Fruit flies are most resourceful creatures. No matter how hot or arid, they seem to have adapted to most niches. But environments that suit one long-established population may not suit another. Dominique Joly explains that male fruit flies from temperate climes become sterile at temperatures where their tropical cousins are perfectly virile. Intrigued by this little-studied phenomenon, Joly and her colleagues were curious to discover the genetic basis of the tropical flies' fecundity, and found that the insect's insignificant Y chromosome makes all the difference to its fertility at high temperatures(p. 2735).
But first they needed to know if the temperate insect's fertility problems were really down to the male; after all, there could be many reasons for the flies to fail to produce offspring, other than male sterility. Having collected two temperate populations of Drosophila melanogaster from towns in northern France, Jean David and Brigitte Moreteau also gathered tropically adapted insects from Kenya and India, before returning to France to rear the insects at 30°C. After allowing the flies to mate, Celine Rohmer waited to see what colour eggs the hot parents produced: brown eggs are fertilized while white eggs are unfertilised. Sure enough, the tropical males had no problems fertilizing the females at 30°C. But while the French flies failed at the sultry temperature, they succeeded at cooler temperatures. And when she took a closer look at the heated temperate flies' sperm, it became clear why they were infertile. The sperm heads were stunted in comparison with temperate fertile males from cooler incubators.
So what had the tropical males got that the temperate males lacked? The team suspected that the male's Y chromosome lay at the root of the temperate insect's problem, even though it only contains 15 genes. Knowing that she needed to isolate the chromosome to discover whether it was the culprit, Joly decided to replace the temperate insect's Y chromosome with a tropical Y chromosome. Then Joly and her colleagues looked to see whether the offspring took after the tropical genes from their Y chromosome or temperate genes inherited from their mother. Sure enough, the insects became fertile at temperatures where temperate males became sterile. The Y chromosome seemed to contain most of the genes that protect tropical males from their temperate cousin's fate.
Curious to find out which genes protect the tropical flies from sterility,the team decided to monitor how temperate males reared at high temperature recovered their fertility at 21°C, to see which stage of spermatogenesis had been affected by the heat. The males' fertility returned within 8 days,significantly less time than spermatogenesis. The heat had not destroyed the temperate males' germ cells. It had somehow disrupted the sperm's maturation,while the tropical males' Y chromosome protected them from damage. And when the team took an in-depth look at the sperms' development during a hot spell,they realised that they had failed to elongate, and the DNA in the sperm's head was incorrectly packaged.
Joly suspects that dynein genes on the tropical fly's Y chromosome could endow the insect with high temperature fertility, but she hasn't ruled out the possibility that other Y chromosome genes confer protection too.