Unravelling the epigenetics of sex
The recently awarded $1.25 million ARC grant means another 4 years of funding for the Pogona Lab, a collaboration between the University of Canberra, UNSW, two premier medical institutes QIMR Berghofer and the Garvan Institute, CSIRO and the Autonomous University of Barcelona.
“The grant means job opportunity for post docs and research students. We can expand and diversify, and consolidate skills and expertise, while driving innovation and breaking new ground in research developmental programming,” Arthur said.
The Pogona Lab is working with a native Australian lizard, the central bearded dragon, to search for the elusive switch that makes their embryonic sexual development sensitive to temperature.
A novel model
Central bearded dragons adapt very well to captivity which makes them easy to house and breed.
“Dragons lay up to 35 eggs per clutch and 4 or 5 clutches per year, which provides a ready supply of eggs and embryos for study. But importantly for us, their sex determination is influenced by temperature but in a very unusual way,” Arthur said.
Central bearded dragons have Z and W chromosomes that determine sex. ZW individuals are female and ZZ individuals are male. This is the opposite of human sex chromosomes, where females have a homogenous pair of sex chromosomes (XX) and males have a heterogeneous pair (XY).
“When you push the egg incubation temperature to the upper extremes in the dragon, temperature overrides chromosomal sex and you get females that have male chromosomes. This means ZZ males reverse sex and develop as females,” he said.
“In a number of other reptile species, it's just temperature that determines the sex. There are no sex chromosomes to reverse. This unusual trait of sex reversal in dragons provides unique opportunities for study.”
Understanding epigenetics
Sex reversal in dragons is a concrete example of epigenetics (genetics "above" the genes), where environmental factors can modify gene expression and result in variable phenotypes. Arthur’s team is using advanced molecular techniques to understand exactly how epigenetic mechanisms reverse sex in dragons.
“We want to understand at the molecular level how the cell senses temperature and how this influences the decision to become one sex or the other, and how it modifies the genes and pathways involved in subsequent sex differentiation,” he said.
The team’s research is important not just in the context of global climate change. Things like habitat clearing and the removal of insulating grasses can also impact the incubation temperature inside the nests of lizards. And the research has applications far beyond understanding sex in lizards.
“Epigenetic processes involved in vertebrate sex determination are highly conserved across species. The mechanisms that determine sex in dragons are the same mechanisms that can go awry in some human cancers,” Arthur said. "This provides us with an exceptional pool of targeted inhibitors of cellular processes drawn from studies of cancer to apply in our dragon work, and an exciting opportunity to better understand how these processes function in an entirely novel context.
“New discoveries lead to unexpected innovation. For example, the studies of the Mexican gila monster led to a novel new drug to treat diabetes – the residency time in the body of an influential molecule common across many vertebrates is much greater in the gila monster version than in the human version. Without intending to overpromise, you never know where your discoveries will lead". There is such an important role for blue sky fundamental research.
It is equally important to be flexible and develop diverse capability so that we are able to meet future challenges. The swift response to SARS-CoV-2 came about from scientists across the world using their capabilities, developed in a wide range of contexts, to tackle a new challenge and deliver important outcomes in a very short period of time. You need only consider the value of phylogenetics in the fight against SARS-CoV-2 to be convinced of this. Building a robust set of capabilities, opportunities for a new generation of scientists, use of cutting-edge technologies, along with innovation and flexibility to explore is what I think made the Pogona work attractive to the ARC in the latest round.”
A bright future for the Pogona Lab
The Pogona Lab has already delivered many papers on the genetics of sex determination in dragons, setting the groundwork for a new assault on the less tractable epigenetics of sex.
“Early career researchers and PhD students supported by the last ARC grant achieved great outcomes. For example, Sarah Whitely examined gene regulation in ZZ individuals that were reversed to female under the influence of temperatures and ZZ individuals that became males under the influence of the sex chromosomes to reveal the fundamental, and very different, pathway to becoming female in the dragon", Arthur said.
“We had a paper published recently in PNAS that was 12 years in the making, led by postdocs Xiuwen Zhang and Susan Wagner that proposed a role for chromosome conformation in the differential expression of isoforms generated by a candidate sex determining gene in the dragon.
“Up to this point we have made great strides with correlative results from transcriptomic studies. This ARC grant will allow us to take the next step, which is manipulative experiments to get to the causation. Can we manipulate the process by which the cell detects temperature and switch temperature sensitivity on and off? We plan to utilise live cell imaging available at ANU to confirm our interventions to manipulate calcium fluxes and then apply these interventions in organ culture see what happens to sex under different temperatures.
We hope that by undertaking manipulative experiments funded under this grant, that we will be able to generate new and definitive results on the epigenetic mechanisms that reprogram the developmental processes governing sexual fate and to continue to publish in the top journals and influence on the direction of research on thermolabile sex determination into the future. We are also keen to see members of our team to ultimately get jobs in the best labs across the world (congrats to Susan for landing a great job with the Max Planck Institute in Germany), which in turn will facilitate connections and collaborations for years to come.”
Concluding remarks
Scientists have long had an interest in sex. “It all began when scientists started to ask ‘Why are there two sexes?’ If you have been successful in the fitness stakes, why dilute your winning formula by admixing your genes with those of another individual? The equal sex ratio in births has intrigued scientists since their first serious census in Europe in the 1700's. Is it Divine providence – such that "every Male may have its Female, and of a proportionable Age"? What are the mechanisms that determine sexual fate of the embryo? – a vast amount of research has been directed at this, with very substantial advances in our knowledge.
But the mechanisms of thermosensitivity of sex determination, widespread in vertebrates, have eluded us for the 50 years since its discovery in an African dragon lizard. With the recent insights on a role for highly conserved epigenetic machinery conserved across vertebrate lineages, including humans, we feel we are on the cusp of some important discoveries. These will contribute to those advances being made in other complementary labs working on fish in Spain and turtles in the US. This is what gets us up in the mornings!”
You can read plain English blog posts about each of the team’s research papers on Arthur Georges blog.
By Paula Doyle and Andrea Wild