In this species, high incubation temperature during egg development reverses genotypic males ZZ into phenotypic females; so females can be ZZ or ZW, but males are always ZZ. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo.
Copeia — In some species, including most mammals and insects, sex is determined solely by chromosomes; in other species, sex is a matter of environmental conditions.
When separate sexes are favored, the transition can occur via several evolutionary pathways. Intermediate gametes do worse than small ones in terms of mobility and numbers, and worse than large ones in terms of provisioning. Read Now. Mechanisms of Development. To date, only one example of the reversal of an ancient sex chromosome back to an autosome has been characterized.
PLoS Genetics. Georges A Female turtles from hot nests: is it duration of incubation or proportion of development at high temperatures that matters? Estrogen can override temperature and induce ovarian differentiation even at masculinizing temperatures.
Warner DA, Shine R The adaptive significance of temperature-dependent sex determination: experimental tests with a short-lived lizard. Sex-determining mechanisms are broadly categorised as being based on either genetic or environmental factors. See Subscription Options Already a subscriber?
An alternative hypothesis of adaptive significance was proposed by Bulmer and Bull in  and supported by the work of Pen et al. However, in the bipotential gonads of those turtles raised at male-promoting temperatures, Sox9 expression was retained in the medullary sex cords destined to become Sertoli cells Spotila et al.
J Evol Biol 21 2 —