Mystery Solved: Researchers Reveal How Male Bearded Dragons Grow Ovaries in Hot Weather

Stress-induced sex determination is just another wonder of the animal universe.

Native to the arid landscapes of Australia, the central bearded dragon (Pogona vitticeps) is a fascinating species. It has genetic sex determination, but when incubated at high temperatures, genetic males sex reverse and develop as females. (Whiteley SL et al., 2021, PLOS Genetics)

(CN) — If you can’t stand the heat, grow ovaries. This is true at least when it comes to bearded dragons.

As a stress response to heat, bearded dragons that had male chromosomes at conception grow ovaries and develop into females. Research published in PLOS Genetics on Thursday analyzes the underlying genetic process that kicks on in response to environmental stress.

“The most exciting component of this work is the discovery that the mechanism involves ubiquitous and highly conserved cellular processes, signaling pathways and epigenetic processes of chromatin modification,” said Sarah Whiteley, an author on the paper in a statement. Whiteley is a researcher at the Institute for Applied Ecology at the University of Canberra in Australia.

“This new knowledge is bringing us closer to understanding how temperature determines sex,” she added. “It is a very exciting time to be in biology.”

In many fish and reptiles, environmental favors influence sex determination. The central bearded dragon, Pogona vitticeps, exhibits sex assignment by both genetic determination and environmental influence. Females can develop from ZW chromosomes, or from ZZ chromosomes — which would otherwise produce males — under high temperatures.

An Australian team of researchers collected eggs during the 2017-2018 breeding season and incubated them for 10 days before splitting them up. ZZ bearded dragon eggs were heated to 96 degrees Fahrenheit to initiate the stress-induced sex swap, while a separate clutch of ZW females and ZZ males developed at 82 degrees.

Researchers then sequenced transcriptomes of gonads at each stage of development to compare the influence of different temperature conditions. An expression of messenger RNA, transcriptomes change depending on an organism’s stage of development and environment, offering researchers a glimpse at the road map steering the development of the lizard’s sex characteristics.

Researchers identified 700 unique genes in ZZ males, 500 unique genes in ZW females, and 998 shared genes between them. The secret to ovary development lies somewhere among these pieces.

“There are large gene expression differences between normal ZWf females and ZZf sex reversed females early in development, before the bipotential gonad differentiates into an ovary. These differences are most pronounced early in development and diminish as development progresses,” the study authors wrote in their paper.

Previous researchers put forth the CaRe hypothesis, supposing the whole Rube Goldberg machine starts when TRP channels in cell membrane sense temperature increases and instruct cells to produce more calcium ions, Ca2+. Meanwhile, mitochondria produce an excess of reactive oxygen species, or ROS. Together the increase in calcium ions and oxygen stress “trigger a suite of alterations in gene expression including chromatin remodeling, which drives sex reversal.”

By mapping out early active genes in ZW and ZZ females, the researchers verified that the processes begin differently in each, driven by different sets of genes, but the processes later converge during the production of ovaries. The newly identified genes, which will be key in subsequent research, include hormone receptors and early growth responders

A Discovery Grant from the Australian Research Council funded this research.

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