TORONTO — On the quest to find more about how living things evolved different sexes, researchers in Japan came across something interesting: a type of freshwater algae that has three sexes, all of which can breed with each other.
The discovery builds on research that Hisayoshi Nozaki, an associate professor at the University of Tokyo, has been conducting for 30 years. That’s how long he has been travelling to the Sagami River and collecting samples from the river and artifical lakes along it.
Researchers came to the conclusion described in a research paper published in the journal Evolution on Monday by studying samples Nozaki collected in 2007 and 2013.
The algae is called Pleodorina starrii algae (P. starrii), and had previously been thought to have two sexes which mated with each other — a mating system called heterothallic.
However, researchers found evidence that P. starrii can be male, female and a third sex, which researchers are calling ‘bisexual’ due to the fact that this third sex has both male and female cells.
It’s a big discovery considering how small these organisms are: this type of algae are 32 or 64 celled organisms. A species of algae or fungi has never been found to have three sexes before.
“It seems very uncommon to find a species with three sexes, but in natural conditions, I think it may not be so rare,” Nozaki said in a press release.
Researchers separated the algae into colonies made up of just male or female P. starrii. When isolated and deprived of resources, the colonies reproduced asexually, meaning they could reproduce through cloning of themselves.
“Isolated colonies of one sex can be mixed with isolated colonies of another sex and the ratio of their offsprings’ sexes can be useful clues for researchers to understand the genetics of sex determination,” the release explained.
Male colonies are recognizable by the clouds of sperm they release into the water, which then swim along until they encounter the larger female colonies in order to start a new generation of algae.
The male version of the algae was discovered in 2006 by the team, and researchers named the male gene ‘otokogi,’ a Japanese word that means “manly,” according to the release. In 2010, a female gene was discovered and named ‘hibotan,’ which means ‘scarlet peony,’ in honour of a character from a 1960s series of films about a sword-wielding woman with a red peony tattooed on her shoulder.
Researchers discovered the third sex by performing numerous mating trials until they identified a “bisexual-factor” gene. The genetically bisexual P. starrii produced both male and female sexual colonies when isolated and made to reproduce through cloning. When researchers looked closer, they found that the bisexual-factor gene could be transmitted to the next generation, making this different than an isolated mutation.
Interestingly, P. starrii that are genetically bisexual also have otokogi genes along with bisexual-factor genes, and can produce viable male or female colonies by mating with any other P. starrii colonies. This bisexual-factor gene is thought to be located on a different chromosome than the otokogi and hibotan genes.
Bisexual-factor genes have been found in genetically female P. starrii as well, but researchers believe the gene is only “active” when paired with otokogi genes. Genetically male P. starrii only have otokogi genes.
Although algae are quite different than humans, this type of long-term research allows us to get closer to understanding the ways that evolution develops different sexes.
“This finding was possible because of our very long-term experience of going on field collection trips and our practice growing and studying algae,” Nozaki said. “Continued, long-term studies are very important to unveil the true nature of species in the natural world.”