For more than 30 years, Hisayoshi Nozaki had been studying algae samples from freshwater systems close to Tokyo. But even he, an associate professor at the University of Tokyo with extensive experience studying algae, was surprised to find a new species that evolved three different sexes, which can breed in pairs with each other. It’s the first time that such an alga has ever been found.

When two sexes are just not enough

The earliest lifeforms that evolved on Earth billions of years ago reproduced asexually — essentially cloning themselves. Later, some organisms evolved sexual reproduction, which requires two parents to each contribute a gamete (sex cells like sperm and eggs) in order to produce offspring with unique genetic characteristics.

Both modes of reproduction have their pros and cons, but how exactly this transition took place is not very well understood.

Some organisms are hermaphrodites, being equipped with both male and female reproductive organs due to very unusual gene expression. However, the new kind of green algae found by Nozaki, known as Pleodorina starrii, is no hermaphrodite. It has three distinct sexes: male, female, and a third sex that the Japanese researchers call bisexual in recognition of the fact that it can produce both male and female sex cells (but with normal expression of genes unlike hermaphrodites).

“It seems very uncommon to find a species with three sexes, but in natural conditions, I think it may not be so rare,” said Nozaki.

For Nozaki, the peculiar algae found in Lake Sagami and Lake Tsukui are of great interest. They may help scientists uncover how early primitive organisms evolved into individuals that sexually reproduce.

In normal conditions, P. starrii grow into spherical colonies composed of 32- or 64-celled organisms that have small mobile (male) and large immobile (female) sex cells. Male colonies are recognizable by the packets of sperm they release into the water. These sperm swim until they encounter a female colony, where they combine with the female cells to produce a new generation.

But during new experiments, the Japanese researchers separated P. starrii colonies into male and female and then deprived these isolated colonies of nutrients. In isolation the colonies reproduce asexually, forming clones with the same genotype. If they’re separated and deprived of nutrients at the same time, the colonies are forced to reproduce sexually.

The researchers found that some P. starrii individuals have bisexual-factor genes that produce normal male or female colonies when they reproduce sexually with other P. starrii colonies. Genetically male P. starrii have only the OTOKOGI male-type gene (a Japanese word meaning “manly”) and genetically female algae can have either only the female-type HIBOTAN genes or both HIBOTAN and the bisexual-factor genes. The researchers suspect that the bisexual factor may only be active in the presence of the “manly” OTOKOGI.

Although algae are very different from humans, this investigation may allow scientists to gain a better grasp of the ways that evolution shape different sexes we recognize as male or female in our own species.

“This finding was possible because of our very long-term experience of going on field collection trips and our practice growing and studying algae. Continued, long-term studies are very important to unveil the true nature of species in the natural world,” Nozaki commented.

The findings were published in the journal Evolution.