Despite having sex with males, females of a Japanese stick insect species produce offspring that are genetically identical to themselves. Combined with other evidence, it suggests that the males of the species have become sterile.
The females of some stick insect species, including a type of Japanese stick insect (Ramulus mikado) have evolved to produce eggs that can turn into female embryos without requiring fertilisation by sperm. This process, called parthenogenesis, means males gradually become less common in the population.
Rare males of other parthenogenetic species are known to mate with females, giving rise to offspring that inherit DNA from both parents. But it is unclear whether the very rare R. mikado males are still able to reproduce with females.
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To find out more, at the National Institute for Basic Biology in Japan and his colleagues collected 15 female and four male R. mikado individuals from the wild. They placed the males in cages with females and verified that all males were able to mount and insert their penis into females.
Then, the researchers sequenced the DNA of eggs subsequently laid by females and compared this with the genetic sequences of the males and females that mated. All eggs were female clones of their mothers. When Nozaki and his colleagues analysed the testes of one of the males, they found that the sperm were highly defective and unlikely to be ejaculated into the females during mating.
The defective sperm probably result from the accumulation of genetic mutations that degrade the males鈥� ability to produce healthy sperm, a trait that became less important as the species transitioned to asexuality, the researchers write in their paper. It seems that the species has now lost its ability to sexually reproduce, they say.
The researchers only looked at the sperm from one male, so further analysis of more individuals is needed to confirm the findings, says at the University of Lausanne in Switzerland. But given that the male insects are incredibly rare, the small sample size is understandable, he says.
Where new males come from if males are sterile and females are cloning themselves to produce only females isn鈥檛 known for sure, but Brandt thinks the females sometimes produce males through parthenogenesis. 鈥淭he precise mechanism of how a female can produce a male via parthenogenesis in this case is probably not known,鈥� he says.
鈥淥rganisms that have lost a trait are a great opportunity to study how that trait is built during development,鈥� says at the University of Lausanne. For example, by comparing sterile R. mikado males with fertile males from similar species, researchers could identify the genetic factors causing sterility, says Lavanchy.
Such insights could also reveal new ways to control pests through introducing sterile individuals into populations, he says.
bioRxiv
