ABOVE: A gametophyte of the brown alga Desmarestia dudresnayi that has both male and female reproductive structures Max Planck Research Institute for Biology

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While many organisms fall into one of two sexes, others are co-sexual, a version of hermaphroditism that involves having both male and female reproductive organs. Multiple species of brown algae (class Phaeophyceae), for example, have evolved over time from a male/female binary into co-sexual hermaphrodites. Susana Coelho, a biologist at the Max Planck Research Institute for Biology Tübingen in Germany, says that while the ability to “auto-fertilize” results in reduced genetic diversity compared to other mating systems, it can also lessen the pressure to find partners in the vast ocean. 

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To understand how brown algae switched to hermaphroditism, Coelho and her team studied eight algal species: four that are co-sexual and four closely related species that have retained two sexes. The researchers used RNA-seq to compare gene expression in each species, finding that hermaphrodites shared more sex-biased genes—genes that are typically expressed differently between the sexes—with females than with males, indicating that females most likely made the transition to co-sexuality. “It was quite cool . . . to see that it was always the female [that] acquired the male function,” Coelho says. The team also found that, as species transitioned, each appeared to experience similar shifts in gene expression. Sixty-one percent of orthologous genes shared across all eight species—sex-biased or not—showed similar changes in patterns of expression in the co-sexual species compared with the binary species. 

This was the most interesting result, says Hisayoshi Nozaki, a biologist at the University of Tokyo who was not involved in the study, noting that this research is the first to examine the genetics of the transition to hermaphroditism in these algae. He adds that a “further, fruitful” step would be to do whole-genome sequencing to learn more about how the seaweeds’ sex chromosomes have changed. 

G.G. Cossard et al., “Selection drives convergent gene expression changes during transitions to co-sexuality in haploid sexual systems,” Nat Ecol Evol, 6:579–89, 2022.