Among mammals sex is usually defined by the presence or absence of the sex specific chromosome Y. In many, but not all, fish species there is also a chromosomal background to sex determination. Several fishes, including most salmonids, have heterogametic males and homogametic females, similar to the mammalian XY/XX-system [1
]. Other species, such as Poecilia
, have homogametic males and heterogametic females (ZZ/ZW), which also is the case for birds [4
]. Some species of the Poecilid platyfish Xiphophorus
, utilize a system with three sex chromosomes [5
]. In yet other species sex determination is influenced by environmental factors such as the temperature surrounding the developing embryo [6
]. Hermaphroditism is also a common feature of several fish species. Several studies have shown that species with genetic sex determination can be directed to produce genetically sex-reversed offspring. This is accomplished either by treating the fish with hormones, which can induce sex reversal in synchronous hermaphroditic fish [9
] and masculinization/feminization in gonochoristic species, or by incubating embryos in certain temperatures or pH [11
]. The proportion of males usually increases with temperature whereas lower temperatures favour females. In the case of pH, species differences have been observed.
There are few studies of sex determination in fish and the genetic mechanisms behind sex determination in fish remain largely unknown. Environmental factors, including endocrine disrupters such as diethylstilbestrol, PCBs or dioxins, can affect both teleost and mammalian reproductive systems, but do not seem to alter sex ratios or cause sex reversals in mammals. This indicates that mammalian sex determination is more strictly genetic, and shows less gonad plasticity than teleosts. However, it has been observed that a number of genes, both sex-linked and autosomal, display dosage effects in mammals (Table ), suggesting that allelic variants could account for differences in gene function.
Chromosomal location and dose effects. Several genes involved in mammalian sex determination have dose effects leading to sex reversal.
While the developmental mechanisms by which the mammalian gonads are formed have been thoroughly studied and several genes involved have been identified, only a few of these genes have been identified in fish. The functions of these genes have not been fully elucidated in fish and both conserved and divergent functions between mammals and fish have been suggested. As zebrafish is an important vertebrate model for developmental biology it is vital that the basic developmental mechanisms of sex determination are further studied in this species. In the present review we discuss the roles of genes involved in sex determination with a focus on the potential role of FTZ-F1 genes in zebrafish sex determination and differentiation. From the present knowledge of these genes in zebrafish we attempt to present a model for zebrafish sex determination and differentiation.