Genotypic Sex Determination Systems could be Adaptations to Extreme Temperature Environments in Reptiles and to Endothermy in Mammals and Birds

Abstract

In some vertebrates species, environmental temperatures (TSD) determine their sex determination. In others, it is controlled by genomic mechanisms (GSD). One hypothesis suggests that GSD systems could have evolved in ectothermic TSD species to escape the geographical limitation imposed by environmental temperatures. Recently, it has been found that TSD reptiles species tend to breed at warmer temperatures than GSD species, especially in habitat with four months of warm temperatures. Here we obtain the pivotal temperature (the one that generates equal ratios of male and females) in 53 reptiles species (from four orders: sphenodontia, crocodilia, testudines and squamata) and compare it with the environmental temperature in the nest during the breeding season in 100 TSD reptiles species and 78 GSD reptiles species. Our results show that GSD species statistically breed in temperatures that would cause a sex bias if they had TSD systems, whereas species with TSD breed in a similar range of temepratrues to the pivotal ones. Additionally, we also found that the body temperature of more than 1,200 endothermic species statistically exceeds the pivotal temperature suggesting that GSD is necessary for endotherms to avoid sex bias that could lead to extinction. Finally, we observed that one of the 100 most invasive species worldwide, Trachemys scripta elegans, a turtle species with TSD, has never been able to establish in countries with less than four months of warm temperatures, confirming a restriction in the geographic range in TSD species caused by extreme temperatures. Altogether, these results suggest that GSD could have evolved as an adaptation to avoid the biased sex ratios that extreme temperature may cause in species with a TSD system.