Some male insects have lost the ability to reproduce but still have sex

While sexual reproduction is the predominant strategy among animals, some species have evolved the ability to reproduce asexually through parthenogenesis. In these species, females can produce offspring without mating. 

However, even in predominantly asexual species, rare males occasionally appear. What remains unclear is whether these males retain reproductive functions or if they are merely evolutionary remnants. 

Understanding this question could provide crucial insights into the evolution and persistence of reproductive strategies in nature.

A new study, published in the journal Ecology, addresses this mystery by examining the rare males of Ramulus mikado, a species of stick insect in Japan where parthenogenetic reproduction is the norm. 

The research was 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.

The results provide compelling evidence that these males, despite their mating attempts, have completely lost their reproductive function.

Males with no reproductive function

The researchers closely studied the behavior and physiology of these rare males and found that, although they actively engaged in mating with females, they did not contribute genetically to the next generation. 

Genetic analysis confirmed that no male-derived genes were passed on to the offspring, indicating that these males are effectively sterile.

The findings revealed that, although the males display typical stick insect morphology and mating behaviors, they do not produce functional sperm. This means that sexual reproduction cannot be restored in the species, even in the presence of males. 

Interestingly, female reproductive organs associated with sexual reproduction showed signs of degeneration, further suggesting that the species has been evolving without sexual reproduction for a long time.

Finding rare males to study

One of the major hurdles in conducting this study was obtaining specimens of these rare males.

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

The collected males were then observed in controlled conditions, where their mating behavior was recorded. Additionally, researchers conducted detailed morphological and histological analyses to examine their reproductive anatomy.

Loss of reproductive traits in males

The study raises important questions about the long-term evolutionary trajectory of asexual species.

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

“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.”

This suggests that once a species becomes fully dependent on parthenogenesis, the re-emergence of sexual reproduction may become impossible. Over time, genetic changes accumulate that further reinforce the asexual mode of reproduction, making the reversal to sexual reproduction unlikely.

“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,” said Nozaki.

“How has R. mikado managed to survive for such a long time? This remains an intriguing mystery for future research.”

Long-term viability of asexual reproduction

Biologists have long debated the evolutionary fate of asexual organisms. Many studies suggest that asexual reproduction, while advantageous in the short term, can be an evolutionary dead end due to the absence of genetic recombination. 

Without recombination, harmful mutations can accumulate, and the population may struggle to adapt to environmental changes or resist emerging pathogens.

Yet, the persistence of R. mikado challenges this assumption. If this stick insect species has survived for hundreds of thousands of years without sexual reproduction, it raises the question of what mechanisms allow it to maintain genetic diversity and adaptability. 

One possibility is rare genetic mutations that provide variation even in the absence of recombination. Another explanation could be environmental stability, where selective pressures have remained relatively constant, reducing the need for genetic diversity. 

There is also the potential for cryptic mechanisms of genetic exchange, such as occasional hybridization events with related species – though no evidence of this has been found in R. mikado.

These possibilities open new avenues for research into the sustainability of asexual reproduction in nature.

Future research directions

The study on R. mikado is part of a broader effort to understand how parthenogenetic species evolve and persist. Future research may focus on identifying the genetic changes responsible for male sterility and exploring whether other parthenogenetic stick insect species have undergone similar irreversible transitions.

Additionally, scientists are interested in investigating how R. mikado avoids the pitfalls associated with asexual reproduction, such as genetic stagnation and vulnerability to environmental shifts. Comparative studies with other stick insect species, both sexual and asexual, could provide valuable insights into these evolutionary dynamics.

By continuing to study rare cases like R. mikado, researchers hope to unravel the complexities of reproductive evolution and better understand the forces shaping biodiversity in nature.

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