快猫短视频

If female flies choose to lay their eggs in an unusual location there can聽be surprising knock-on effects聽for the shape of male fly genitals.

Spotted-wing drosophila (Drosophila suzukii), originally from Asia but now found in Europe and North America, are a major pest. While most fruit flies lay their eggs on decaying fruits, females of this species deposit theirs inside unblemished fruits including strawberries, grapes and plums. To pierce the tougher skin of ripening fruit, the females have evolved a modified version of the egg-laying organ 鈥� the 鈥渙vipositor鈥� 鈥� which is longer than those of related species and serrated like a saw.

But the females鈥� large ovipositors are a bit of an obstacle for males during sex. Aya Takahashi and colleagues at Tokyo Metropolitan University, Japan, investigated by examining the mating positions and genital anatomy of 顿.听蝉耻锄耻办颈颈聽and a closely related sister species, 顿.听蝉耻产辫耻濒肠丑谤别濒濒补. They encouraged pairs to copulate and then flash-froze them with dry ice.

Male fruit flies generally use various sharply pointed genital structures to hold on to their partners during mating. But because of the length of the ovipositor in spotted-wing drosophila, the males鈥� genitalia can only grasp its tip.

Genital mismatch

So males have had to develop other ways to stabilise themselves while mating, probably using other genital structures. Takahashi and colleagues found that they have longer, thicker bristles on their anal plates, which come into contact with the ovipositor during copulation.

Because of their mismatched genitals, females of D. subpulchrella have great difficulty if they try to mate with males of D. suzukii. 鈥淐opulation of this combination occurs occasionally in captivity, but the pairs tumble around for the whole period, indicating an apparent incompatibility,鈥� says Takahashi.

How variants of one species evolve to become two separate species that don鈥檛 interbreed is a key question in evolutionary biology. Takahashi says the evolution of different genitalia in these flies, resulting from one group becoming specialised to lay eggs on intact fruit, shows an example of how this might happen.

She thinks there are likely to be many similar stories in other insect species, since ovipositors are subject to natural selection related to various ecological factors, but are also usually involved in copulation.

A co-author of the study, Yoshitaka Kamimura, won an Ig Nobel Prize in 2017 for discovering insects in which the females have male-like genitalia and the males have female-like genitalia.