Graduate School of Frontier Sciences, The University of Tokyo Department of
Integrated Biosciences

'Sex' is an essential part of reproduction for all species that reproduce sexually, including humans. The question of how sex is determined has been one of the key biological questions since the time of Aristotle to the present day. At the same time, 'sex' is also a matter of concern for many people. The eternal theme of love, which is a theme that can be found in novels, dramas, songs, paintings, sculptures and other creative works, is just a derivative trait that has arisen alongside the evolution of sex. As the recognition of the term “sexual minority (LGBTQ)” spreads, we are entering an age when society as a whole should reconsider “what sex is”. Researching “what sex is” is not only important from a biological perspective, but also has the potential to enrich our lives and overcome social issues. In the Department of Biological Control, we are conducting research from a wide range of perspectives, from the molecular level to the ecological level, in order to uncover the mysteries of sex.

The word “sex” generally refers to the male and female sexes. In biology, the sex of an individual that produces sperm is referred to as male, and the sex of an individual that produces eggs is referred to as female. Since sex is an essential trait for sexual reproduction, it is closely related to all biological phenomena, from the microscopic to the macroscopic level.

In our laboratory, we are conducting research to uncover the mysteries of sex by looking at sex phenomena from the perspective of the three Ss: System (mechanism), Symbiont (symbiotic bacteria), and Species (species).

The appeal of the system that determines sex (sex determination mechanism) is that the dominant role of determining sex is carried out by the action of a single gene, and this manifests itself in all aspects from appearance to metabolism, immunity, lifespan and behavior. There are many species of insects that exhibit such pronounced sexual dimorphism that they have even been mistakenly classified as different species. The scientific name for the spongy moth, which our laboratory studies, is Lymantria dispar, and it is said that this name was given because the male and female moths have such markedly different appearances.

Another attractive aspect of the sex determination mechanism as a system is its transient nature. Despite the fact that sex is a universal trait for all species that use sexual reproduction, the mechanisms that determine sex and sex-determination genes show a tremendous diversity. The more we research, the more new mechanisms that overturn preconceived ideas we find. In fact, through our research using silkworms, we discovered that small RNAs that inhibit transposon transfer and splicing inhibitors play an important role in sex determination.

The further appeal of the sex determination mechanism as a system lies in the fact that it combines flexibility and robustness. Despite the fact that sex determination systems are prone to change, the output of producing males and females does not change. This means that the renewal from the old system to the new system is carried out smoothly. In order to clarify this mechanism, we are conducting research focusing on the Japanese oak silkmoth. It is known that the Japanese oak silkmoth has different types of sex determination genes between subspecies and between regional populations. We expect that we will be able to capture the process of the birth of a new sex determination system using the Japanese oak silkmoth.

Examples of symbionts (symbiosis bacteria) and pathogenic microorganisms hijacking the host's sex determination mechanism and causing a bias in the sex ratio have been observed in humans, birds, insects, and plants. It is possible that this kind of takeover by parasites and the struggle between the host and the parasite to resist it has led to the diversity of sex determination mechanisms. It has long been known that the sex determination gene shows functional differences between regional populations, subspecies, and closely related species.

We have previously identified the sex-determination gene in the spongy moth and found that there are significant interspecific and intraspecific differences in the copy number of this gene. Furthermore, we have discovered a new species of spiroplasma that parasitizes the spongy moth's close relative, the Lymantria umbrosa. Spiroplasmas are known to cause male-killing, and the spiroplasma we discovered is also known to be a male-killing type. We will investigate the relationship between sex-determination genotypes and resistance to spiroplasmas in the sponty moth and also examine the correlation between the range of spiroplasma infection in the field and the distribution of sex-determination genotypes, to verify whether the parasitism of symbiotic bacteria can be a driving force for the diversification of sex-determination mechanisms.

Species are important pieces that make up an ecosystem. Understanding the mechanisms that lead to species extinction is an important theme for the protection of endangered species and the maintenance of biodiversity. We are focusing on a phenomenon called “reversal of speciation” as a factor in species extinction. “Reversal of speciation” is a phenomenon in which two different species revert to the same species. Reversal of speciation is thought to occur when the opportunity for inter-species hybridization increases due to the dissolution of geographical isolation or environmental change.

We discovered through field surveys that the hybrid between Lymantria umbrosa and L. dispar can occur at a frequency that can cause a reversal of speciation. On the other hand, when these two species are crossed, we found that female-to-male sex change in the female occurs, resulting in female lethality and the appearance of intersex individuals, and that the sex ratio is biased towards males. We have determined that interspecific differences in sex-determination genes are involved in this. Furthermore, we have also found that a similar phenomenon occurs in the wild. The bias in the sex ratio in hybrids can trigger reproductive isolation and promote speciation. We have hypothesized that the diversification of sex-determination genes has an adaptive significance in suppressing the reversal of speciation and maintaining species identity, and we are conducting research to verify this hypothesis.