Research Interests

Ecological Developmental Biology; Sociobiology; Insect Physiology

Phenotypes of organisms are not determined completely genetically, but vary according to environmental factors (phenotypic plasticity). Some organisms express several discrete adaptive phenotypes (polyphenism). Polyphenism can be explained as the modification of postembryonic development to produce alternative phenotypes. Social insects possess a few types of individuals (castes) in their colonies, to which specific tasks are allocated. In a colony, different castes possess different characteristic morphologies, such as the enlarged mandibles of soldiers for defense, or the wings of alates for dispersal. These caste-specific body parts are exaggerated or reduced during postembryonic development by responding to extrinsic cues, such as physical environments and/or social interactions among colony members. In my laboratory, we are studying on the polyphenism in ants, termites and aphids, in terms of the developmental mechanisms of phynotype-specific characters. We are working on these topics, in terms of the alteration of body plan in response to environmental signals, and trying to understand the evolutionary process of the interaction between ontogeny and environment.


Termite Research

Termites (order Isoptera) are hemimetabolous social insects, having various castes specialized in colony tasks. Those includes sterile individuals such as soldiers specialized in defense. We can see various caste-specific morphological features like mandibles or frontal projections in soldiers, for instance. Those are produced in the course of soldier differentiation. In the case of alate development, compound eyes and legs are produced or modified during postembryonic development. Our recent studies using molecular techniques isolated genes related to soldier differentiation and analyzed the expression profiles of those genes in order to understand the mechanism of caste differentiation and the link between molecular and social evolution.

 

Solders show aggressive attacking behavior against predators or intruders.

Ant Research

Ants (order Hymenoptera: family Formicidae) have adapted to the ground ecosystem and possess a large number of workers that lose their wings. However, this does not mean that ants have lost their wings genetically, because they can still produce winged individuals (alates) in response to environmental factors in their annual life cycles. The developmental fates of castes are probably determined by the pattern-formation genes in the early stage of postembryonic development, but we found that the apoptotic degeneration occurs in the wing primordia of future workers. As apoptotic wing degeneration has been observed in two phylogenetically distant groups of ants, this phenomenon is suggested to be conserved in many ant species.

 

Aphid Research

Aphids display divergent adult phenotypes, depending on environmental conditions experienced during their embyonic and nymphal stages in their complex life cycle.  However, the developmental mechanisms producing such polyphenic traits according to the extrinsic stimuli still remain uncovered.  On the other hand, aphids show vigorous fertility by means of the parthenogenetic viviparous reproduction, and produce clonal individuals without genetic variations, providing us exellent experimental materials.  In this study, we tried to induce winged/wingless parthenogenetic individuals in two aphid species Acyrthosiphon pisum and Megoura crassicauda, by manipulating the density conditions, in order to analyze the developmental mechanism underlying the wing polyphenism.  We successfully induced two different wing types and made it clear that the critical stages determining the wing types were different between the two species.  Furthermore, we are analyzing the ultrastructures of wing primordia by a scanning electron microscope and the histlogical characteristics by making cross sections.  In both species, wing buds can be observed at the third-instar stages, although it was suggested that the developmental programs should be launched prior to this stage.  We have started to clone several developmentally important genes (e.g., wg, dpp, hh etc.) to uncover the developmental mechanism and evolutionary processes of aphid wing polyphenism.

Daphnid Research

"Polyphenism", that produces alternative phenotypes depending on the environmental stimuli, is known to play an important role in increasing fitness under unstable emvironments.  Cladocerans, especially Daphnia provide us a model system studying the evolution of polyphenic development. We are currently working on the molecular basis underlying Daphnia polyphensim, mainly focusing on two representative phenomena, neckteeth formation in D. pulex and male induction of D. magna.

In D. pulex, juveniles form neckteeth in response to kairomones, released by predatory Chaoborus larvae. To reveal the developmental mechanism of defensive morph, we observed detailed embryogenesis and postembryonic development. The kairomone-exposed embryos possessed the thickened epidermis at the back of head, where the neckteeth would be formed. The direct exposure on embryos and neonates showed that the reception and developmental mechanisms are still working even at the postembryonic stages. Investigations on growth rate and reproduction suggested that the there are several developmental regulations in response to kairomone.  We are currently trying to screen genes that up- or down regulated through this developmental process, using differential display.

D. magna is a good material to analyze the switch between sexual and asexual reproduction.  The induction of sexual generation is also triggered by environmental stimuli, therefore, this phenomenon is also a case of polyphenism. It is possible to induce male production artificially by the application of methylfarnesoate, juvenile hormone and their analogues. It seems worth to analyze genes responding to those chemicals. Here, we are also applying differential display to uncover molecular mechanism underlying the process of male induction.

 

    

Daphnia pulex        Defensive phenotypes (right) are induced by the presence of predators.