Temperature-dependent Sex Determination(TSD) in Reptiles
Sex determination involves the formation of testes in males or ovaries in females through gonadal differentiation during early embryonic development to establish the sex of an organism. Sex determination can occur in different forms, such as genetic sex determination (GSD) and environmental sex determination (ESD), depending on the species of the organism. Mammals and birds typically rely on GSD through the male (XX/XY) and female (ZZ/ZW) heterogametic systems respectively for sex determination, whereas reptiles rely on either GSD, temperature-dependent sex determination (TSD) – a form of ESD whereby offspring sex is determined after conception based on the temperature during egg incubation – or both GSD and TSD simultaneously to determine sex.
The huge variation and complexity in sex determination mechanisms among reptile species are due to multiple transitions between modes, such as GSD and TSD, that have occurred during evolution (Sarre et al., 2011, p. 391). Previously, GSD and TSD are thought to be fundamentally distinct mechanisms that are mutually exclusive (Janzen and Paukstis, 1991). However, Bull (1981) and Shine et al. (2002) suggest that the interactions between genes and environment have caused phylogenetic transitions from GSD to TSD in certain reptiles because of temperature effects on the genotypic system which override GSD. Thus, in terms of sex determination, these reptile species become more temperature-sensitive as some animals may even go through sex reversal due to extreme environmental conditions and elimination of Y or W chromosomes, indicating that genetic influence on sex determination is lost as the reptiles begin to depend on TSD mechanisms (Bull, 1981, PokornÁ and Kratochvíl, 2009). According to Elf (2003, p. 349), there are three types of TSD patterns, including male-female (MF), female-male (FM), and female-male-female (FMF), that represent the different types of TSD outcomes depending on the reptile species. MF represents TSD with more males being produced at low temperatures and more females produced at high temperatures whereas FM is the reverse of MF, but at intermediate temperatures, ratios of males to females are produced for both MF and FM TSD patterns (Elf, 2003, p. 349). In FMF, more females are produced at low and high temperatures while more males are produced at intermediate temperatures (Elf, 2003, p. 349). Many reptile species, including sea turtles, leopard gecko (Eublepharis macularius) and Australian central bearded dragon lizard (Pogona vitticeps),were discovered exhibiting TSD which indicates that temperature is an extremely important factor for sex determination in reptiles (Gutzke and Crews, 1988, Quinn et al., 2007). Hence, drastic changes in environmental conditions such as changes in global climate will heavily affect the reptiles that rely on TSD mechanisms, potentially causing issues such as sex reversal, abnormal offspring development, imbalanced sex ratios, and ultimately extinction.
The continuous rise in global temperature due to global warming will affect the population dynamics of certain reptile populations that rely on TSD because sex reversal can be induced when eggs are incubated at certain temperatures and imbalanced sex ratios can result consequently. Sex reversal is the development of genotypically normal but phenotypically feminized males or masculinized females. Quinn et al. (2007) studied sex reversal in reptiles by incubating eggs of P. vitticeps at a range of temperatures and demonstrated that genotypic males (ZZm) incubated at high temperatures were sex-reversed into phenotypic females (ZZf). Thus, more females than males were produced under high temperatures, indicating that populations will risk having imbalanced sex ratios which can affect population dynamics. One restriction in the study is the mortality of embryos that were incubated at below 22°C which leads to great uncertainty in terms of sex reversal in low incubation temperatures (Quinn et al., 2007). Sex-reversed reptiles are still viable and fertile whereby sex-reversed mothers (ZZf) would produce more temperature-sensitive offspring than normal ZW mothers when mated with ZZ males (Holleley et al., 2015). Nonetheless, only ZZ offspring can be produced due to the lack of W chromosome in sex-reversed mothers, and therefore incubation temperature is essential for sex determination in ZZ offspring (Holleley et al., 2015). Holleley et al. (2015) suggest that overproduction of females and rapid elimination of W chromosome in animal populations are due to exposure of eggs at high incubation temperatures and sex-reversed females having increased fecundity with their potential to sex reverse being heritable.
Although Quinn et al. (2007) and Holleley et al. (2015) showed that sex reversal caused by high incubation temperatures can lead to imbalanced sex ratios in reptile populations, their findings are not fully applicable to all reptile species as both studies were focused only on the species P. vitticeps. For instance, Ferguson and Joanen (1982) demonstrate that more males are being produced at higher incubation temperatures while females are produced at lower incubation temperatures for the American alligator (Alligator mississippiensis) species which show contrasting results to the studies by Quinn et al. (2007) and Holleley et al. (2015). All three studies share a common limitation in which the eggs were only incubated and studied under constant temperatures in laboratory conditions but not under fluctuating temperatures that mimic the natural environmental conditions. Therefore, further studies need to be designed to mimic the natural conditions to determine more accurate sex ratios of reptile populations in the wild. Nevertheless, these studies have shown that changes in temperature causing sex reversal can ultimately lead to imbalanced sex ratios. These studies also suggest that TSD mechanisms vary according to reptile species, hence further research is required to determine how TSD works specifically in other reptile species.
Moreover, climate change can affect the development of reptile offspring produced by TSD reptiles in terms of behaviour and morphology. The effects of temperature change on offspring development can be potentially threatening to the survival of reptiles as their normal development will be disrupted, thus affecting reproductive success. Regardless of offspring sex, leopard geckos (Eublepharis macularius) that rely on TSD mechanisms with the FMF pattern will produce larger and more aggressive offspring when eggs are incubated at male-biased intermediate temperatures than at female-biased low and high temperatures (Flores et al., 1994, Gutzke and Crews, 1988). The methodology of the study by Flores et al. (1994) was well-designed, particularly the techniques used during egg incubation and animal housing. In the experiment, eggs were required to be incubated at temperatures 26°C, 30°C, 32.5 and 34°C, however, they may not survive through constant incubation at 34°C as it is potentially the lethal temperature of leopard geckos (Flores et al., 1994). Hence, the eggs were first incubated at 32°C, then at 34°C the next day, and lastly back to 32°C one to three days before hatching to ensure the survival of eggs (Flores et al., 1994). As for housing of the geckos, the incandescent light installed above each cage and fluorescent lights will be turned on and off alternately in cycles for thermoregulation which is vital for the survival of reptiles (Flores et al., 1994). Also, the exposure of geckos to controlled environments with constant temperatures at early life stages can increase offspring survivorship, but to accurately mimic natural environmental conditions, geckos were later exposed to daily thermal cycles after they reach 10 weeks of age (Flores et al., 1994). These findings show that when female offspring are incubated at male-biased temperatures, they will become masculinized in terms of behaviour and morphology as they develop after hatching3, suggesting that temperature is an essential factor for offspring development in reptiles. However, further studies and information are needed to determine the possible effects of altered offspring development, including changes in body size and aggressive behaviour, on survival and reproductive success of reptile populations when offspring are incubated at either male- or female-biased temperatures.
Gutzke and Crews (1988) claimed that the elevated levels of testosterone (T), low levels of oestrogen and slow sexual maturation in female offspring resulted from incubation at male-biased temperatures while male offspring incubated at female-biased temperatures had increased oestrogen levels. However, Tousignant et al. (1995) found that presence of T in females is due to ovarian activity instead of egg incubation temperature because similar T levels were present in females regardless of incubation temperature, hence contradicting the results produced by Gutzke and Crews (1988). Both studies appeared to have different methodologies for measuring sex steroid levels: Gutzke and Crews (1988) tested blood samples of females weighing at least 25g that were taken after behavioural testing whereas Tousignant et al. (1995) tested samples of 50-week old, sexually mature females before behavioural testing. Tousignant and Crews (1995) showed that majority of female geckos weighed 25g at around 25-30 weeks of age and were sexually immature. This suggests that the hormone levels discovered by Gutzke and Crews (1988) are irrelevant to reproductive physiology in female offspring because hormone levels will alter significantly only when sexual maturation is reached. Thus, further experimentation is required to identify whether egg incubation temperatures will affect reproductive physiology and secondary sexual development during later life stages of reptile offspring.
Furthermore, the increase in global temperature can lead to declination of offspring production and ultimately extinction of species because extreme temperatures can cause detrimental effects on egg and offspring survival, especially of animals that rely on TSD mechanisms such as sea turtles. Sea turtles as egg-burying reptiles are very sensitive to temperature whereby eggs incubated at high temperatures will typically hatch as females and vice versa in most species, therefore many sea turtle populations are female-biased following the global temperature rise due to global warming. Imbalanced sex ratios will negatively impact most animal populations due to increased risks of species extinction. Conversely, Santidrián Tomillo et al. (2015) suggest that for sea turtle populations, the increase in female production as global temperature is rising can be advantageous in some cases because many offspring will die at extreme temperatures, hence the increased female population is needed to promote natural population growth which prevents extinction of species. However, when incubation temperature reaches near lethal levels, the mechanism is inefficient because sea turtles are unable to survive at extremely high temperatures (Santidrián Tomillo et al., 2015). Santidrián Tomillo et al. (2015) ran simulations for TSD and GSD models under nest temperatures that were reconstructed according to temperatures from previous years, then compared the growth rates of TSD and GSD models to determine effectiveness of TSD as compared to GSD in sea turtle populations. Simulations for future climate change were also ran with increasing temperatures that mimic the rising global temperature on TSD and GSD models to study potential effects of climate change on sea turtle populations (Santidrián Tomillo et al., 2015). The simulations were used to study natural conditions and to predict future outcomes in the most accurate way for the understanding of TSD mechanisms in sea turtle populations. Adaptation of sea turtles to climate change is vital so that thermal tolerance is increased to prevent issues such as imbalanced sex ratios, abnormal offspring development and low hatching success caused by high incubation temperatures in sea turtle populations (Miller, 1985).
In conclusion, TSD mechanisms in reptiles can be beneficial or disadvantageous to reptile populations depending on the environmental factors, species factors and state of populations. Reptile populations relying on TSD must adapt to altered environments through behavioural and physiological changes or move to other more suitable environments for survival and successful reproduction. Otherwise, population dynamics can be greatly affected by imbalanced sex ratios, abnormal offspring development and eventually species extinction that can result under extreme environmental conditions. Thus, future research on TSD mechanisms in other reptile species is needed to fully understand TSD in reptiles as TSD works differently depending on animal species.
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