For humans, sex is a simple matter of chromosomes: two Xs and we become female; one X and a Y and we develop into males. But things aren’t so straightforward for many lizards – many studies have found that the temperature of the nest also has a say, even overriding the influence of the chromosomes. But the full story of how the lizard got its sex is even more complicated. For at least one species, the size of its egg also plays a role, with larger eggs producing females, and smaller ones yielding males.
The discovery comes from Richard Shine’s group at the University of Sydney. In earlier work, they showed that if the Eastern three-lined skink (Bassiana duperreyi) incubates its eggs at low nest temperatures, XX carriers develop into males regardless of their chromosomes.
Now, Rajkumar Radder, a former member of Shine’s team, has shown that the amount of yolk also determines the sex of a skink, but only at low temperatures. By deliberately adding and removing yolk from eggs using a syringe, he managed to alter the sex of the hatchlings. This degree of complexity is totally unprecedented – it means that for the skink, sex is a question of its chromosomes, the temperature it was reared under and the amount of yolk it had.
Earlier this year, I wrote about how the human obsession with size is reshaping the bodies of other species at an incredible pace. Unlike natural predators that cull the sick, weak and unfit, human fishermen prize the biggest catches and throw the smallest ones back in.
As a result, fish and other species harvested by humans are shrinking, often within a few generations, and are becoming sexually mature at an earlier stage. These changes are bad news for populations as a whole, for smaller individuals often have lower odds of survival and produce fewer offspring.
But David Conover from Stony Brook University has found a silver lining in this tale – selectively harvesting fish can lead to dramatic changes in body size, but these changes are reversible. Release them from the pressure of constant hunting, and some of the animals start to rebound to their previous state.
Conover spent ten years raising a commonly harvested species called the Atlantic silverside in six captive populations, each containing about 100 individuals. Every year, the fish produced a new generation and for five years, Conover would remove 90% of the fish, either by taking the largest ones, the smallest ones or randomly selected individuals. In every other way, the fish were all reared under exactly the same conditions. This constant upbringing ensured that any changes to their bodies would be the result of genetic influences rather than environmental ones.