Morphogenesis and gene expression in the soldier-caste differentiation of termites
Since almost all termite species possess a soldier caste, there must be ubiquitous mechanisms of soldier differentiation throughout isopteran species. In order to reveal the caste differentiation mechanisms, observations during the soldier morphogenesis and identification of soldier specific gene expression are thought to be important. In this article, I summarize research approaches for analyzing caste differentiation in termites, and introduce two of our studies in Hospitalitermes medioflavus (Termitidae) and Hodotermopsis japonica (Termopsidae).¶Colonies of the nasute termite H. medioflavus have soldiers with a frontal projection (nasus) on the head, from which defensive substances are secreted. During soldier differentiation from male minor worker to presoldier, the most dynamic morphogenesis occurs. In the presumptive nasus epithelium of minor workers, a disc-like structure termed "soldier-nasus disc" rapidly develops to form the nasus of presoldiers. This rapid growth is associated with two folding layers of cuticle and epithelium.¶To identify genes specifically expressed in soldiers of the damp wood termite Hodotermopsis japonica, a differential display using RT-PCR was tried, comparing mRNA from the heads of soldiers and pseudergates. An identified gene candidate termed SOL1 was expressed specifically in terminally differentiated mature soldiers, and the product of the gene was suggested to encode a novel protein with a putative signal peptide at the N-terminus. This gene was shown to be expressed in the mandibular glands which actually develop during the soldier differentiation. Thus, these molecular techniques are applicable to reveal the proximate mechanisms of caste determination in termites and other social insects.
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