Dragon Sex

There are two mechanisms for the determination of gender in reptiles, genotype sex determination (GSD, with XX and XY chromosomes) and temperature-dependant sex determination (TSD, with ZZ and ZW chromosomes). Utilising both mechanisms makes reptiles highly desirable subjects within genetics.

The Australian Central Bearded Dragon was used by the Australian National University (ANU) in an experiment to further study sex determination in reptiles. The Bearded Dragons were previously described as having TSD, therefore temperature overrides genetic sex determination during embryonic development; high and low temperatures result in femaleness and moderate temperatures ensures maleness. The scientists at the ANU discovered some fascinating findings about this process which may result in disastrous consequences for future generations.

Originally it was believed that the W chromosome was the basis for female phenotype expression, however according to this study the W chromosome is not required for femaleness. Their new theory describes a ‘master’ sex determining gene – too much of the Z chromosome and the reptile becomes male (ZZ), too little and it becomes female (ZW). Both situations are dependant on optimal operating temperatures.

Because the sex is reliant on temperature, increases in environmental temperature (as is the case with climate change) may distort sex ratios, or at worst ending reproduction when no males exist. The researchers are now beginning to look at other reptiles to see how widespread this type of sex determination is and to hopefully on how we can maintain the sex ratio’s for a positive future outlook for our reptilian friends.

Posted by Tania Burke
Student # 41163702

References

Primary
Quinn, A. E., Georges, A., Sarre, S. D., Guarino, F., Ezaz, T. & Marshall Graves, J. A. (2007). Temperature sex reversal implies gene dosage in a reptile. Science, 316, 411.

Secondary
Hayes, J. (2007). Heat prevails over genes in lizard sex. Cosmos Magazine. http://www.cosmosmagazine.com/node/1249 (Accessed 25/05/07).

Young, E. (2007). Genes vs heat - a reptile sex trigger. New Scientist. http://www.newscientist.com/channel/life/dn11669-genes-versus-heat--a-reptile-sex-trigger.html. (Accessed 25/05/07).

Interesting Links
Bryner, J. (2007). Warming climate reverses sex of lizards. Live Science. http://www.livescience.com/animals/070419_lizard_sex.html. (Accessed 25/05/07).