#100 Unraveling the Mysteries of Evolution through Tiny Spider Mites

Assistant Professor SATO Yukie, Faculty of Life and Environmental Sciences

When you hear the word "mite," you probably do not have a positive impression. After all, ticks and blood-sucking mites are indeed unpleasant creatures. However, there are also some mites that break down organic substances in soil, and others that attack pests. There is also a group of mites called spider mites that attach themselves to plant leaves. On crops and garden plants, spider mites are considered pests, but the majority of them live where people never notice them. Prof. Sato's research focuses on spider mites that exhibit what can be called social behavior.

Anyone who hears the term "social insect" would probably think of insects such as ants, bees, and termites. Indeed, ants have an advanced social structure with colonies consisting of a queen ant, all-female worker ants, and male ants whose only job is mating. However, other insects also exhibit various kinds of social behavior. Spider mites, which belong to the same large group as spiders, have been found to have budding social behavior. When Prof. Sato started her studies at Hokkaido University School of Agriculture, she wanted to do research on animals in some way or another, and chose the Lab of Animal Ecology for her graduation research. Under the tutelage of then-professor Yutaka Saito, an expert in spider mites and particularly mites of the genusStigmaeopsis, she conducted research onStigmaeopsis miscanthi, a mite that infests Chinese silver grass.

The reason these mites are called spider mites is because they produce silk like spiders (but from their mouth, not from their abdomen like spiders). Almost all spider mites use this silk to attach their bodies to the plants they infest or to create a coarsely woven nest called a shelter to protect themselves from predatory mites.Stigmaeopsis mites systematically spin silk into depressions in leaf veins to create an amazing tunnel-shaped shelter where the colony lives. They do almost all their daily activities from feeding and breeding to raising their young in this shelter. Their shelter even has a designated bathroom area, and all the individuals keep the shelter clean. Prof. Sato says it touches her heart to watch young mites that have just hatched toddle their way to the bathroom to do their business.

S. miscanthi mites have a particularly developed social structure amongStigmaeopsis mites. They raise their young collectively, and the males fight to protect their fellow mites when predatory mites such asPhytoseiidae invade the shelter. It is believed that unique genetic sex determination systems are what produce the advanced social behavior of insects such as ants and bees. Human men and women both have the same genome size (a set of the minimum number of chromosomes necessary for life) with two sets of chromosomes each. In ants and bees, in contrast, males have one set of chromosomes whereas females have two sets. This system is called haplodiploidy. Females develop from fertilized eggs, and males from unfertilized eggs. The details are too long to explain here, but this is believed to be the reason why worker ants and worker bees are all female. Mites are actually haplodiploid as well, which is a factor that gives them the capability to have a developed social structure.

The area around Sugadaira Research Station is a treasure trove of resources for research. The fields of Chinese silver grass are tall and golden before the grass is harvested in the fall.

S. miscanthi colonies have a skewed male-to-female ratio: 80 to 90% are female. In some places, males in the same colony kill each other, dominate the colony, and create a harem. The frequency at which males kill each other (aggression) is now known to be related to how cold the nesting site gets in the winter. Prof. Sato believes that this can be explained by an evolutionary theory called kin selection theory (the idea that natural selection favors the traits maximizing inclusive fitness, which considers not only individual reproductive success but also the reproductive success of related individuals with shared genes) because the ecological characteristics of spider mites should cause relatedness of individuals composing a colony to be higher in cold areas. To prove this hypothesis, she plans to further investigate topics such as differences in ecology and behavior and relatedness of individuals in colonies. By doing so, she hopes to figure out howS. miscanthi, which are believed to have migrated from the south, evolved their unique behavior.

The two-spotted spider mite, a pest of various cultivated plants that is found across the world, has had its genome completely sequenced and has become a new model animal. For this reason, there is a global network of researchers studying spider mites. Peter Schausberger, a professor at the University of Vienna, is currently working in Prof. Sato's lab at Sugadaira Research Station as a fellow conducting joint research. There are noS. miscanthi mites in the Sugadaira area, but you can easily find spider mites if you turn over a bamboo leaf. The area has no shortage of not only spider mites, but also other resources for student lab work and nature workshops for the general public. Prof. Sato initially did not intend to become a researcher, and wanted to get a private sector job. However, her graduation research opened her eyes to the joy of studying the ecology of living things. She wants to share this joy with as many aspiring researchers as possible.

S. miscanthi shelters expanded into a tunnel shape. You can see adult mites, juvenile mites, eggs, and the bathroom area (brown area). The white objects are shed skins. The adult females (mites to the lower right of the bathroom area) are reinforcing the shelter's web by spinning silk while moving in a zigzag pattern. This also helps to clean the floor of the shelter (underside of the leaf) because dust and shed skins get stuck to the web that serves as the ceiling of the shelter.

A shelter ofStigmaeopsis saharai mites on the underside of a bamboo leaf. This species does not expand their shelter, but instead builds a new one nearby. They build their bathroom area outside the shelter (the blackish area near the entrance to the lower right of the shelter).

Article by Science Communicator at the Office of Public Relations