Scientists discover first known species of algae with three distinct sexes
Although we may consider ourselves far from green algae blobby, we’re not really that different.
An explosion of algae a few hundred million years ago this is believed to be what allowed all human and animal life to evolve, and all in all, only about a billion and a half years ago between us in terms of evolution.
What’s more, according to a team of Japanese researchers, algae may actually help us understand how different sexual systems – like male and female – evolved in the first place.
Researchers at the University of Tokyo and a number of other Japanese universities have found that a type of green algae called Pleodorina starrii has three distinct sexes – “male”, “female” and a third sex which the team called “bisexual”. This is the first time that a species of three-sex algae has been discovered.
“It seems very rare to find a species with three sexes, but under natural conditions I think it may not be that rare”, said one of the researchers, Tokyo University biologist Hisayoshi Nozaki.
Algae is not a very specific scientific classification. It’s an informal term for a huge collection of different eukaryotic creatures that use photosynthesis for energy. They are not plants, as they lack many plant characteristics; they are not bacteria (although cyanobacteria are sometimes called blue-green algae); and they are not mushrooms.
Because algae are such a large and diverse group, there is a lot of variation in how they feed on them, but generally algae are able to reproduce either asexually (by cloning) or sexually (with a partner), depending on the stage of the life cycle they are in. It can be either haploid (with a single set of chromosomes) or diploid (with two sets).
There are also hermaphroditic algae which can change depending on the expression of genes in the organism. Having three sexes, including hermaphrodites, is called ‘threesome‘.
But volvocin green algae P. starrii is still different from that. The bisexual form of this haploid alga has both male and female reproductive cells. The team describes it as a “novel haploid mating system” completely unique to algae.
P. starrii form 32 or 64 same-sex vegetative colonies and have small motile sex cells (male) and large immobile sex cells (female) similar to those in humans. Male sex cells are sent around the world in semen pouches to find a female colony to attach to.
Bisexual P. starrii have both, can form male or female colonies and can therefore mate with a male, female or other bisexual.
Above: Colony of sexually induced male algae (left). Female colony with male sperm packet (center). Female colony with dissociated male gametes (right).
The researchers are particularly excited because other closely related algae have different sex systems, which means the discovery could tell us more about how these sex changes took place.
“Mixed mating systems such as the trioecium may represent intermediate states of evolutionary transitions between dioecious (with male and female) and monoecious (with only hermaphrodites) mating systems in diploid organisms”, the team write in their new paper.
“However, haploid mating systems with three sex phenotypes within a single biological species have not been reported previously.”
For 30 years, Nozaki had collected algae samples from the Sagami River outside of Tokyo. Samples taken from lakes along this river in 2007 and 2013 were used by the team for the new discovery.
The team separated the algae colonies and tricked them into sexually reproducing by depriving them of nutrients, finding that bisexual algae had a “bisexual factor” gene separate from the male and female-specific genes previously discovered.
Bisexual cells also had the male gene, but can produce either male or female offspring.
“The coexistence of three sex phenotypes in a single biological species may not be an unusual phenomenon in wild populations”, the researchers conclude.
“Further field collection studies may reveal the existence of three sex phenotypes in other species of volvocin.”
The research was published in Evolution.