Australia's central bearded dragons have a particularly strange method for determining their sex, and a new study has found things are even more bizarre than previously realized. Dragons become female not as a result of a crucial gene's sequence like most animals, but because of the way nearby stretches of DNA interfere with its coding – something never seen before.
Humans, like other mammals, determine our sex genetically – even if it is not always as simple as the XX=female and XY=male tale told in introductory genetics. Some reptiles also use chromosomes, but for others, sex depends on the temperature at which eggs are incubated.
Central bearded dragons (Pogona vitticeps) have intrigued scientists for 20 years because at ordinary temperatures they use genetics, producing roughly equal numbers of males and females, but females dominate in the heat. Scientists have been exploring this for years, but in the journal Proceedings of the National Academy of Sciences, they have revealed even the genetic processes don't operate the way every other known animal does.
When not relying on temperature, male reptiles have two Z chromosomes and females have a Z and a W. Professor Jenny Graves of Latrobe University told IFLScience; “We thought we'd better look at how dragons do genetic sex determination at normal temperatures. We thought it might be dosage-dependent like birds, where two copies of the gene makes male, and having only one makes a female.”
Finding the relevant gene proved challenging, however. The most likely suspect was nr5a1, involved in sex determination in many species. However, sequencing revealed nr5a1 is identical on the Z and W chromosomes. Nevertheless, the two chromosomes as a whole clearly differ. “We can see down the microscope the W is bigger than the Z because it is full of repetitive sequences,” Graves added.
After seven years' work, Graves and co-authors have concluded nr5a1 is key, but its location is essential. On the Z chromosome, it codes normally with a single mRNA isoform for a protein that makes dragons male. However, on the W it is, in Graves's words; “In a bad neighborhood.” The surrounding sequences cause it to produce not one mRNA isoform, but 16, many of which, the University of Canberra's Professor Arthur Georges calls “truncated”. These truncated isoforms interfere with the normal one, not only on their own chromosome but those produced by the Z. The result is a blocking of the proteins that would otherwise make a dragon male.
“So rather than sex being determined by the base sequence of a sex gene – like in humans, and every other animal we know about – in the dragon lizard it was about the gene’s kinky chromosomal ‘neighborhood’,” Graves said in a statement.
Both Graves and Georges admit they don't yet know how this relates to the temperature-dependent sex determination, if it does, and the strange differences in female behavior and body shape that produces. They're also not sure if there is any evolutionary advantage to doing things this way, with Georges telling IFLScience; “Anything that takes a species in a beneficial direction will be adopted.”
In the same edition of PNAS, Graves has two other papers about the Earth BioGenome Project, an exceptionally ambitious effort to sequence the genomes of every known life form. Graves told IFLScience that one paper discusses the goals, while the other addresses the question of why it is worthwhile to sequence at least 1.8 million species rather than picking one representative from every family.
The papers were submitted separately from the dragon one, and Graves is the only overlapping author, but Graves said the dragon research represents; “A perfect example of why we should sequence everything.”
As far as we know, even P. vitticeps's nearest relatives do not have the same strange sex selection, and if we'd chosen another lizard to sequence we would've missed this strange phenomenon. In addition, Graves notes, the creation of such an immense database provides exceptional opportunity for comparisons between species to explain odd features.
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