Recently whole-genome sequencing data were produced from mullet and assembled into a draft genome sequence in which >30 loci are potentially associated with SD. Further analysis is required to know the involvement of each sex-biased gene for its functional influence on sex. Investigators attempted to know the sex-linkage group in tilapia and found that 2 linkage group i.e. LG1 and LG22 show SD loci, whereas in mullet it was documented that LG8 is the SD loci. Rigorous study with all male population(YY) or knockout of male marker gene, might give conclusive understanding on SD in future. The review highlights the absence of genomic understanding of SD and SDR genes for futuristic direction of research.