A Tasmanian lizard can switch its sex from female to male before birth, making it the first non-egg-laying animal to do so. Spotted snow skink (Carinascincus ocellatus) are sometimes born anatomically male while remaining genetically female, new research has found.
What is the reason for this switch, you might ask? The answer, according to the authors of the paper, published today in Proceedings of the Royal Society B, is temperature.
And the little skinks are in good company when it comes to temperature-based sex determination. While in humans, sex relies on a pair of inherited chromosomes, in reptiles this isn’t always the case. For some, sex depends on the temperature at which eggs are incubated, and for others, it can be a bit of both. Take the central bearded dragon (Pogona vitticeps), for example. Normally, sex is determined genetically, but at temperatures above 32°C (90°F), genetic males become functional females.
This is known as “sex reversal” and happens when the genes that determine sex are also temperature sensitive. So far, it has only been documented in egg-laying animals: fish, amphibians, and reptiles. But that has just changed with the new discovery in spotted snow skink.
C. ocellatus are native to Tasmania and give birth to live young. In nature, at low elevations, the sex ratio of their populations is known to be affected by temperature – there are more females in warm conditions and more males in cooler conditions. In the new study, researchers from the University of Tasmania set out to determine whether these differences in sex ratio are determined by temperature-induced sex reversal.
They trapped 100 newly pregnant females from varying altitudes and kept them at different temperatures in the lab. The skinks were then split into five groups of 20. In one experiment, two groups were allowed access to a heat lamp for either four or 10 hours a day, exposing them to a range of temperatures between 10°C (50°F) when the lamp was off and 20-37°C (68-99°F) when it was on. In another experiment, the remaining groups were kept at constant temperatures of either 33°C, 29.5°C, or 26°C (91°F, 85°F, or 79°F) during the day and 10°C (50°F) overnight.
When the 423 baby skinks were born, each had DNA from their tail sequenced to determine their genetic sex and had their sex organs examined to determine their anatomic sex.
All anatomically female skinks had two X chromosomes, meaning they were all genetically female as well. There had been no male-to-female sex reversal. But the same cannot be said for the anatomically male skinks. Thirty-one, or 7 percent, of the newborns had male sex organs and female (XX) chromosomes.
These “XX males”, as they are deemed in the study, were found in both experiments and were more common in females from lower latitudes and when terrarium temperature was restricted – ie when the heat lamp was turned on for just four hours or when the temperature was controlled at 26°C (79°F).
“Sex reversals were also identified in our high-elevation population,” the authors write. “Sex reversal exhibited strong temperature sensitivity in our high-elevation population consistent with the patterns we observed in the low-elevation population.”
“This provides strong evidence that sex reversals are a key mechanism contributing to temperature-sensitive sex determination in this species.”
It could also explain the female-skewed sex bias observed in warm conditions in C. ocellatus populations. If XX males were to mate with XX females, all offspring would be genetically female as the XX males lack a Y chromosome, leading to potentially lots more females in the second generation, Benjamin Geffroy at the French Institute for the Exploitation of the Sea in Montpellier explained to New Scientist.
[H/T: New Scientist]