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Pleodorina starrii: Scientists Discover a Third Sex in Tiny Algae

Pleodorina starri. algae with three sexes

Scientists from the University of Tokyo have discovered that Pleodorina starrii, a species of freshwater algae, has three sexes. 

It may be the first case discovered for algae or fungi. Previously, scientists have observed many plants and some invertebrates with three sexes due to normal gene expression.

According to University of Tokyo Associate Professor Hisayoshi Nozaki, cases like this may not be rare.

“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. 

Nozaki has been studying the algae for 30 years, collecting samples in manmade lakes along the Sagami River.

According to EurekAlert!:

“Nozaki and his colleagues are interested in Pleodorina starrii because it and its close evolutionary cousins use different sex systems, so they are useful models to study the genetics of how sex evolved. P. starrii had been identified decades prior, but had not been studied in detail.”

Male, Female, and Bisexual Pleodorina starrii

Pleodorina starrii is a 32- or 64-celled organism with three sexes: male, female, and what they called ‘bisexual.’ The bisexual sex can produce female and male sex cells in a single genotype. Notably, they exist due to a normal expression of their species genes. Thus, it’s different than with hermaphrodites, which result from an unusual gene expression.

Previously, scientists thought the algae reproduced with a “heterothallic” system of two sexes.

Moreover, the analysis showed a “bisexual-factor” gene located on a chromosome separate from the male or female sex genes. 

According to the researchers, all three sexes can breed in pairs with each other. When bisexual algae interbreed, they produce viable male or female colonies.

According to CTV News:

“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.”

Reproducing Under Stress

When kept in isolation, the algae tends to grow in spherical colonies of the same sex. Generally, female colonies tend to be larger.

Then, they reproduce asexually, creating clones. 

However, when the scientists applied pressure, the algae began behaving differently. They deprived the colonies of nutrients and saw them begin reproducing sexually instead. Males release sperm that swim to female colonies, beginning new generations.

In Pleodorina starrii, the male sex cells are small and can move faster than larger female sex cells.

Image via CTV News: “Left: Sexually induced male colony of algae. Center: Pleodorina starrii female colony with male sperm packet (arrowhead). Right: Pleodorina starrii female colony with dissociated male gametes (arrowheads).”

The ‘Scarlet Peony’ and the ‘Manly’ Gene

From 2007-2013, the researchers collected Pleodorina starrii in dam lakes along rivers, Lake Sagami and Lake Tsukui. 

First, they found a male gene and named it Otokogi, or “manly.” 

From there, it took until 2010 to identify female genes. 

This time, they chose the name Hibotan or “scarlet peony” after a 1960s action movie, which has a sword-wielding woman, the “Red Peony Gambler” with a red peony tattoo on her shoulder.

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Pleodorina starrii and the Evolution of the First Sexes

In genetically female P. starrii, they may have only the Hibotan gene or a combination of Hibotan and bisexual-factor genes. 

Meanwhile, genetically male P. starrii only have the Otokogi gene.

In genetically bisexual P. starrii, both Otokogi and bisexual factor genes are found. 

Now, the scientists speculate that the bisexual factor “may only be active in the presence” of the Otokogi male gene. However, more research is required to confirm if this is true.

“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 said.

For whatever reason, the scientists apparently didn’t give a catchy name to the bisexual sex. 

Name choices aside; through their research, they hope to understand how the first sexes evolved and the genetics of sex determination.

“Co-existence of three sex phenotypes in a single biological species may not be an unusual phenomenon in wild populations,” the study states. “The continued field-collection studies may reveal further existence of three sex phenotypes in other volvocine species.”

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Featured image: Screenshot via YouTube

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