Reduction of a nymphal instar in a dampwood termite: heterochronic shift in the caste differentiation pathways.

Background: Generally in termites, alates differentiate through multiple nymphal instars which gradually develop wing buds. However, in a dampwood termite, , alates molt directly from a single nymphal instar with short wing buds. In this study, to examine the mechanism underlying the wing formation during the alate differentiation in , histological and morphological observations were carried out on the developmental process of wing formation during the nymphal instar, in comparison with those in , which has two nymphal instars. Furthermore, the expression patterns of genes that are thought to be responsible for wing formation, i.e., wing-patterning genes and genes encoding hormone-related factors, were quantified during alate differentiation and compared between the two species.

Results: The results showed that, in , wings were formed in a complicatedly folded shape, not only inside the wing buds as seen in , but also under the dorsal thoracic cuticle, where the wing tips shifted toward the median thoracic part. Accordingly, the wing expansion pattern also differed from that in . Furthermore, the results of real-time qRT-PCR on overall expression profiles of wing-patterning genes and hormone-related genes suggest that the single nymphal instar in well resembles to the second nymphal instar in . In particular, significant upregulation of () and downregulation of (-) that were observed at the second nymphal instar in apparently occurred during the single nymphal instar in .

Conclusion: The developmental events for wing formation are compacted into a single nymphal instar in , and as a result, the unique wing formation is seen to compensate for the spatial restriction inside small wing buds, leading to the completion of functional wings.