Sex Determination

Sea turtles don't have sex chromosomes.

Temperature both influences sexual differentiation and rate of development of Caretta caretta. As it does with many other chelonian species, within the limits of embryonic tolerance cooler temperatures produce male hatchlings (26 and 28°C, C. caretta; 26°C, C. mydas), and warmer temperatures produce female hatchlings (32°C, C. caretta; 29 and 32°C, C. mydas) and the temperatures between these produce both sexes. The phenomenon of temperature dependent sex determination has important consequences in conservation practices. The middle one-third of the incubation period (stages 22-27) is the sex determining period for C. caretta. Prior to stage 21, the embryonic kidney is not differentiated into a functional mesonephros and there is no visible gonad. Thus, within this critical period, incubation temperature has a particular influence on the growth of urogenital tissue. The rate of embryonic development is faster at the warmer temperatures. A variety of factors (maternal behaviour in choosing a nest site, the zygote's response to temperature in becoming male or female, environmental effects (temporal or spatial temperature of the nesting area) interact to determine the sex ratio. Temperature dependent sex determination (TSD) has been reported in at least five families of Chelonians(7 genera, 72 species). There are three pattern of TSD. -Type A; females produced at lower temperatures, males produced at high temperatures (most crocodilians and lizards). -Type B; females produced at high temperatures, males at lower temperatures (many turtles). -Type C; females produced at low and high temperatures, males at intermediate temperatures(three crocodile, one lizard, and three turtle species).