Comparative Expression Analysis of Olfactory Receptor Genes Among Individuals With Soldier and Worker Caste Differentiation Fates in Termites

Clarifying the mechanisms controlling caste differentiation is a key research topic in the study of termite biology. The damp-wood termite <i>Zootermopsis nevadensis</i> is the only species of termites in which soldier caste differentiation can be observed under natural conditions. In an incipient colony, soldier differentiation requires first-molted 3rd-instar larvae to engage in trophallactic behavior as a recipient from the reproductives. In contrast, other 3rd-instar larvae exhibit worker-like behaviors, such as allogrooming, and molt into later instars without undergoing soldier-specific morphogenesis. Therefore, differences in behaviors between 3rd-instar larvae with different caste differentiation fates are likely due to differences in chemosensory capacity for recognizing reproductives. To investigate this possibility, we focused on the expression patterns of olfactory genes in the main chemoreception organs (antennae) of the 3rd-instar larvae in incipient colonies. We analyzed the duration from the first to the 4th-instar and confirmed that the length of the 3rd-instar period differed significantly between individuals with different developmental fates. RNA-seq analysis of antennal tissues from 3rd-instar larvae revealed that the olfactory receptor genes exhibiting significant expression differences between individuals with different caste differentiation fates were more frequently found in the <i>ionotropic receptors</i> (<i>IRs</i>) gene family (25/132 [18.9%]) than in the <i>odorant receptors</i> (<i>ORs</i>) gene family (6/62 [9.7%]). We paid particular attention to the gene expression differences of the co-receptors (<i>Orco</i>, <i>IR8a</i> and <i>IR25a</i>), all of which are essential for odorant and ionotropic receptor functions. We performed real-time quantitative PCR analysis using antennal tissues from individuals that had spent different numbers of days in the 3rd-instar. The results showed that all co-receptor genes were highly expressed in individuals with a worker differentiation fate from the very beginning of the 3rd-instar. These results suggest that different chemosensory capacities among 3rd-instar larvae are important for regulating caste differentiation fates in this species.