Micro-chromosomes

There may be multiple points in the developmental cascade where environmental influences intervene to reverse sex - the ultimate target is activation of the aromatase gene, which converts testosterone and androstenedione to estrone and estradiol, directing development into the female pathway.

This year, after a painstaking search, Sarre and his colleagues identified tiny Z and W sex chromosomes - micro-chromosomes, in fact - in P. vitticeps, which has genetic sex determination.

By screening amplified restriction length polymorphisms (AFLPs), they identified a 72 base-pair, female-specific marker on the W chromosome. It differs by only a single base from its two Z homologues, but distinguishes genotypically female (ZW) embryos from genotypically male (ZZ) embryos that have changed sex because of high incubation temperatures.

In their Science paper, Sarre's team notes that for many reptiles with temperature sex determination, the male-to-female switch occurs at both ends of the temperature scale - a balanced sex ratio occurs only in a "Goldilocks zone" between 22 and 34 degrees.

They were unable to determine if all P. vitticeps embryos develop as females at incubation temperatures below 22°C, because no embryos survived.

They believe a dosage mechanism, involving a Z-linked gene, or genes, determines sex. With two active copies of the unknown gene, a ZZ karyotype producing twice the dose of a gene product may repress female development.

The ZW karyotype generates in a female embryo, due to inactivation of the Z-linked gene or its protein - which may act alone, or as a component of a transcription complex.

Sarre and his colleagues doubt that the W chromosome harbours a female equivalent of the mammalian SRY gene that initiates female development but aim to investigate that possibility through the application of genomic approaches.

They propose that when incubated at high or low temperatures, both maleness genes in ZZ embryos are progressively deactivated, and development switches to the female pathway - the "default" pathway, as in the mammalian XY system.

"The big question is how such transitions occur - and how such a diversity of sex-determining modes evolved in this one group of organisms," he says.

Sarre's team has bought a bacterial artificial chromosome (BAC) library to hunt for candidate maleness genes in the regions of the Z chromosome flanking the marker sequence on the Z chromosome.

In a 2004 review article in BioEssays, Sarre and colleagues observed that many reptile researchers consider genetic and environmental sex-determination mechanisms to be fundamentally different, and mutually exclusive.

They argued that genetic and environmental sex determination in reptiles should be seen as a continuum of states between species that rely primarily on genetic sex determination, and those in which sex is determined by environment/temperature.

The continuum would include species like P. vitticeps, in which the two mechanisms coexist and interact in some measure, determined by environmental influences, to bring about sex determination.