Pleodorina starrii

Pleodorina starrii is a species of algae that resides in freshwater in Japan. The name starri is in honor of phycologist Richard C. Starr.

Since 2006 it has provided molecular genetic evidence for an evolutionary link between sexes and mating types. It was later confirmed to be the first reportedly trioecious haploid species.

Reproduction
It is trioecious meaning males, females, and hermaphrodites exist in the species.

Reproduction is asexual under normal conditions and creates colonies of clones with the same genotype. Sexual reproduction is induced by low-nitrogen conditions and is anisogamous.

Studies on the species
The species was involved in a 2006 study that provided the first molecular genetic evidence for the evolutionary link between sexes and mating types. A male-specific gene was founded in the species and named "OTOKOGI", meaning manliness or chivalry in Japanese.

The species was previously believed to be heterothallic with males and females, but a 2021 study revealed it also contained hermaphrodites. This study was the first time the sexual system trioecy has been reported in haploid species and it challenged models for the evolution of sexual systems. Although trioecy is viewed as being an evolutionarily unstable transitional state between dioecy and monoecy, a 2023 study revealed this probably not the case in this species. Instead, in this species, trioecy could have evolved due to the reorganization of certain genes.