Abstract
Eusocial insects exhibit reproductive division of labor, in which only a fraction of colony members differentiate into reproductives. In termites, reproductives of both sexes are present in a colony and constantly engaged in reproduction. It has been suggested that the sex ratio of reproductives is maintained by social interactions. The presence of reproductives is known to inhibit the additional differentiation of same-sex reproductives, while it promotes the differentiation of opposite-sex reproductives. In this study, using the damp-wood termite Hodotermopsis sjostedti, physiological effects of male/female reproductives on the differentiation of supplementary reproductives (neotenics) were examined. The results showed that the only male-neotenic condition, i.e., the presence of male neotenics in the absence of female neotenics, accelerated the neotenic differentiation from female workers (i.e., pseudergates). Under this condition, the rise of juvenile hormone (JH) titer was repressed in females, and the application of a JH analog inhibited the female neotenic differentiation, indicating that the low JH titer leads to rapid differentiation. Thus, the only male-neotenic condition that actively promotes reproductive differentiation by manipulating physiological condition of females is suggested to be a mechanism underlying sexual asymmetry in reproductive function, which may lead the female-biased sex allocation of reproductives.
Highlights
Eusocial insects construct sophisticated social systems via the reproductive division of labor, in which only a fraction of colony members occupy reproductive status, while the others, known as workers, are engaged in non-reproductive roles such as brooding, maintaining nest, and foraging[1]
Our previous study showed that the differentiation ratio of female neotenics was increased by the presence of an opposite-sex reproductive and established the efficient induction method of neotenic differentiation[15] (Fig. 1b)
By applying the induction method, the results in this study examining the period to the molt and the differentiation ratio into neotenics clearly showed that the presence of reproductives retarded the differentiation of additional neotenics from the same sex, while it accelerated the differentiation from the opposite sex (Figs. 2, S1)
Summary
Eusocial insects construct sophisticated social systems via the reproductive division of labor, in which only a fraction of colony members occupy reproductive status, while the others, known as workers, are engaged in non-reproductive roles such as brooding, maintaining nest, and foraging[1]. In termites (order Blattodea, superfamily Termitoidea), little is known about the mechanisms that regulate the number of reproductives It has been suggested, though, that the regulatory mechanisms underlying reproductive differentiation in termites differ from those in hymenopterans, since their respective social and developmental systems are so different[4]. The pattern of JH-titer transition is thought to be critical in the differentiation of neotenic reproductives[16,24], the relationship between the presence of reproductives and physiological conditions of colony members has yet to be elucidated. In this study using the damp-wood termite Hodotermopsis sjostedti, which is thought to possess ancestral characteristics such as linear caste developmental pathways[25,26], we investigated the physiological mechanism underlying the additional differentiation of neotenics induced by pre-existing reproductives of both sexes. The JH-titer transitions during the induction periods were quantified and compered among conditions
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