Asato Kuroiwa

The Mystery of X and Y ? the Strange Mechanism That Determines Sex

Asato Kuroiwa , Associate Professor

Faculty of Science/Graduate School of Life Science (School of Science/Department of Biological Sciences (Biology))

High school : Kyoto City Murasakino High School.

Academic background : Doctorate from Nagoya University

Research areas
molecular cytogenetics
Research keywords
genetics, X chromosomes, Y chromosomes, sex determination, spiny rats

What are you aiming to achieve?

Our sex as humans is determined by genetics and by the chromosomes in which genes exist. Our cells contain chromosomes in different combinations, depending on whether we are male or female, with the larger chromosome known as the X chromosome and the smaller one as the Y chromosome. People with two X chromosomes become female while those with one X and one Y become male (Fig. 1). This sex determination function is true not only for humans but for all mammals. There are thought to be around 5,000 kinds of mammals on earth, with almost all of them following the same rules in terms of sex determination mechanisms.

Fig. 1: Two X chromosomes make a female, while one X and one Y make a male. However, spiny rats have no Y chromosomes, and their males and females have only one X chromosome each.

Furthermore, the X and Y chromosomes of mammals used to be a pair of same-sized chromosomes. Ancestral species of mammals are believed to have appeared on earth around 310 million years ago, at which time both X and Y were the same size and are believed to have contained more than 1,100 types of genes. As a result of long periods of evolution, however, the chromosome now called Y was subject to harmful mutations, which accumulated, resulting in the gradual shrinking of the Y chromosome. The original number, more than 1,100 genes, has also been reduced to the roughly 30 types that exist today. In other words, a male with XY has roughly 1,070 genes fewer than a female with XX. Although the number is far smaller, the remaining genes in the Y chromosome have evolved to play important roles, such as in the formation of sperm and the development of the testes, and are vital for the male species. If the genes in the Y chromosome continue to reduce, however, the Y chromosome may be lost, and males may cease to exist, leading, in some theories, to the end of the human race.

I am working to solve the mysteries of the evolution of the Y chromosome and sex determination mechanisms using spiny rats, which possess extremely strange chromosomes, in my research. Spiny rats are a Japanese native endemic species that live in the Nansei Islands. They are mammals, but do not have Y chromosomes, with both males and females instead possessing only one X chromosome each (Fig. 1). I am studying these spiny rats and their strange chromosomes from a wide range of angles.

What sort of equipment do you use to perform what type of experiments?

Spiny rats are an endangered species, and as such have been designated as a protected species in Japan. As a result, it is not easy to use them as subjects for research. We received permission from the government to capture them and work in collaboration with those who are surveying their habitat, getting them to send us the tips of tails of animals that have been captured during surveys. We culture cells from the tips of these tails, creating chromosome specimens, and extract DNA for our experiments. Furthermore, the cultured cells are a valuable genetic resource and so are frozen and stored. Genetic analysis is carried out by molecular biological experimentation. The equipment we use includes clean benches to cultivate the cells, CO2 incubators, optical microscopes to analyze the chromosomes, image analysis devices, and DNA sequencers to determine the base sequence of the genes.

Fig. 2: Capture surveys and spiny rats Fig. 3: Cultivating cells from the tips of their tails
Fig. 4: Cultured cells and experiments Fig. 5: Analysis of the chromosomes

What do you aim to achieve next?

My research began because of a strong desire to understand how sex is determined between males and females. In the case of mammals, the secrets are hidden within the X and Y chromosomes. I want to clarify the mystery of X and Y as much as possible. Furthermore, by researching specific examples within spiny rats, we have begun to make small inroads into explaining differences between X and Y that were not understood until now. For example, by studying spiny rats we have discovered that the loss of a Y chromosome does not necessarily mean the end of males. Of course, we humans are not following the same evolutionary path as spiny rats. However, they are the only model species for the verification of Y chromosome loss that has thus far been found throughout the world. Unfortunately, the destruction of their natural environment and the arrival of imported predator species have resulted in a major decrease in their population. I hope that my research will cause many people to realize that there is this amazingly interesting animal that is native to Japan, and that this will contribute to more proactive measures to conserve them.