As a RISE fellow and undergraduate researcher in John Roe’s lab, Kristoffer Wild (’13) learned a lot about the ecology of box turtles. After graduating from UNC Pembroke, Kristoffer moved on to Austin Peay State University in Tennessee, where he studied fence lizards for his M.S. degree in Biology, followed soon by a teaching position in Memphis. Years would pass, but Wild and Roe continued their collaborations, resulting in three peer-reviewed papers about box turtles, each with Roe as first author. Following in Roe’s footsteps, Wild is now completing a doctoral degree in Applied Ecology from the University of Canberra in Australia. Wild and Roe recently published a paper that examined the genetic structure of the Central Bearded Dragon (Pogona vitticeps). Wild is first author on this paper.
Their paper, entitled “Evolutionary stability inferred for a free ranging lizard with sex-reversal,” was co-authored by Lisa Schwanz, Arthur Georges, and Stephen Sarre, and appears in the peer-reviewed journal Molecular Ecology. The Central Bearded Dragon is a large lizard endemic to Australia. Sex is determined by genetics (sex chromosomes) and by incubation temperature of the embryos, resulting in three sex classes: males (ZZ genotype), females (ZW genotype), and sex-reversed females (ZZ genotype) in which the female phenotype has been induced by high incubation temperatures. Thus, global warming tied to anthropogenic climate change could accelerate the rate of sex reversal.
Wild, Roe and their co-authors tested this hypothesis using data collected during a 16-year (2003-2019) period in southeastern Australia. They used molecular methods and field surveys of wild populations (207 lizards) to determine genotypic sex (ZZ or ZW), phenotypic sex, genetic structure, sex reversal rates, and gene flow. In an intensive radio-telemetry survey of a subpopulation, they gauged differences in behavior, space use, survivorship, and morphometrics among sex classes. Sixty-three adult lizards were fitted with a backpack harness containing a GPS unit, radio-transmitter, and accelerometer, allowing the researchers to monitor lizard locations, behaviors, and fates during an 18-month period.
Their study detected a modest 17% rate of sex-reversal in the larger population, a rate that is not accelerated. Moreover, the rate of sex reversal observed for the second eight years of their study was much less than the rate observed during the first eight years, while little change occurred in the proportion of sex-reversed females. They detected similar movements and home range behaviors for sex-reversed ZZ females and ZW females, as well as similar growth and survival. However, sex-reversed ZZ females had lower reproductive output than ZW females in the field. These results were contrary to observations from laboratory experiments that showed greater fecundity for sex-reversed females. Importantly, nearly every year of their study had average air temperatures in excess of the threshold temperature for sex reversal, signaling that continued warming could accelerate sex reversal in the future, and making it evident that nesting temperatures and other local factors should be the focus of future study. The large scope of their study provides valuable insights into sex determination in the Central Bearded Dragon and other reptiles.
“More than 100 species of squamates (lizards and snakes) have undergone an evolutionary transition from genetic sex determination to temperature-dependent sex determination in the past”, says Roe, “and we were able to study this process in the field as it happens in real time”. “We did not find evidence that such a transition is happening in bearded dragons under current conditions, but we cannot rule it out in the future if temperatures and other environmental stressors continue to rise”.
Kristoffer Wild is on course to defend his thesis in the coming months. Thanks to influential research experiences in the John Roe Lab, as well as research opportunities in graduate school, no doubt he will continue to pursue important work in ecology. And maybe one day, Wild will mentor undergraduate students, who like himself, become excited about asking really cool questions about nature, and follow in their mentor’s footsteps.
Dr. John Roe is a herpetologist and Professor in the UNCP Department of Biology, and winner of the 2020 Undergraduate Research Mentor Award given by the Pembroke Undergraduate Research and Creativity Center Council (PURC). He teaches undergraduate courses in Field Zoology, General Zoology, and Environmental Science, while mentoring students in field and laboratory research. He has co-authored more than 50 papers, including several with undergraduate researchers, many of whom have continued their ties to his lab, long after graduation.