Embryonic sex determination (i.e., the mechanism that determines whether embryos develop into males or females) is a fundamental biological process that has significant implications in determining sex ratios in natural populations. Many vertebrates, including mammals, birds, amphibians, and some reptiles exhibit genotypic sex determination (GSD), where males or females are produced based on the sex chromosomes that an embryo receives from its parents. However, this familiar method for determining the sex of offspring is not the only process that has evolved in vertebrates. Many species of reptiles (and fish) rely on environmental factors to determine the sex of embryos. Temperature is by far the most common environmental characteristic that determines sex in certain species of reptiles, but recent research has also shown that other factors (e.g., egg size) can influence sex ratios. For species with temperature-dependent sex determination (TSD), there are two potential patterns determining how temperature impacts development. First, a gradient of males to females or females to males can be produced as the temperature changes from low to high, with each sex being exclusively produced on one of the two temperature extremes. Second, females can be produced at both temperature extremes and males at temperatures in the middle. For either method to be successful, nests at a population level (or even within a single nest) must experience different incubation temperatures that allows both sexes to be produced.
In reptiles, the evolutionary history of embryonic sex determination is not clearly understood. TSD has been documented in several groups, including crocodilians, tuatara, and the majority of turtle species. TSD is thought to be the ancestral state for reptiles, but it is difficult to define clear evolutionary pathways because many groups of squamates (lizards and snakes) exhibit GSD. To further complicate our understanding, there have likely been multiple transitions between the two mechanisms in some lizard groups. Some of these lizard groups now have species that exhibit TSD while others undergo GSD.
Adding to the complicated evolutionary history of sex determination in reptiles, it has also been difficult for researchers to identify the mechanisms that favor TSD over GSD. Generally the temperature that produces each sex should produce individuals with higher fitness than if individuals of the same sex were produced at a different temperature. In practice, this can be difficult or nearly impossible to test. How do you produce one sex at a temperature that does not allow for that sex to develop? In a handful of studies, the answer has been to manipulate hormones in the embryos to artificially produce the desired sex no matter the incubation temperature. These studies have generally supported the idea that producing each sex at the ‘correct’ temperature increases that sex’s fitness over individuals produced at the ‘wrong’ temperature, but the exact reasons why this is the case remain unclear in most cases. A final complicating factor is that nests in nature experience temperature fluctuations that may impact the sex ratios produced. The effects of temperature variation through time as well as temperature variation within a nest must be considered when studying TSD.
Despite the difficulty in designing studies to examine fitness effects of TSD, one thing is clear, relying on temperature to determine sex ratios has widespread ecological implications for these species. A direct reliance on temperature means that there is a high likelihood of having some nests or perhaps even some years when sex ratios are significantly biased. This has already been documented in some turtle species. The effects of climate change will continue to impact this delicate balance, and species with TSD will be forced to adapt to changing temperatures (it is debatable whether or not this is even possible) or face the consequences of skewed sex ratios.
Of particular interest to the Longleaf Savannas Initiative this year is sex determination in Spotted Turtles (Clemmys guttata). Spotted Turtles exhibit TSD with males produced at temperatures ranging from 22° to 27° C and females produced at temperatures of 30° C. This is particularly interesting given the significant latitudinal variation that Spotted Turtle populations cover. Southern population will likely be the first to experience the effects of climate change as nesting conditions during the summer push temperatures closer to producing all females. This year we will be placing temperature loggers in Spotted Turtle nests to collect the first temperature data from nests in Georgia. These efforts and continued population monitoring will allow us to assess the impacts of changing temperatures on sex ratios of new Spotted Turtle generations.