Abstract
Caste polyphenism in social insects is regulated by social interactions among colony members. Trophallaxis is one of the most frequently observed interactions, but no studies have been conducted identifying the intrinsic factors involved in this behaviour and caste differentiation. Dopamine (DA) has multiple roles in the modulation of behaviours and physiology, and it produces species-specific behaviours in animals. Here, to verify the role of DA in termite soldier differentiation, we focused on the first soldier in an incipient colony of Zootermopsis nevadensis, which always differentiates from the oldest 3rd instar (No. 1 larva) via a presoldier. First, brain DA levels of the No. 1 larva at day 3 after its appearance were significantly higher than day 0. Second, DA synthesis gene expression levels were extraordinarily high in the No. 1 larva at day 0–1 after appearance. Finally, injection of a DA receptor antagonist into the No. 1 larva resulted in the inhibition of presoldier differentiation. Behavioural observations of the antagonist or control-injected larvae suggested that brain DA and signalling activity regulate the frequencies of trophallaxis from reproductives and presoldier differentiation. Because trophallaxis is a social behaviour frequently observed in natural conditions, the role of DA should be investigated in other social insects with frequent trophallactic and allogrooming behaviour.
Highlights
Phenotypic plasticity, which is defined as the ability to express adaptive phenotypes in response to variable environmental conditions from a single genotype, may drive adaptive evolution [1]
We focused on the ontogeny of the No 1 larvae in incipient colonies of Z. nevadensis to examine the proximate mechanisms of presoldier differentiation
The results showed that the frequencies of proctodeal trophallactic behaviour from both reproductives to the No 1 larvae were decreased by the injection of a DA receptor antagonist
Summary
Phenotypic plasticity, which is defined as the ability to express adaptive phenotypes in response to variable environmental conditions from a single genotype, may drive adaptive evolution [1]. Polyphenism is an extreme case of phenotypic plasticity, and alternative distinct phenotypes are produced depending on extrinsic factors. Polyphenism normally involves an alteration in behaviours related to life history as well as morphological. There are several important studies on the neurochemical basis of behavioural changes in polyphenisms [3,4], the regulatory mechanisms of behavioural changes accompanying the morphological alterations are still unknown
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