An international research team led by Max Planck scientists describes how different species of brown algae switched from being males and females to hermaphroditism. The hermaphrodites likely originated when females from the ancestral species acquired male characteristics. The results are now being published in the journal Nature Ecology & Evolution; this work on the brown algal model organisms represents a key step towards a better understanding of the molecular mechanisms behind transitions between sexual systems.
Humans are just like blackbirds, asparagus, or hemp: typically, they come as males and females. Other organisms present as hermaphrodites who have both male and female reproductive organs in the same individual: roses, for example, or earthworms. Yet there are also species who live in mixed populations of males, females, and hermaphrodites. While humans and many more determine sex with sex chromosomes, the sex of clownfish and other organisms is decided purely by their environment. In short, the diversity of sexual system across the tree of life is immense, and their evolution often involves transitions between different sexual systems.
Hermaphrodites look similar to females
An important, yet hitherto poorly understood transition is the switch from the male/female dichotomy to hermaphroditism. This is why an international team of researchers now studied how four species of brown algae reversed from a male/female system to hermaphroditism – independently of each other. In each of the four cases, the scientists compared two species that shared a common ancestor, one species representing the ancestral state with separate sexes and a hermaphroditic species.
“Even though these four events occurred completely independently, the mechanisms that drove them and the functions of genes involved appear to be similar,” says Susana Coelho, scientific director at the Max Planck Institute for Biology in Tübingen and lead author of the study. “Each time, it seems that a female individual acquired genes related to male development.” Indeed, the hermaphrodite algae resemble closely their female ancestors.
But the process is not as simple as merely acquiring new male genes. During the transition to hermaphroditism, many genes related to male development underwent rapid evolutionary changes. Despite the turnover of genes related to male development, the function of these genes proved very similar, even across only distantly related species. “This suggests that there are shared requirements for male versus female development,” concludes Coelho.
Dating and mating in the ocean can be tricky
The transition from one sexual system to another can have profound evolutionary and ecological consequences: “Self-fertilization of hermaphrodites is comparable to mating with your sibling – you lose genetic diversity.” Yet hermaphroditism does also have some advantages: “As far as we know, there are no pollinators in the ocean, and male and female gametes are released in the seawater” explains Coelho. “If a brown algae species has males and females, the reproductive cells of both sexes need to find each other on their own – and that’s not always easy in the huge ocean. It therefore seems much more practical for algae to be hermaphrodites who can fertilize themselves.”
Brown algae have been evolving independently from animals and plants for more than a billion years. They display a fascinating array of complex morphologies, types of life cycle, and reproductive modes and are increasingly being used as models to address developmental and evolutionary questions. “In our brown algae species, the transition between sexual systems happened rather fast and recently, and the evolutionary distances between the species are short,” states Coelho. “This makes brown algae the perfect model organisms for understanding the mechanisms behind such impactful shifts.”
Guillaume G. Cossard, Olivier Godfroy, Zofia Nehr, Corinne Cruaud, J. Mark Cock, Agnieszka Lipinska, Susana M. Coelho:
Selection drives convergent gene expression changes during transitions to co-sexuality in haploid sexual systems | Nature Ecology & Evolution on 21 March 2022. DOI 10.1038/s41559-022-01692-4
Prof. Dr. Susana Coelho
Director, Department of Algal Development and Evolution
Max Planck Institute for Biology Tübingen