Scientists discovered on November 12 that male and female split-fan kelp can reproduce asexually without fertilization when potential mates are scarce, revealing unexpected reproductive flexibility in brown algae during laboratory experiments published in PLOS Biology.
Giant brown algae typically rely on a complex life cycle involving microscopic stages where females release eggs and males release sperm.
While unfertilized eggs developing into offspring was previously documented, researchers found that male reproductive cells can also develop independently into new individuals, according to a study led by marine biologists at the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany and the Max Planck Institute for Biology Tübingen.
How do kelp switch between sexual and asexual reproduction?
Laboratory trials showed that microscopic male and female Laminaria pallida grown in separate dishes produced offspring without fertilization. When researchers sequenced RNA from the samples, they observed that male gene expression underwent a significant shift during asexual development.
“Our findings highlight that kelp are even more flexible in their reproduction than previously appreciated,” said Daniel Liesner, a marine biologist at GEOMAR and lead researcher on the project who said that finding males capable of asexual reproduction was completely unexpected.
Researchers also tested whether the physical presence of the opposite sex influenced the process by growing male and female kelp in the same petri dish separated by a fine mesh. Both sexes reproduced asexually at much higher rates when isolated from compatible partners, while laboratory-grown kelp favoured sexual reproduction in the presence of opposite-sex members.
“This suggests that kelps have much greater flexibility in how they reproduce than previously appreciated, they can respond to the absence of a mate by increasing their capacity for asexual reproduction in both female and male sexes,” explained Susana Coelho.
Scientists suggest the algae may detect nearby partners through chemical signals or mating cues that diffuse through moving water. Olivier De Clerck, a seaweed biologist at Ghent University in Belgium, said that the ability of male reproductive cells to develop without fertilization is particularly striking because sperm cells are small and possess few resources to support development compared to eggs.
Why do wild kelp populations rarely show asexual reproduction?
Field surveys present a sharp contrast to laboratory results. When the team examined 523 wild Laminaria pallida sporophytes collected along the Atlantic coasts of South Africa and Namibia, they found no unambiguous genetic signatures or clear evidence of asexual reproduction occurring in those natural populations.
“This disconnect between lab and field results was one of the most puzzling aspects of our study,” Coelho reflected, while noting that the physical presence of potential mating partners actively suppresses asexual development.
Reina Veenhof, an ecologist at the Scottish Association for Marine Science in Oban, UK, pointed out that the study helps explain how changes in gene activity allow kelp to reproduce in different ways, though researchers emphasize it would be premature to describe the process as a proven survival strategy in natural kelp forests.
What role does this flexibility play in marine ecosystems?
Kelp forests provide vital food and shelter for marine life along rocky coastlines, facing continuous pressures from changing ocean temperatures, grazing, and habitat fragmentation. The study’s authors note that while asexual reproduction is less vigorous, the mechanism provides a reproductive backup and a conditional alternative to sex that facilitates persistence when mating is not possible or sexual reproduction faces constraints.
Retaining sexual pathways simultaneously allows the kelp to secure the long-term benefits of sex whenever mates become accessible. For small or isolated populations, this life-cycle plasticity and reproductive flexibility could offer just enough advantage to keep the next generation going until a suitable partner arrives.
“Our findings show that kelps have much greater flexibility in how they reproduce than previously appreciated. Remarkably, even male gametes can develop into new individuals without fertilisation, providing a potential reproductive backup when mating partners are scarce. This flexibility may help kelp populations persist under challenging environmental conditions,” the researchers said in the PLOS Biology study titled “Life-cycle plasticity enables conditional asexual reproduction in a kelp.”
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