Steroids & Temperature Influence Sex Determination in Gekko japonicus


Sex determination is a developmental process altering undifferentiated gonads into testes or ovaries. Vertebrates have two types of sex determination: genotypic sex determination (GSD), in which offspring sex is determined at the time of fertilization by genetic factors and environmental sex determination (ESD), where environmental factors act after fertilization at a critical time in embryonic development to determine offspring sex. 

Temperature dependent sex determination (TSD) is one form of ESD. TSD is widespread in reptiles including all crocodilians, tuataras, many turtles and some lizards. The discovery that in TSD species temperature is not the only factor influencing sex determination suggests that other factors, especially maternal influences via yolk steroid hormone deposition, can influence the end result of the sexual differentiation process, although the influence (direction and/or magnitude) of a given steroid hormone may be species-specific. For example, eggs with elevated levels of corticosterone are more likely to produce daughters in the Jacky dragon, Amphibolurus muricatus, and sons in the three-lined skink, Bassiana duperreyi.

Ding et al. (2012) incubated eggs of the Japanese gecko Gekko japonicus at three temperatures, and measured yolk testosterone (T) and 17β-estradiol (E2) levels at three time points in embryonic development (oviposition, 1/3 of incubation, and 2/3 of incubation), to examine whether maternal influence on offspring sex via yolk steroid hormone deposition is significant in the species. Eggs incubated at 24 °C and 32 °C produced mostly females, and eggs incubated at 28 °C almost a 50:50 sex ratio of hatchlings. Female-producing eggs were larger than male-producing eggs. Clutches in which eggs were incubated at the same temperature produced mostly same-sex siblings. Yolk T level at laying was negatively related to eggs mass, and yolk E2/T ratio was positively related to egg mass. Their data in G. japonicus show that maternal influence on offspring sex via yolk steroid hormone deposition is significant; incubation temperature affects the dynamics of developmental changes in yolk steroid hormones; influences of yolk steroid hormones on offspring sex are secondary relative to incubation temperature effects; and offspring sex correlates with an interaction between incubation temperature and yolk steroid hormones.

Citation
Ding G-H, Yang J,  Wang J, Ji X. 2012. Offspring sex in a TSD gecko correlates with an interaction between incubation temperature and yolk steroid hormones. Naturwissenschaften 1-8,