MINI REVIEW Exploring the evolution of environmental sex determination, especially in reptiles

Sex-determining mechanisms (SDMs) can be categorized according to the primary causal agent of sex determination. Under environmental sex determination (ESD), sex is established by a nongenetic cue (e.g. nutrient availability) experienced within a discrete period after conception (Bull, 1983). In contrast, genotypic sex determination (GSD) occurs when sex is established by genetic factors (e.g. sex chromosomes) at conception. Understanding and distinguishing between ESD and GSD is important because the evolutionary dynamics of genes and primary sex ratio, for example, should differ fundamentally between these two categories of SDMs (Valenzuela et al., 2003). The great diversity of SDMs has thus long puzzled evolutionary biologists. It is easy to envision how 1 : 1 primary sex ratios and GSD are selectively linked. Because each sex must provide half the genes to subsequent generations, frequency-dependent selection on the primary sex ratio is strong (Fisher, 1930) and so meiotic separation of sex chromosomes provides a painless solution to the problem. It is perhaps surprising then to learn that ESD (and, potentially, adversely biased sex ratio) is phylogenetically widespread, occurring in such diverse taxa as plants, nematodes, echiurid worms, amphipods, fish and amniote vertebrates (Korpelainen, 1990,1998). Despite the possible Fisherian fitness cost of biased sex ratio under ESD, the selective pressures favouring ESD over GSD in most of these taxa are understood (Bull, 1983). For instance, photoperiod induces production of males earlier, and females later, in the season in the amphipod (Gammarus duebeni). Because male fitness improves more than female fitness with increased size, ESD is adaptive in this system by permitting males to experience a longer growing season than females (McCabe & Dunn, 1997). The adaptive significance of ESD is not so clear for amniote vertebrates, however, the clade in which this SDM is perhaps most common. Most amniote vertebrates have GSD, but a small highly diverse set of taxa in this clade possesses ESD. In these latter animals, ESD occurs as temperature-dependent sex Correspondence: Fredric J. Janzen, Department of Ecology, Evolution and Organismal Biology, Iowa State University, 253 Bessey Hall, Ames, IA 50011-1020, USA. Tel.: 515 294 4230; fax: 515 294 1337; e-mail: [email protected]