Primordial Germ Cells, New Stars of Turtle Sex Determination

An individual’s sex may be determined by genetic elements, as in humans. However, the sex of some other animals may be determined by the environment. One such environmental cue is the temperature the embryos experience during their development. Temperature-dependent sex determination (TSD) is common in reptiles, including some turtles. Red-eared slider turtles (Trachemys scripta elegans) have been used as a model TSD species for decades.

Illustration of a red-eared slider turtle by Karl Bodmer from 1865 (Public Domain)

Developing gonads of this turtle are at first bipotential—neither male nor female, but capable of committing to either ovary or testis fate. This commitment is brought about by—initially also bipotential—supporting cells of the gonad and their differentiation into granulosa cells, or Sertoli cells, respectively. Multiple signals affect the differentiation of these cells. One of the stronger cues is the temperature experienced during the thermosensitive period of embryonic development. Red-eared slider embryos incubated at 26°C develop into males, while a slightly higher temperature (31°C) produces females. An intermediate temperature of 29°C results in offspring at a 1:1 sex ratio. Other signals, such as the presence of steroid hormones, can override the effects of temperature and affect sex determination. Thus, the current model of turtle TSD primarily focuses on bipotential supporting cells and the signals affecting them.

Model ray-finned fishes (zebrafish and medaka) display genotypic sex determination; however, manipulation of germ cell number may result in sex reversal. To date, nobody has studied the effect of germ cells in reptiles or other amniotes. Thus, a team of researchers, including Barbora Straková from the Department of Ecology, focused on the effects of temperature on germ cells of red-eared slider embryos, as well as on how germ cells influence an individual’s sex.

Embryos differed in germ cell number when incubated at different temperatures: gonads of embryos incubated at a higher temperature (31°C) contained more germ cells than embryos incubated at a lower temperature (26°C). The effect of temperature on germ cell number was most significant early in the development — even before the differentiation of supporting cells. Moreover, germ cell number seems to be under direct influence of temperature; bipotential gonads of embryos incubated at 26°C (expected to develop into testes) developed into ovaries when treated with oestrogen. However, these ovaries still contained fewer germ cells.

The effect of germ cell number on sex determination is apparent in embryos incubated at 29°C, i.e., embryos that should develop into males and females in a roughly 1:1 sex ratio.  When treated with chemicals that deplete the gonads of germ cells, embryos showed a markedly skewed sex ratio in favour of males. In most cases, a higher number of germ cells results in the embryo committing to a female fate. However, the effect of germ cell number is not absolute — females hatch from eggs incubated at 31°C despite lower germ cell number.

Some aspects of turtle sex determination are similar to those observed in model ray-finned fish (actinopterygians), where the effect of germ cell number may override genotypic sex determination and where a higher number of germ cells supports commitment to a female fate. Supporting cells play a more important role in turtle TSD, albeit germ cells may prove more important under environmental temperature fluctuation.

Thus, the authors postulate a new model of TSD for amniotes, one where sex determination results from a “parliamentary decision” of multiple cell types integrating various signals. Moreover, the direct effect of temperature on germ cell number may provide an evolutionary advantage by increasing female reproductive potential, as females are presumed to benefit from higher germ cell numbers more than males.

Tezak, B., Straková, B., Fullard, D. J., Dupont, S., McKey, J., Weber, C., & Capel, B. (2023). Higher temperatures directly increase germ cell number, promoting feminization of red-eared slider turtles. Current Biology.

Kateřina Bezányiová