A species of freshwater algae collected from lakes in Japan has done something unusual: evolve to have three different sexes; and they all reproduce in pairs. All three sexes are male, female, and bisexual, which the researchers say is because the third sex can produce both male and female sex cells in a single genotype.
It is not a hermaphroditic organism because it has developed its sex cells under normal genetic expression, whereas a hermaphroditic individual that can produce male and female sex cells generally exists due to abnormal genetic expression.
The discovery is based on research that Hisayoshi Nozaki, an associate professor at the University of Tokyo, has been conducting for 30 years. During this time, he has traveled to the Sagami River collecting samples from the river and nearby man-made lakes.
Sexual peculiarity
Algae represent a large and diverse group of eukaryotic creatures that use photosynthesis for energy. This special algae is called Pleodorina starrii, and was previously thought to have two sexes that mated with each other, a mating system known as heterothalic. However, the researchers found evidence that P. starrii can be male, female, and of a third sex.
“Isolated colonies of one sex can be mixed with isolated colonies of another sex, and the sex ratio of their offspring can be useful clues for researchers to understand the genetics of sex determination ,” the authors explain.
Male colonies are recognized by the transparent packets of sperm they release into the water. The sperm packets swim to a female colony, then divide into individual sperm cells that enter the individual female cells and combine to produce a new generation. And the ‘bisexual factor’ gene is located on a separate chromosome in male and female sex cells.
It’s a great discovery considering how small these organisms are: they barely have 32 or 64 cells. Never before has a species of algae or fungi been found to have three sexes. The team describes it as a “new haploid mating system” entirely unique to algae.
And it is that, traditionally, algae can reproduce asexually (cloning themselves) or sexually (with a partner), depending on the stage of the life cycle in which they are. It can be haploid (with a single set of chromosomes) or diploid (with two sets).
But the researchers found that genetically bisexual P. starrii produced male and female sex colonies when isolated, and reproduced through cloning. Taking a closer look, they found that the bisexual factor gene could be passed on to the next generation, making it different from an isolated mutation.
According to scientists, although algae are quite different from humans, they could help us understand how different sexual systems, such as male and female, evolved.
Referencia: Three sex phenotypes in a haploid algal species give insights into the evolutionary transition to a self-compatible mating system Kohei Takahashi, Hiroko Kawai-Toyooka, Ryo Ootsuki, Takashi Hamaji, Yuki Tsuchikane, Hiroyuki Sekimoto, Tetsuya Higashiyama, Hisayoshi Nozaki First Published : 12 July 2021 EVOLUTION
