S u m m a r y :
High temperatures cause embryos of bearded dragon lizards to transform into females, and now we know why! The new findings explaining the sex swap are published in the journal Science Advances.
Heat changes male lizards into female!
“It’s so hot, let me change sex,” says the bearded dragon! As a matter of fact, it would not be the first reptile whose sex is ‘determined’ by temperature. While scientists have observed this phenomenon time and again, they have not been able to decipher the ‘how’ behind the sex switch. The new findings now show that it might just be the result of temperature-generated changes in the synthesis of specific proteins.
Sex change is not something new to the cold-blooded world. Several species of both reptiles (like crocodiles and marine turtle) and fish don’t even have sex chromosomes which allows them to turn into whichever sex that is adequate as per the temperature; sex is decided by the temperature of egg incubation. Bearded dragon lizards are even more unusual, though, because they do have sex hormones, and so their sex determination is subject to not only chromosome combinations but also to temperature, explains study author Clare Holleley from the University of Canberra.
“They have two sex chromosomes, but they also have this temperature override,” Holleley says.
Start off as male, and then hatch as female
Holleley and her team have been working on the bearded dragon for years now. Last year, they explained that bearded dragon embryos with male chromosomes would change sex to hatch as females when the temperature exceeded 32°. Below this temperature, embryos with two Z chromosomes turn into males while those with a Z and a W develop into females; however, higher temperatures cause the male ZZ dragons to become females instead. The team has finally understood how this is done.
Sex reversed by Jumonji genes
Holleley and her colleagues have focused on comparing two types of female bearded dragons: those that are females because of their genes, and those that are female because of temperature. The aim was to find genetic differences between the two. They, thus, collected the RNA of normal females, normal males, and the sex-reversed female. Samples were taken from different parts of the organisms: the brain, reproductive organs, among others.
The results show that the sex-reversed females would switch on the activity of a range of genes, two of which are members of a group of genes called the Jumonji family. The latter are documented to exert influences on sex differentiation in other animals; for instance, a Jumonji gene plays a role in testes development in some male mammals.
Changed RNA in sex-reversed lizards
Another interesting finding is changed RNA in the lizards. RNA is the complementary of DNA, and it carries the information from the latter; it is used to translate the data encoded in the DNA to make proteins. Normally, during this process, certain pieces of the RNA will be edited (removed). However, in the sex-reversed lizards, one of these RNA sections in the two Jumonji genes was not removed. Upon investigating further, the researchers found that these RNA pieces carried chemical codes that functioned as stop signs: when the translation apparatus would reach these sections, the whole process of RNA translation from the two genes would be halted.
Heat affects proteins
Does this mean that the different RNA leads to no protein being made or to the synthesis of other modified proteins in dragons incubated at high temperatures? A potential answer might lie in the fact that proteins coded in Jumonji genes control other genes that are behind developmental processes—genes which are turned on and off according to environmental conditions like heat. So, apparently, high temperature affects the proteins made by the two bearded dragon Jumonji genes, with the latter, then, interacting with proteins of sex-related genes. This question remains unanswered though, building up for new research, according to Holleley.