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Sex as a formality: Study shows male stick insects have lost their reproductive function

While most animals reproduce sexually, some species rely solely on females for parthenogenetic reproduction. Even in these species, rare males occasionally appear. Whether these males retain reproductive functions is a key question in understanding the evolution of reproductive strategies.

A new study in Ecology by a research team led by Assistant Professor Tomonari Nozaki from the National Institute for Basic Biology, Professor Kenji Suetsugu from Kobe University, and Associate Professor Shingo Kaneko from Fukushima University provides insight into this question. The researchers focused on the rare males of Ramulus mikado, a stick insect species in Japan, where parthenogenesis is predominant. Their analysis of male reproductive behavior reveals new findings.

Males engage in mating but do not contribute genetically

The rare males actively mated with females, just like typical males. However, genetic analysis confirmed that no male-derived genes were passed on to their offspring. The study further revealed that these males do not produce functional sperm, rendering them incapable of restoring sexual reproduction in the species.

Their findings showed that the males exhibited the typical morphological traits of stick insects and engaged in mating behaviors with conspecific females. However, despite their reproductive attempts, they were completely sterile. At the same time, the female reproductive organs associated with sexual reproduction showed signs of degeneration.

The challenge of finding rare males

One of the biggest hurdles in this research was obtaining specimens.

"In fact, we have never personally encountered a male R. mikado in the wild, despite extensive field collections of stick insects across Japan," said Dr. Suetsugu. "To overcome this, our research team collaborated with museums and citizen science groups, successfully collecting seven males over four years."

The team then recorded their mating behavior and conducted morphological and histological observations.

Evolutionary implications of lost male reproductive function

"Surprisingly, the rare males of this stick insect have completely lost their reproductive function," remarked Dr. Kaneko.

"Unlike female sexual traits, the loss of male sexual traits is generally thought to take an extremely long time. In many other species, even rare males often retain their reproductive capabilities. Our findings suggest that R. mikado has relied solely on parthenogenesis for such an extended period that even neutral mutations have accumulated, leading to the complete loss of male reproductive traits."

Dr. Nozaki added, "This study demonstrates that parthenogenesis in R. mikado has become irreversible. Although asexual reproduction is often considered evolutionarily short-lived due to the lack of genetic recombination, our previous research estimated that this species has persisted for hundreds of thousands of years. How has R. mikado managed to survive for such a long time? This remains an intriguing mystery for future research."