Sex in Dragons

The molecular basis of genetic and environmental sex determination

	Identified cryptic sex chromosomes in the central bearded dragon Pogona vitticeps using state-of-the-art molecular cytology and genetic techniques. They have cryptic ZW chromosomes, cryptic because they involve the microchromosomes and cannot be readily identified using standard chromosome bandings, requiring advanced cytogenetic techniques, such as comparative genomic hybridisation (CGH).
	Used AFLP techniques to isolate sex specific DNA sequence from the Bearded Dragon, and expanded these fragments to provide sex chromosome specific probes for fluorescent in situ hybridisations (FISH).
	Incubated dragon eggs at moderate to high temperatures to cause sex reversal, and demonstrated this conclusively using our sex specific markers and cytogenetic techniques.
	Used sex chromosome specific FISH probes through comparative mapping to provide the first evidence of independent origins of female heterogamety (i.e. non-homologous ZZ/ZW sex chromosome systems) among closely related dragon lizards.
	Developed W chromosome-specific paints in central bearded dragon by chromosome microdissection.
	Used these sex chromosome paints in FISH to identify that Z chromosomes in this species are also microchromosomes, slightly smaller than that of W chromosome and and with different repeat contents. In addition we also developed Z chromosome specific paints.
	Used these Z and W sex chromosome paints, to provide first evidence of underlying sex chromosomes in a TSD dragon lizard which also have sex microchromosomes.
	Constructed a genomic BAC library from a female Pogona vitticeps and screened this library with the sex specific markers and isolated BAC clones from the sex chromosomes. These sex chromosome BACs are being sequenced, once fully sequenced, the findings promise to identify novel sex determining sequences/genes in this species.
	Screened BAC library for candidate vertebrate sex chromosomal genes and discovered that dragon lizard sex chromosomes are not homologous to that of snakes, or gecko or birds and therefore evolved independently, a significant findings to support idea of multiple evolution of sex chromosomes among reptiles.

We now plan to expand our work to investigate transitions between GSD and TSD and between XY and ZW systems of sex determination. We also plan to identify and characterise the homologous chromosome and gene(s) in closely related reptiles, both TSD and GSD. We hope then to discover how the sex determining pathways in the related GSD and TSD species differ and reconstruct how transition between GSD and TSD occur in evolution. Our work will provide new insights to the mechanisms of sex determination in vertebrates.

The potential payoffs for understanding reproductive processes in other vertebrates have driven a huge interest in the mechanisms of sex determination. However, research to date has not capitalised on the immense opportunities offered by the study of the diverse range of sex determining mechanisms in reptiles, where the link between the genotype and phenotype is more complex. Most studies have been restricted to mammals, which are relatively conservative in their mode of sex determination. In a context where insights are often derived from oddities and exceptions to the rule, "studies on sex determination in non-mammalian vertebrates will undoubtedly throw light on the human situation" (Mittwoch, 2000).

Publications to emerge from our work are available here .