Research

　Insect legs are composed of several segments, which is the coxa, trochantor, femur, tibia, tarsus, and pretarsus, from the proximal to distal direction. Among these, the tarsus is further subdivided into 1-5 subsegments called the tarsal segment 1-5 from the proximal to distal direction. The number of tarsal segments varies among different insect species. For example, in the foreleg of the male diving beetle, the first to third tarsal segments are flat and expanded to form a sucker-like structure, in the longicorn beetle, the first to third tarsal segments are flat and heart-shaped ventrally, and in the mosquito, each tarsal segment is very thin and elongated. Thus, the tarsi of adult insect legs are highly diverse and evolutionarily variable traits and are good materials for approaching the mechanisms of shape-making, including the evolution and diversity of shapes.

　We are trying to understand how the final shape of the adult leg of Drosophila is formed by using live imaging to observe in real-time the changes in the developing leg during the pupal stage when the final shape is formed. Recent our studies have revealed that each cell changes its shape much more dynamically than previously thought during the process of final shape formation, thereby creating the overall shape.

　By understanding the molecular mechanisms that cause the dynamic cell shape change and how it leads to the formation of the final shape, and in turn, through comparisons with insect species of different shapes, we would like to gain a better understanding of the shape-making mechanisms.

The body surface of insects is covered with a kind of extracellular matrix called the cuticle. The cuticle is a rigid structure that protects the contents of the body from environments. Because of this rigidity, the new cuticle must be formed and the old cuticle must be shed, when the body grows during post-embryonic development or undergoes a dramatic change in its shape by metamorphosis. During this process, which is called molt or eclosion, the old cuticle is not broken randomly, but always cleaved at a fixed position. In other words, some kind of the "cut here line" is pre-formed during the cuticle formation.

The "cut here line" on the cuticle has been little studied, even though it is of great importance not only in insects but in all molting arthropods. Drosophila pupae are encased in an enclosing pupal case called the puparium, and we are studying the operculum ridge, the "cut here line" of the puparium that is cleaved during eclosion, in order to understand the structure that functions as the "cut here line," the mechanism of its formation, and the mechanism that determines the position of the "cut here line." 

　Since the position of the "cut here line" varies widely among the major arthropod taxa and often differs among insect species, we expect that the study of the "cut here line" will lead to a better understanding of the evolutionary process of insects and as arthropods as a whole.

The cuticle is a type of extracellular matrix composed of chitin, cuticular proteins, and other substances secreted by epidermal cells. There are various types of cuticular proteins, and the properties of the cuticle vary greatly depending on which cuticular proteins are used. Mutants of cuticular proteins can have drastic changes in body shape. In these mutants, the cells themselves are normal, but the body shape is altered due to the different properties of the cuticle, the extracellular matrix. In other words, the body shape can be changed by changing the type of cuticular protein used. This may be the reason why different species of the same insect have different body shapes, ranging from thin and elongated like the walking stick to round like the ladybird. Our goal is to understand how the properties of the cuticle are regulated by cuticular proteins and how these cuticle properties regulate body shape.