Social interactions affecting caste development through physiological actions in termites

Abstract

A colony of social insects is not only an aggregation of individuals but also a functional unit. To achieve adaptive social behavior in fluctuating environmental conditions, in addition to coordination of physiological status in each individual, the whole colony is coordinated by interactions among colony members. The study on the regulation of social-insect colonies is termed “social physiology.” Termites, a major group of social insects, exhibit many interesting phenomena related to social physiology, such as mechanisms of caste regulation in a colony. In their colonies, there are different types of individuals, i.e., castes, which show distinctive phenotypes specialized in specific colony tasks. Termite castes comprise reproductives, soldiers and workers, and the caste composition can be altered depending on circumstances. For the regulation of caste compositions, interactions among individuals, i.e., social interactions, are thought to be important. In this article, we review previous studies on the adaptive meanings and those on the proximate mechanisms of the caste regulation in termites, and try to understand those comprehensively in terms of social physiology. Firstly, we summarize classical studies on the social interactions. Secondly, previous studies on the pheromone substances that mediate the caste regulatory mechanisms are overviewed. Then, we discuss the roles of a physiological factor, juvenile hormone (JH) in the regulation of caste differentiation. Finally, we introduce the achievements of molecular studies on the animal sociality (i.e., sociogenomics) in terms of social physiology. By comparing the proximate mechanisms of social physiology in termites with those in hymenopterans, we try to get insights into the general principles of social physiology in social animals.

Compound eye formation in the termite Incisitermes minor (Isoptera: Kalotermitidae)

“…Observations from this study failed to produce characteristics separating late I. minor larvae in to either a worker or nymphal caste, consistent with the notion that I. minor follows a basically linear pathway (Cabrera and Rust 1999). Thus, late larvae would act as workers (reviewed in Watanabe et al 2014). As observations of molts among late-larval workers would be necessary to describe individual developmental plasticity, pseudergates could not be effectively identified in this report.…”

“…1). This observation also demonstrates the exceptional developmental plasticity seen within this clade (reviewed in Watanabe et al 2014). …”

“…Multiple reports have demonstrated that social physiology and environmental factors both play key roles in caste-related polyphenism and differentiation in termites (reviewed in Watanabe et al 2014). Thus, most groups employ statistical analysis comparing measurements of specific body parts from every individual in an existing colony when describing casterelated developmental pathways (e.g., Okot-Kotber 1985;Roisin andPasteels 1986, 1991;Noirot and Pasteels 1987;Lainé and Wright 2003;Miura et al 1998Miura et al , 2000Roisin and Lenz 1999;Koshikawa et al 2002;Hanus et al 2006;Katoh et al 2007;Neoh and Lee 2009;Roisin and Korb 2011).…”

“…For example, in termites that follow a bifurcated pathway, late larvae begin to show signs of differentiation into either a worker (neuter) or nymphal (reproductive) caste. However, in many termite species that follow a basically linear pathway, late larvae act as workers and remarkable developmental plasticity has been observed in particular larvae known as Pseudergates (reviewed in Watanabe et al 2014). Pseudergates are individual larvae that have been reported to depart from the traditional progressive developmental pathway via molts that either preserve their present developmental stage as a worker, or realign them toward a different caste position.…”

Expressions of juvenile hormone biosynthetic genes during presoldier differentiation in the incipient colony of Zootermopsis nevadensis (Isoptera: Archotermopsidae)

“…Caste differentiation in termites is an example of polyphenism; alternative phenotypes are generated through changes in the developmental process in response to environmental cues during the postembryonic period (Hartfelder and Emlen 2012;Miura 2005;Nijhout 2003;Roisin 2000;Watanabe et al 2014). Generally, in insects, different phenotypes resulting from alterations in developmental processes are regulated by the endocrine system (Hartfelder and Emlen 2012;Nijhout 1999).…”