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The lizard known as Eulamprus tympanum, or the southern water skink, is native to mountainous regions of southeastern Australia, not the Red River Valley near Crookston, Minnesota. But the unique biology of this species of lizard has brought together a scientist from the University of Sydney in Australia and Pamela Elf, assistant professor of biology at the University of Minnesota, Crookston. In support of some of the work related to this collaboration, Elf has received the first funding ever awarded to a U of M-Crookston faculty member from the National Science Foundation (NSF). An endocrinologist who has studied both birds and reptiles in her work, Elf specifically focuses on possible roles that hormones play during the development of the embryo and in behavior of offspring after birth.

A mother's contribution

"I'm interested in studying the maternal contributions [made] during embryonic development that impact the embryo," says Elf. Using reptile species, she studies the effects of both the hormones that the mother deposits into the yolk as the egg is formed (for egg-producing species; see accompanying box) and those in the maternal circulation, to which embryos are also exposed (for species that have a placenta). A few years ago, Elf began to study the influence of yolk hormones in egg-laying reptiles that use temperature-dependent sex determination (TSD). In TSD reptiles, the sex of the offspring is determined by the temperature at which the embryos are incubated during development. For egg-laying TSD reptiles, this means that the temperature at which the eggs in a nest are incubated decides the sex of the offspring.

This reliance on environmental factors to determine sex stands in sharp contrast to the method of sex determination used by mammals, which occurs at the time of fertilization and is based on the inheritance of X and Y sex chromosomes. Also in contrast to mammals, reptiles are cold-blooded, meaning that their body temperature mimics that of their surrounding environment. To keep their body temperature relatively constant, reptiles choose to bask in the sun or seek shade as needed. Just as a reptile moves within its surroundings to determine its body temperature, nest-site location within the habitat influences the temperature at which reptile eggs will be incubated.

Elf has examined the role of hormones in TSD in egg-laying reptiles and observed a correlation between temperature and levels of the female sex hormone estradiol in the yolk of eggs. Specifically, she found that eggs incubated at the female-producing temperature had the greatest concentration of estradiol in the yolk, while those incubated at the male-producing temperature had the lowest concentration. Based on these results, Elf has postulated that temperature is not the only factor in the sex determination scheme of egg-laying reptiles; the influence of hormones appears to be an additional key factor.

A one-of-a-kind lizard

Over the past few years, Elf published her studies of yolk steroid hormones and sex determination in egg-layers like snapping turtles and alligators. At the same time, half a world away, University of Sydney doctoral student Kylie Robert was publishing her own work on TSD in reptiles. In her studies on the southern water skink, an ovoviviparous species (see accompanying box), Robert had observed the first instance of TSD in a reptile that gives birth to live young. Rather than nest location being the source of the incubation temperature, the mother herself chooses her location within her habitat to control her own body temperature and, in doing so, the sex of her offspring. Until Robert's work was published, TSD was thought not to exist in live-bearing reptiles.

Robert read Elf's work and contacted her about the possibility of collaborating together to explore hormonal contributions to TSD in the southern water skink. Robert sent some initial samples from her southern water skinks to Elf for analysis. Based on intriguing preliminary results from these analyses, Robert was awarded a Human Frontiers in Science Program fellowship to perform post-doctoral studies in Elf's lab. She subsequently collected samples from the Australian skinks and brought them to Crookston. Through NSF support to Elf, Robert and Elf analyzed the samples. According to Elf, "preliminary data indicate that, in this ovoviviparous TSD lizard, temperature appears to exert a greater influence on circulating maternal plasma levels of hormones, particularly estradiol, than on yolk levels of hormones." Because the skink embryos are exposed to the maternal circulation (see accompanying box), maternal plasma hormones may be the link between temperature and the embryo in the sex determination process.

Mysteries to unlock

What makes a female skink migrate to a particular temperature within her habitat, allowing her then to produce a particular sex ratio of her offspring? Robert found that, for the southern water skink, females adjust the ratio of the sex of their offspring according to the ratio of males and females in the surrounding population. A pregnant female housed with male skinks will move to a female-producing temperature, and vice versa. It is ideal for colonies in the wild to maintain a balanced sex ratio, yet while the skinks' behavior to try to maintain that ratio has been documented, the trigger for the behavior remains unknown. Scientists like Elf and Robert can only speculate whether a pregnant female reacts to male behavior, pheromones, or other cues to tell her to choose one temperature, and therefore one particular ratio of offspring, over another. Based on their successful collaboration to date, the two scientists plan to continue working together to explore the mysteries of sex determination in the southern water skink.



Pamela Elf's faculty Web profile:
www.crk.umn.edu/faculty/E/Pam_Elf.htm