Abstract
In termites, i.e. a major group of eusocial insects, the soldier caste exhibits specific morphological characteristics and extremely high aggression against predators. Although the genomic background is identical to the other non-aggressive castes, they acquire the soldier-specific behavioral character during the course of caste differentiation. The high aggressiveness and defensive behavior is essential for colony survival, but the neurophysiological bases are completely unknown. In the present study, using the damp-wood termite Hodotermopsis sjostedti, we focused on two biogenic amines, octopamine (OA) and tyramine (TA), as candidate neuromodulators for the defensive behavior in soldiers. High-performance liquid chromatographic analysis revealed that TA levels in the brain and suboesophageal ganglion (SOG) and the OA level in brain were increased in soldiers than in pseudergates (worker caste). Immunohistochemical analysis revealed that TA/OA neurons that innervate specific areas, including the mandibular muscles, antennal nerve, central complex, suboesophageal ganglion, and thoracic and/or abdominal ganglia, were enlarged in a soldier-specific manner. Together with the results that pharmacological application of TA promoted the defensive behavior in pseudergates, these findings suggest that the increased TA/OA levels induce the higher aggressiveness and defensive behavior in termite soldiers. The projection targets of these soldier-specific enlarged TA/OA neurons may have important roles in the higher aggressiveness and defensive behavior of the termite soldiers, inducing the neuronal transition that accompanies external morphological changes.
Highlights
Division of labor has been independently acquired in various social-insect lineages, but the neurophysiological mechanisms have been studied in only a few lineages—mostly in social hymenopterans
We focused on the biogenic amines, especially OA and TA, as candidate neuromodulators responsible for the higher aggressiveness of termite soldiers in the dampwood termite Hodotermopsis sjostedti Holmgren
The present study revealed that TA/OA systems potentially responsible for the defensive division of labor in the damp-wood termite H. sjostedti
Summary
Division of labor has been independently acquired in various social-insect lineages, but the neurophysiological mechanisms have been studied in only a few lineages—mostly in social hymenopterans. In some social insects, including bees, aphids, and termites, individuals with unique morphologies and high aggression, i.e., soldier castes or guards, specialize in colony defense [1,2,3]. The different castes of social insects share similar (sometimes identical) genetic backgrounds with other colony members, but epigenetically acquire stable high aggression and defensive behaviors during postembryonic development in a castespecific manner [1, 2]. In the lower termites like H. sjostedti, olderinstar larvae playing roles of workers have the potential to differentiate into other castes Such individuals with a broad range of developmental options are currently termed pseudergates, originally defined as individuals without wing buds that develop regressively from nymphal instars [8, 20]. We performed pharmacological experiments to examine the effects of OA and TA application on defensive behavior in termites
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