June 4, 2009
Weekly News UpdateIs it a Boy or a Girl? For Some Reptiles, It May Depend on Egg Size
Whether an animal develops into a male or female is in most cases determined by specialized sex chromosomes. In most mammals, for example, males have an X chromosome and a Y chromosome, and females have two X chromosomes. Because they can give either an X or a Y chromosome, male mammals determine the sex of their offspring. For some reptiles, such as turtles, ambient environmental temperature has an impact on sex determination. Research indicates that turtle eggs in cooler nests tend to develop into males and eggs in warmer nests tend to develop into females.
Researchers in Australia recently discovered an interesting twist to sex determination in one type of reptile. Studies of the lizard Bassiana duperreyi showed that when the lizard's eggs were exposed to extreme low temperatures, the sex of the embryos switched, regardless of chromosomes, sometimes even resulting in XX males and XY females. Further research showed that large eggs tended to result in female offspring and smaller eggs tended to result in male offspring.
The scientists initially gave little thought to this discovery, assuming that the correlation between egg size and sex was an anomaly. However, further research indicated that egg size did indeed have an impact on sex. The scientists were surprised to find that when yolk was removed from an egg, the embryo had a tendency to switch to a male, even when the embryo had female chromosomes. Conversely, when yolk was added to an egg, the embryo had a tendency to switch to a female. According to Dr. Richard Shine, one the Australian scientists involved with the study, the researchers were "gobsmacked" by these results and look forward to the opportunity to further research the mechanics behind this interesting phenomenon.
The results of the scientists' research was published in the June 4, 2009 online edition of the journal Current Biology. Scientists who contributed to the paper included Rajkumar S. Radder, David A. Pike and Richard Shine, all from the University of Sydney, and Alexander E. Quinn from the University of Canberra.