Sex determination of lizards depends on temperature

The ambient temperature determines the sex of the lizard species Niveoscincus ocellatus (spotted skink) in low-lying regions. In more elevated areas, genetic factors determine sex. This is the conclusion of researchers headed by the Groningen professor Ido Pen. The results of the research were published by Nature online on 27 October.

The researchers analysed field data from a type of viviparous lizard, Niveoscincus ocellatus or spotted skink, found on the Australian island of Tasmania, for sex ratios and reproductive success.  An embryo of the skink can grow into either a male or a female. The researchers compared a coastal population with a population at 900 m above sea level. Although the lizards all belong to the same species, their methods of sex determination differed. Temperature turned out to be the determining factor in the coastal population, whereas for the population at a higher elevation genetic elements determined sex.

Females

From an evolutionary point of view, it is beneficial for lizards to have temperature-dependent sex determination, according to Pen. This is because temperature partly predicts the reproductive success of a species because heat has a positive effect on the youngsters’ growth.  Females in particular realise greater reproductive success with higher temperatures, as they are the ones who produce the young. Pen: ‘If their first year is favourable, they can start reproducing at an earlier age. On an evolutionary scale, a species that has temperature-dependent sex determination can profit from this: females are born in warm periods, males during colder ones.’

Mountains

Nevertheless, the mountain population appears to have evolved genetic sex determination and not the favourable temperature-dependent system. Pen explains this by pointing out that in the mountains extremely warm years alternate with extremely cold years: ‘The differences in temperature are too great. In a warm year you’d get only females and in a cold year only males.  That’s not favourable for reproduction either. That’s why we find genetic sex determination in the mountains.’  Along the coast there is a mild climate with relatively small fluctuations in temperature, resulting in a good ratio between males and females.   

Model

What is remarkable is that one single species has two different mechanisms for sex determination. Pen used a mathematical model to analyse the field observations in order to predict which sex determination system would evolve. ‘There are in fact two opposing forces. On the one hand it is favourable to have temperature-dependent sex determination. On the other hand, fluctuations between years work against that system. We have succeeded in developing a model that can use field data to precisely predict which of the two systems will evolve.’   

Experiments

The results from the field data were also tested by the researchers in a controlled environment. To this end they caught pregnant female lizards from both the coastal and the mountain populations and then exposed them to a warm lamp for four or ten hours respectively.  These experiments confirmed the existence of a genetic and a temperature-dependent sex determination system. After four hours of heat treatment, the coastal lizards gave birth to virtually only males, after ten hours mainly females. However, heat treatment had no effect on the lizards from the mountain population: the sex ratio of the young was fifty-fifty after both four and ten hours of heat treatment.  

Sex determination

The question of whether an embryo will develop into a male or a female has interested biologists for decades. In mammals, such as humans, the sex chromosomes X and Y determine sex. These chromosomes are very different to each other; the Y chromosome is strongly degenerated. In reptiles, the sex chromosomes are virtually identical. According to Pen, this is an indication that such transitions occur more often between different sex determination systems in reptiles: ‘With warm-blooded animals it is not possible to have temperature-determined selection because the embryos grow in a constant temperature. These temperature differences are much greater with cold-blooded animals and so selection can be based on them.’