Researchers TAKAHASHI Kohei


Research Fellow,

The University of Tokyo


Higashiyama Group

Research Fields
Evolutionary biology, Phycology, Genome biology
Haploid species, Mating systems, Sex determination, Reproductive isolation, life-history evolution, Inbreeding

The elucidation of molecular genetic basis for evolution of trioecious green alga using whole genome analysis

I'm interested in the molecular genetic mechanisms of the evolutionary dynamics generating the diversity of mating systems. Many evolutionary biologists have studied how different mating systems have evolved since the time of Charles Darwin. There are two basic mating systems: The dioecious mating system that includes male and female sex phenotypes and the monoecious mating system with only bisexual sex phenotype producing both male and female gametes. The evolutionary transition of these mating systems is often recognized across eukaryotes. In addition to these mating systems, the coexistence of unisexual (male or female) and bisexual individuals called intermediate or mixed mating systems is known. Among the mixed mating systems, the coexistence of three sex phenotypes (male, female, bisexual) in a single species, ‘trioecy’, is rarely found in diploid mating systems such as flowering plants and invertebrates. On the other hand, in haploid mating systems including algae and fungi, mating system like ‘trioecy’ had not been reported until recently.

The volvocine green algae is a model group exploring the molecular genetic mechanisms of transition of mating systems. In this group, heterothallic (male and female) and homothallic (only bisexual) species coexist within closely related taxa. I have recently found that a volvocine green alga Pleodorina starrii has haploid ‘trioecious’ mating system that has three sex phenotypes: male, female, and bisexual phenotypes (Takahashi et al. 2021 Evolution). Intercrossing experiments among P. starrii three sex phenotypes also revealed a genetic system that may determine the three sex phenotypes based on two independent loci: sex-determining region (SDR) in UV sex chromosomes that determine sex in haploid mating systems and autosomal bisexual factor that only bisexual phenotype has.

Comparative genomic analyses among three sex phenotypes revealed that males and bisexuals had the same ‘male SDR’ whereas females had a ‘female SDR’. In the volvocine algae, male SDR harbors the MID gene that determines mating type minus dominantly and female SDR harbors FUS1 gene that acts as the mating type plus gamete-specific membrane adhesion protein. Interestingly, MID is duplicated as three paralogs and FUS1 translocated to the autosomal region and all three sex phenotypes have this gene. expression pattern of MID is almost no difference between males and bisexuals whereas the expression of FUS1 was strictly suppressed in males but not in bisexuals. According to these results, the reorganization of SDR with changes of existence states of sex-specific genes such as MID, FUS1 should be the fundamental genomic basis of trioecy in this species. In addition to this genomic basis, different gene regulation mechanisms might have evolved in males and bisexuals, thus allowing them to be evolutionarily viable sexes while having similar sex-determining genes.(Takahashi et al. 2023 Commun. Biol.).

Field sampling with Higashiyama lab students in Lake Sagami. 2022.

The asexual colony of Pleodorina starrii (left) and schematic drawing of "sex chromosome revolution" in P. starrii revealed in Takahashi et al. 2023 (right).

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