Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com Review Article Sex Dev 2009;3:118–135 DOI: 10.1159/000223077 Environmental Effects on Fish Sex Determination and Differentiation J.F. Baroiller a H. D’Cotta a E. Saillant b a CIRAD, Upr20, Dept. Persyst, Campus International de Baillarguet, Montpellier, France; b The University of Southern Mississippi, Dept. Coastal Sciences, Gulf Coast Research Laboratory, Ocean Springs, Miss., USA exist. Temperature regulates the expression of the ovarian- aromatase cyp19a1 which is consistently inhibited in tem- perature masculinized gonads. Foxl2 is suppressed before cyp19a1. Recent in vitro studies have shown that foxl2 acti- vates cyp19a1, suggesting that temperature acts directly on foxl2 or further upstream. Dmrt1 up-regulation is correlated with temperature-induced male phenotypes. Temperature through apoptosis or germ cell proliferation could be a criti- cal threshold for male or female sex differentiation. Copyright © 2009 S. Karger AG, Basel Overview on Sex Determining Systems in Fish Fish, with almost 30,000 species representing half of all vertebrates [Helfman et al., 2009], have colonized nearly all aquatic habitats (from freshwaters to hyper-sa- line waters, polar waters to tropical swamps or geother- mal hot waters, from deep sea to mountain lakes, or from alkaline to acidic water). To match this array of habitats they have developed an amazing variety of adaptive re- sponses such as the diversity of mechanisms by which fish can become a male or a female individual. There are 2 main types of sexuality in fish: 1) hermaphroditism in which at least some individuals in a population will pro- duce both sperm and eggs either simultaneously or at successive life stages [Baroiller et al., 1999; Devlin and Key Words ESD  Fish  Genes  Gonads  Growth rate  GSD  Hypoxia  Sex determination/differentiation  Temperature Abstract Environmental factors affect the sex ratio of many gonocho- ristic fish species. They can either determine sex or influence sex differentiation. Temperature is the most common envi- ronmental cue affecting sex but density, pH and hypoxia have also been shown to influence the sex ratio of fish spe- cies from very divergent orders. Differential growth or devel- opmental rate is suggested to influence sex differentiation in sea bass. Studies in most fish species used domestic strains reared under controlled conditions. In tilapia and sea bass, domestic stocks and field-collected populations showed similar patterns of thermosensitivity under controlled con- ditions. Genetic variability of thermosensitivity is seen be- tween populations but also between families within the same population. Furthermore, in the Nile tilapia progeny testing of wild male breeders has strongly suggested the ex- istence of XX males in 2 different natural populations. Tilapia and Atlantic silverside studies have shown that temperature sensitivity is a heritable trait which can respond to direction- al (tilapia) or frequency dependent selection. In tilapia, tran- sitional forms within a genetic sex determination (GSD) and environmental sex determination (ESD) continuum seem to Received: February 14, 2009 Accepted: April 28, 2009 J.F. Baroiller CIRAD, Upr20, Department Persyst, Campus International de Baillarguet FR–34398 Montpellier (France) Tel. +33 46 75 93 951, Fax +33 46 75 93 825, E-Mail baroiller@cirad.fr © 2009 S. Karger AG, Basel 1661–5425/09/0033–0118$26.00/0 Accessible online at: www.karger.com/sxd