Who needs sex? Kelp reproduce asexually when no mate is nearby

According to Nature, researchers published a study on 12 November in PLoS Biology detailing how some types of seaweed can take reproduction into their own hands when potential mates are scarce. Max-Planck-Gesellschaft reported that the study was conducted by researchers from the Department of Algal Development and Evolution at the Max Planck Institute for Biology Tübingen, focusing on Laminaria pallida, a kelp with a complex life cycle found along the Atlantic coasts of South Africa and Namibia. Kelps form towering underwater forests that support countless marine species. Their life cycle includes a microscopic stage consisting of tiny filaments that produce eggs or sperm cells, where sex normally takes place. It has long been known that unfertilized eggs can develop into new kelp without fertilization through a process called parthenogenesis. However, while studying cultures of Laminaria pallida, researchers discovered that male reproductive cells can also develop into new kelp without fertilization, revealing an unexpected route to asexual reproduction. Daniel Liesner, lead researcher on the project and marine biologist at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany, stated, “Finding that males can reproduce asexually was completely unexpected.” Liesner added, “We found that males can produce smaller, egg-like gametes which soon initiate asexual cell divisions, showing that parthenogenesis is not only a female trait.” Olivier De Clerck, a seaweed biologist at Ghent University in Belgium, noted that the ability of male reproductive cells to develop without fertilization is striking because sperm cells are small and have few resources to support development. However, Daniel Liesner also pointed out that compared with kelp produced through sexual reproduction, those developing without fertilization grow more slowly and show developmental abnormalities, which may reduce their chances of survival. Researchers analyzed gene activity and found that during asexual reproduction, gene expression in males changed dramatically, making their overall pattern of gene activity increasingly resemble that of females. Susana Coelho, Director of the Department of Algal Development and Evolution, stated that male and female cells may use similar developmental programmes to produce a new generation without fertilization. When researchers travelled to Namibia and South Africa to conduct a population genetic study, they tested more than 500 individuals in the wild. Nature reported that 523 individual kelp showed no clear evidence of asexual reproduction, and Max-Planck-Gesellschaft noted that among the more than 500 tested individuals, they identified no clear signature of clonality in the wild. Back in Tübingen, however, more than 90 per cent of the gametophyte isolates collected were able to reproduce asexually. Susana Coelho called this disconnect between lab and field results one of the most puzzling aspects of the study, asking why the trait is so widely maintained within populations if no evidence of it is found in the wild. To explore this paradox, researchers designed an experiment exposing gametophytes of one sex to a medium including the other sex while preventing gamete contact. The results showed that both male and female kelp produced more offspring in the absence than in the presence of the other sex. Susana Coelho explained that this suggests kelps have much greater flexibility in how they reproduce than previously appreciated, responding to the absence of a mate by increasing their capacity for asexual reproduction in both sexes. Nature reported that when lab-grown in the presence of a member of the opposite sex, kelp favoured sexual reproduction, suggesting they can detect nearby partners through chemical signals. Reina Veenhof, an ecologist at the Scottish Association for Marine Science in Oban, UK, noted that the study helps to explain how changes in gene activity allow kelp to reproduce in different ways. Ultimately, sex remains the preferred option producing more vigorous offspring, but for small or isolated populations, asexual reproductive flexibility provides a backup plan to keep the next generation going until a suitable partner arrives.