Fish Gonadogenesis. Part I: Genetic and Environmental Mechanisms of Sex Determination

Abstract

Many species of fish produced in aquaculture or for the ornamental fish trade exhibit sexual dimorphism in growth, age at maturity, secondary sexual characters, or other traits of interest. This has led to a desire to produce populations of only one sex for commercial ongrowing. Although direct sex reversal via manipulation of sex differentiation is used commercially (e.g., in tilapia aquaculture), in most cases there is a need to understand the sex determination system to some extent and manipulate this to produce monosex fish. Sex determination is the genetic or environmental process that establishes the sex (gender) of an organism, whereas sex differentiation is the process by which an undifferentiated gonad is transformed into an ovary or a testis. Fish are the most diverse group of vertebrates in terms of sex determination, and the number of fish species of interest to aquaculture keeps increasing. Together, these aspects explain the growing interest to understand how sex determination and differentiation produce the sex ratio. This review concentrates on recent research using the tools of molecular biology to broaden our understanding of the different aspects related to fish sex determination, both in model fish species and in species of commercial importance.

Keywords:

• sex ratio
• sex chromosomes
• sex-determining loci
• sex-specific markers
• sex-linked markers
• environmental sex determination
• temperature-dependent sex determination
• aromatase
• QTL

ACKNOWLEDGEMENT

This work has been carried out within the project AQUAFUNC (EU-SSA-022685) with financial support from the Commission of the European Communities. We wish to thank D. M. Power and K. Sundell and the AQUAFUNC project for providing the opportunity to write this review. Thanks are due to N. Ospina-Álvarez for assistance with the bibliography. We are grateful to the two anonymous referees for comments and suggestions that helped to improve the manuscript.

Notes

^aA sex-modifying locus also operates, at least in some populations (see text);

^bSex chromosome differentiation in only one population (Nanda et al., 1992);

¹ Genetic or genotypic because the sex of the organism is determined by its genotype. Since even ESD species have a certain degree of genetic influence; we use the term genotypic.

² Also referred to as 2-factor systems when referring to the sex factors, i.e., the segregating units that provide the basis for sex determination, i.e., X and Y in a XX/XY system.

³ Although at the population level there can be three or more different sex factors, however, in a given individual there is a maximum of two. Thus the multifactorial system can be seen as a special case of a monofactorial/2-factor system.

⁴ A sex–linked marker is located on the same chromosome as a sex–determining gene but recombination can still occur between the two loci, altering the pattern of association of alleles. A sex–specific marker lies in the non–recombining region around the sex–determining gene.

⁵ Caused by alleles, normally rare, of autosomal loci that can influence or override the master (see chromosomal) gene(s) (adapted from definition given by Nanda et al., 2003).