Fitness-related effects of sex reversal in the agile frog

In mammals and birds (warm-blooded animals), the composition of sex chromosomes initiates the direction of sexual development. For example, in mammals, individuals with two copies of the X chromosome develop into females, while those with one copy of the X and one copy of the Y chromosome become males. While the presence of sex chromosomes was verified in many amphibian, reptile and fish species, experiments over the recent decades lead to the recognition that sexual development may be influenced by both internal and external factors in these cold-blooded animals. Results of these experiments indicated that if embryos or larvae are exposed to unusually high or low temperatures, the environmental conditions may override the effects of sex chromosomes. In such a case, for example, an individual carrying two copies of the X chromosome may become male instead of female. This phenomenon is called environmental sex reversal. More recently, sex-reversed individuals of some cold-blooded species have been found in nature, suggesting that sex reversal can not only be induced under artificial conditions, but may occur in wild populations as well. Because extreme temperature conditions are becoming ever more frequent due to climate change, environmental sex reversal is also expected to become more common in nature. However, we know disturbingly little about the effects of sex reversal on both individual and population level. Are sex-reversed individuals as viable as others? Can sex-reversed individuals breed and produce offspring similarly to others? These are questions important on the individuals level. However, the answers are important for the persistence of the population as well, because the perspectives of future generations depend on them. If sex reversal becomes frequent, there may be a deficit of one sex. In theory, XX individuals might all become males, and if this happens, there will be no females left and the population will go extinct. This project is focusing on the agile frog, an amphibian species of which sex-reversed XX males are known to occur in the wild. Our goal is to collect a variety of information on the prospects of sex- reversed animals for survival and mating success both by inspecting wild populations and monitoring the development of individuals in captivity. We expect that our results will enable predictions on the future of agile frog populations under the current climate change and will contribute to a better understanding of the evolution of sexual development.

In mammals and birds (warm-blooded animals), the composition of sex chromosomes initiates the direction of sexual development. For example, in mammals, individuals with two copies of the X chromosome develop into females, while those with one copy of the X and one copy of the Y chromosome become males. While the presence of sex chromosomes was verified in many amphibian, reptile and fish species, experiments over the recent decades lead to the recognition that sexual development may be influenced by both internal and external factors in these cold-blooded animals. Results of these experiments indicated that if embryos or larvae are exposed to unusually high or low temperatures, the environmental conditions may override the effects of sex chromosomes. In such a case, for example, an individual carrying two copies of the X chromosome may become male instead of female. This phenomenon is called environmental sex reversal. More recently, sex-reversed individuals of some cold-blooded species have been found in nature, suggesting that sex reversal can not only be induced under artificial conditions, but may occur in wild populations as well. Because extreme temperature conditions are becoming ever more frequent due to climate change, environmental sex reversal is also expected to become more common in nature. However, we know disturbingly little about the effects of sex reversal on both individual and population level. Are sex-reversed individuals as viable as others? Can sex-reversed individuals breed and produce offspring similarly to others? These are questions important on the individual's level. However, the answers are important for the persistence of the population as well, because the perspectives of future generations depend on them. If sex reversal becomes frequent, there may be a deficit of one sex. In theory, XX individuals might all become males, and if this happens, there will be no females left and the population will go extinct. This project focused on the agile frog, an amphibian species of which sex-reversed (XX) males are known to occur in the wild. Natural pigment patterns allowed us to monitor both captive and free-ranging individuals and collect a variety of information on the prospects of sex-reversed animals for survival and mating success. We found only subtle differences between XX and XY males in skin colouration, and they were indistinguishable in most parameters. However, we found differences in testosterone levels between the two male types. Additionally, multiple evidence together suggest that XX males are overall less successful breeders. Our findings enable more accurate predictions on the future of agile frog populations under the current climate change and contribute to a better understanding of the evolution of sexual development.