Yuki Ishikawa

Abstract: Numerous cases of predator-induced polyphenisms, in which alternate phenotypes are produced in response to extrinsic stimuli, have been reported in aquatic taxa to date. The genus Daphnia (Branchiopoda, Cladocera) provides a model experimental system for the study of the developmental mechanisms and evolutionary processes associated with predator-induced polyphenisms. In D. pulex, juveniles form neckteeth in response to predatory kairomones released by Chaoborus larvae (Insecta, Diptera). Previous studies suggest that the timing of the sensitivity to kairomones in D. pulex can generally be divided into the embryonic and postembryonic developmental periods. We therefore examined which of the genes in the embryonic and first-instar juvenile stages exhibit different expression levels in the presence or absence of predator kairomones. Employing a candidate gene approach and identifying differentially-expressed genes revealed that the morphogenetic factors, Hox3, extradenticle and escargot, were up-regulated by kairomones in the postembryonic stage and may potentially be responsible for defense morph formation. In addition, the juvenile hormone pathway genes, JHAMT and Met, and the insulin signaling pathway genes, InR and IRS-1, were up-regulated in the first-instar stage. It is well known that these hormonal pathways are involved in physiological regulation following morphogenesis in many insect species. During the embryonic stage when morphotypes were determined, one of the novel genes identified by differential display was up-regulated, suggesting that this gene may be related to morphotype determination. Biological functions of the up-regulated genes are discussed in the context of defense morph formation. It is suggested that, following the reception of kairomone signals, the identified genes are involved in a series of defensive phenotypic alterations and the production of a defensive phenotype.

Abstract: One of the key characters of social insects is the division of labor, in which different tasks are allocated to various castes. In termites, one of the representative groups of social insects, morphological differences as well as behavioral differences can be recognized among castes. However, very little is known about the neuronal and molecular bases of caste differentiation and caste-specific behavior. In almost all termite species, soldiers play defensive roles in their colonies, and their morphology and behavior are largely different from workers (or pseudergates). Therefore, we predicted that some genes linked to defensive behavior and/or those required for neuronal changes are differentially expressed between workers and soldiers, or during the soldier differentiation, respectively. Using the brain and suboesophageal ganglion (SOG) of the damp-wood termite Hodotermopsis sjostedti, we first screened genes specifically expressed in soldiers or during soldier differentiation by the differential display method, followed by quantitative real-time polymerase chain reaction. No distinctive differences in expression patterns were detected between pseudergates and soldiers. In the course of soldier differentiation, however, five genes were found to be up-regulated in brain and/or SOG: 14-3-3epsilon, fibrillin2, beta-tubulin, ciboulot, and a hypothetical protein containing a SAP motif. Some of these genes are thought to be associated with cytoskeletal structure or motor-associated proteins in neuronal tissues. The identified five genes could be involved in soldier-specific neuronal modifications, resulting in defensive behaviors in termite soldiers. The temporal expression patterns of these genes were consistent with the neuronal changes during soldier differentiation, suggesting that molecular machineries, in which the identified factors would participate, play important roles in behavioral differentiation of termite soldiers.

Abstract: In social insects, the division of labor among castes is one of the fundamental natures for establishing social behaviors. In termites, because of their hemimetaboly, the regulations of postembryonic development should be important to regulate appropriate caste ratios in a colony. Although the caste differentiation can be triggered by social interactions among colony members, the interactions via primer pheromones and the physiological mechanisms underlying caste determination or differentiation are poorly understood. In this study, therefore, we propose a model for the regulatory mechanism controlling caste differentiation, focusing on the physiological states and inhibitory interactions among individuals. The proposed model incorporates an internal physiological system like JH and ecdysone actions, together with two types of inhibitory interactions by pheromones secreted by soldiers and alates. The results of computer simulations based on the proposed model suggested that the two types of interactions were sufficient to enable a colony to maintain the appropriate caste ratio. These results should help to find the primer pheromones and to infer their functions in the caste differentiation in termites.

Abstract: Social insects exhibit a variety of caste-specific behavioral tendencies that constitute the basis of division of labor within the colony. In termites, the soldier caste display distinctive defense behaviors, such as aggressively attacking enemies with well-developed mandibles, while the other castes retreat into the colony without exhibiting any aggressive response. It is thus likely that some form of soldier-specific neuronal modification exists in termites. In this study, the authors compared the brain (cerebral ganglion) and the suboesophageal ganglion (SOG) of soldiers and pseudergates (workers) in the damp-wood termite, Hodotermopsis sjostedti. The size of the SOG was significantly larger in soldiers than in pseudergates, but no difference in brain size was apparent between castes. Furthermore, mandibular nerves were thicker in soldiers than in pseudergates. Retrograde staining revealed that the somata sizes of the mandibular motor neurons (MdMNs) in soldiers were more than twice as large as those of pseudergates. The enlargement of MdMNs was also observed in individuals treated with a juvenile hormone analogue (JHA), indicating that MdMNs become enlarged in response to juvenile hormone (JH) action during soldier differentiation. This enlargement is likely to have two functions: a behavioral function in which soldier termites will be able to defend more effectively through relatively faster and stronger mandibular movements, and a developmental function that associates with the development of soldier-specific mandibular muscle morphogenesis in termite head. The soldier-specific enlargement of mandibular motor neurons was observed in all examined species in five termite families that have different mechanisms of defense, suggesting that such neuronal modification was already present in the common ancestor of termites and is significant for soldier function.

Abstract: Termites are one of the major groups of social insects, which comprise alates, workers (pseudergates), and soldiers within a species. These castes have different roles and behaviors, and undertake division of labor to increase the inclusive fitness of their colony. On the basis of the different behavioral repertoires, caste-specific neural modifications are predicted, such as modification of sensory systems, i.e., inputs into the nervous system. This study evaluated these sensory-system differences based on mechanoreceptive sensilla length among castes of the damp-wood termite Hodotermopsis sjostedti. We found that soldiers and alates had longer sensilla than pseudergates, and that this variation among castes differed with body region. Specifically, the differences were particularly conspicuous on head capsules and pronota, while sensilla on mouthparts and legs were of similar lengths among castes. It is proposed that soldiers and alates use these long mechanoreceptive structures to sense faint vibrations, an important capability for their defensive role in detecting enemies and cracks in nest wood.