Evolutionary Strategies of Diverse Wildlife

The Great East Japan Earthquake that occurred in 2011 (Heisei 23) resulted in a giant tsunami that struck the Tohoku Region of Japan. This led project associate professor Jun Kitano, who was studying the “three-spined stickleback” (photo, right panel), a fish inhabiting the cold water mainly in Hokkaido and Tohoku areas, to investigate the changes that occurred in this fish, resulting from the tsunami-related destruction of its native habitat. Professor Kitano cites this as his motivation for joining the “systems resilience” project. We would like to present some of our latest achievements.

Many strange creatures in the world!

This world is home to several creatures displaying incredibly flashy shapes, whose shapes serve diverse purposes and lead to diverse behaviors. For example, bowerbirds gather tree branches to build large objects. Surprisingly, these objects are not nests, but are intended to attract females to the nest. The flashier the object, the more popular the male. My motivation of research is “Why is there such a strange creature?” I wanted to know how these creatures evolved. So, I chose a fish, belonging to the Gasterosteidae family, called the three-spined stickleback, to discover the genomic structure leading to these differences in behaviors and shapes. The size of the three-spined stickleback genome is very small compared to that of humans, at about 1/7 of the human genome, and perform several courtship behaviors, which were some of my major reasons for choosing this fish species for my research.

Tsunami as an environmental factor affecting the evolution of three-spined sticklebacks

There is a city of Otsuchi in Iwate Prefecture faced with Pacific Ocean. This place is famous for a novel by Hisashi Inoue called“Kiri Kiri Jin.” There are two rivers flowing through this city: the Otsuchi and Kozuchi Rivers. The three-spined stickleback live in the upstream of the Otsuchi River; but this area was covered by large quantities of rubble during the tsunami. Naturally, we believed that this population of three-spined stickleback was wiped out. But apparently, they survived. The rubble was removed relatively quickly by the volunteers and the defense forces; in addition, this houses a number of fresh springs. So we suspected that this species of fish survived by seeking shelter in the mouth of these springs. We also observed some new ponds composed of spring water that had been invaded by the three-spined stickleback. When the genetic component of these fish was analyzed, we found these fish to be hybrids of the upstream three-spined stickleback, which never migrate, and the marine three-spined stickleback that does migrate. So, we could witness the actual movement of creatures in real time, which is one of very important evolutionary forces.

Systems resilience project

Speciation and sex chromosomes

Besides the tsunami project, I am studying the mechanisms of species formation (speciation) using the Japan Sea and the Pacific Ocean sticklebacks. Generally, a species is defined according to a so-called Biological Species Concept: a group of animals, within which individuals are randomly mating, but are reproductively isolated from members of another such group. So, speciation is the process of the formation of reproductive isolation. However, reproductive isolation is not always 100%, and one cannot draw a clear line between species and populations. At a glance, one cannot distinguish between the three-spined stickleback from the Sea of Japan and the three-spined stickleback residing in the Pacific Ocean. But their DNA is completely different. If we crossbreed a female from the Sea of Japan with a male from the Pacific Ocean, it leads to hybrid male sterility, wherein the hybrid males cannot produce sperms. But in another direction of crossbreeding, hybrid sterility does not occur. In other words, these two fish are not perfect species yet, but are now in the process of speciation. Therefore, we believe this fish makes an ideal model system to study the process of speciation.

We discovered from the genetic analysis of the three-spined stickleback, that only the three-spined stickleback from the Sea of Japan display a fusion between a Y chromosome and the 9th autosome, resulting in a new type of sex chromosome, i.e., “neo-Y chromosome.” Furthermore, the remainder of the 9th autosome evolved as a “neo-X chromosome”.. Since sex chromosomes for female are XX and males are XY, there is a theory predicting that genes advantageous to one sex are accumulated in sex chromosomes. We actually found genes for several species- and sex-specific traits on neo-sex chromosomes. Based on this information, we made a hypothesis that the evolution of sex chromosomes is linked to species differentiation.

So, in order to test the generality of this hypothesis, we began a study of medaka fishes (photo, top panel). The genes on the sex chromosomes of mammals are basically the same for humans, chimpanzees, etc.; however, the composition of Y chromosome that determines the male sex is completely different between closely related medaka species. The sex chromosomes of Indonesian medakas mostly possess the XX and XY structures; however, the genes on these chromosomes are often different between closely related species. We are trying to analyze if the evolution of genes on the sex chromosomes is associated with the evolution of sex-related differences, such as the flashy color of males, which may important for species differentiation. So, we are currently conducting genetic and genomic analysis of these fishes.

(Text in Japanese: Jun Kitano, Rue Ikeya. Photographs: Mitsuru Mizutani. Published: March 10, 2015)