Reproductive organ development along the caste differentiation pathways in the dampwood termite Hodotermopsis sjostedti

One of the principal features of eusocial insect societies is the reproductive division of labor, which involves developmental regulation of the reproductive organs. However, although the regulation of caste development is important for establishing social structure in termites, one of the major eusocial insect groups, little is known about reproductive organ development during caste differentiation. Therefore, in this study, we investigated the anatomy and histology of the reproductive organs at various developmental stages during caste differentiation in the dampwood termite Hodotermopsis sjostedti, in which the differentiation of reproductive and non-reproductive castes occurs in late larval instars. We found that the reproductive organs gradually developed during the younger instars in both sexes. However, in females, there were larger variations in ovarian development between individuals in the seventh-instar larvae (i.e., pseudergates), which may reflect variability in their caste fates. By contrast, the testis size did not show such heterogeneity in male pseudergates. Interestingly, presoldiers and soldiers possessed relatively well-developed ovaries or testes containing sperm. When differentiating into the two types of reproductives, i.e., alates and neotenics, the reproductive organs developed rapidly, but the developmental timing of organ growth differed between these as well as between the sexes. Thus, reproductive organ development along the caste differentiation pathways appears to be regulated in a caste- and sex-specific manner.