High temperatures can create “superfemales” lizards

In animals like turtles, lizards and other reptiles, individual sex is influenced, or even determined, by external variants, mainly temperature. As Earth is going through climate changes, population dynamics of these species might be subject to dramatic effects.

Central bearded dragons (Pogona vitticeps) are large lizards found in Eastern Australia, whose sex is determined by their chromosomes and environment’s temperature. Embryos of these species, that are genetically determined as males, can turn into females when eggs are incubated at high temperatures. In this case, the individual has chromosomal characteristics that are typical of a male, but they develop into what scientists are calling “superfemales”. The name might make you imagine some new superheroin, but what are these “superfemales” in reality and why does it matter?

In most species, sex differences in morphology, physiology and behavior are caused by sex-linked genes. When sex determination is so greatly influenced by the environment, it can lead to the emergence of organisms that exhibits a mixture of male-like and female-like traits. A group of researchers from Australia have studied 20 sex-reversed females, 55 males and 40 regular females of beardead-dragons in captivity. They observed that sex-reversed females retain some of their masculine attributes, such as long tails, high body temperature and a peculiar behavior: superfemales are more active and remarkably bolder than “normal” females. In nature, this behavioral characteristic might make them able to better explore their surroundings and enhance feeding rates. Then, in better conditions than regular females, superfemales could also have greater reproductive activity and possibly lay eggs in greater number.

Unlike humans, in central bearded dragons, as well as many other reptiles, chromosomal females are the heterozygous sex (ZW) while males are homozygous (ZZ). Sex reversed females, therefore, are homozygous females that maintain male chromosomes and many male-like personality characteristics, at the same time that they are females in terms of reproductive biology.

superfemalefig1

Figure 1. Result of an experiment to assess Central Bearded Dragons (Pogona vitticeps) boldness. The figure represents the time taken for the lizard to emerge from a shelter and data are shown for three sexes (ZW females, ZZ males and ZZ sex- reversed females). Sex-reversed females take less time to emerge, indicating a bolder behavior.

This set of features, which represents “the best of both worlds”, are driving scientists worried about a possible extinction of regular females, especially in the scenario of climate changes. Along some generations, behavioral differences between ZW females and sex-reversed ZZ females could affect the relative frequency of these types of individuals within a population. If sex-reversed females’ bold behavior really confers them a higher fitness in nature, these individuals’ frequency would increase. This scenario could rapidly drive a local population from genetic to complete environmental sex determination, and to total loss of ZW females (and W chromosome). With global temperatures increasing over the years, scientists are afraid of a possible extinction of dragons’ populations.

superfemalefig2

Figure 2. Central Bearded Dragon. © Arthur Georges. In: http://www.sciencemag.org/news/2016/06/third-sex-lizards-could-outcompete-their-normal-female-cousins.

However, there’s no reason to panic yet. All study so far was conducted under controlled conditions in laboratory. Researchers highlight that this study revealed plasticity of traits once regarded as fixed, species-specific characteristics, indicating that sex in dragons is a “much more complicated matter than we have assumed”. Next step is to understand the bearded-dragons’ population dynamic in the wild – which could be a completely different story, considering all interactions between environmental and social variables which individuals are exposed to. Moreover, field studies on the ecology of these lizards could answer some very general questions in reproductive biology.

Bruna de Oliveira Cassettari

References:

Li H, Holleley CE, Elphick M, Georges A, Shine R. 2016 The behavioural consequences of sex reversal in dragons. Proc. R. Soc. B 283: 20160217. http://dx.doi.org/10.1098/rspb.2016.0217

Sex in dragons: a complicated affair. 8 June 2016. In: http://sydney.edu.au/news-opinion/news/2016/06/08/sex-in-dragons–a-complicated-affair.html

Figure 2. Central Bearded Dragon. © Arthur Georges. In: http://www.sciencemag.org/news/2016/06/third-sex-lizards-could-outcompete-their-normal-female-cousins

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