For three decades, Professor Hisayoshi Nozaki has collected algae samples from the Sagami River an hour outside of Tokyo, Japan. This work was part of a research project aimed at gaining a deeper understanding of sexes. Now, that research is finally paying off.
Samples first collected in 2007 and 2013 from a dam lake show a strange species of algae. What makes the algae species, Pleodorina starrii, interesting is that it comes in three sexes.
Individuals of each of the sexes breed in pairs. The third sex which scientists are calling ‘bisexual’ can produce both male and female sex cells.
This may sound like a case of hermaphroditism but it’s subtly different. Hermaphrodites are indeed organisms that produce both male and female sex cells, however this is due to abnormal gene expression.
In the case of P. starrii, the third sex is completely normal. A few other plants and some invertebrate animals have three sexes, although this is the first case of an algae with three sexes.
“It seems very uncommon to find a species with three sexes, but in natural conditions, I think it may not be so rare,” explained Professor Nozaki.
There are even ancient organisms with sexes referred to as ‘plus’ and ‘minus’ rather than male and female. Later in evolution, sexes evolved to be quite different from one another, such as in mammals the small sperm cells and large egg cells.
P. starrii is a good model organism for studying the evolution of sex because, it has a different reproductive strategy than its closest relatives, representing an evolutionary change.
To study their reproduction, colonies of different sexes are reared in the lab and forced to reproduce sexually. Males grow sperm packets which are released, and the sperm swim until they come into contact with a female colony.
The researchers discovered that this third sex in P. starrii has previously isolated both the male and female genes in the algae. It is their hope to continue research and make more discoveries about the nature of sex.
“This finding was possible because of our very long-term experience of going on field collection trips and our practice growing and studying algae,” said Professor Nozaki.
“Continued, long-term studies are very important to unveil the true nature of species in the natural world.”
The study is published in the journal Evolution.