Skewed soldier sex ratio in termites: testing the size-threshold hypothesis

Abstract

Social insect colonies contain multiple phenotypes, i.e. castes, and this caste polyphenism is often linked to sexual dimorphism. Unlike social hymenopterans, both termite sexes are diploid and contribute to helper-type tasks within the colony. Nonetheless, a biased sex ratio is frequently observed in termites, especially in soldiers. To explain this bias in soldier sex ratio, Matsuura (2006) postulated the existence of a size threshold for workers molting into soldiers. Under the influence of sexual size dimorphism (SSD), such a threshold was considered to indirectly favor one sex. We conducted a literature survey of sex ratio among termite soldiers and tested this size-threshold hypothesis using data for 67 termite species from a variety of termite lineages. We demonstrated the existence of a size threshold for individuals molting into soldiers, resulting in the acquisition of soldiers of only one sex in species exhibiting strong SSD. In species exhibiting weak SSD, the size threshold skews the sex ratio of soldiers, but does not necessarily cause the loss of one sex. Finally, we observed a prevalence of single-sex soldiers in the Termitidae, regardless of SSD, suggesting that the ancestral developmental mechanisms that constrain soldier differentiation from one sex are maintained in certain extant species.

Secondary queens in the parthenogenetic termite Cavitermes tuberosus develop through a transitional helper stage

“…Sexual specialization in soldiers is a common feature in termites (Bourguignon, Hayashi, & Miura, ; Noirot, ). In species with phragmotic defence, in which soldiers use their large head as a plug to block the tunnels (Eggleton, ), a proposed hypothesis is that there is a size threshold below which workers cannot develop into soldiers.…”

“…Therefore, species with sexual size dimorphism (SSD) in the neuter castes should display a soldier sex ratio biased in favor of the larger sex (Matsuura, ). While this hypothesis holds for many lower termites, unisexual soldiers are common across Termitidae species that do not display SSD (Bourguignon et al, ). Accordingly, our results reveal an absence of sexual dimorphism in larvae and workers of C. tuberosus .…”

“…In T. hospes, the apterous line is composed of three worker instars, and soldier development initiates only from females of the first worker instar (Noirot, 1955), showing that this developmental scheme is conserved among the Termes-group (sensu Jones & Eggleton, 2011). Sexual specialization in soldiers is a common feature in termites (Bourguignon, Hayashi, & Miura, 2012;Noirot, 1955). In species with phragmotic defence, in which soldiers FIGURE 2 Nymph-like individuals of C. tuberosus with soil-filled gut and translucent wingbuds "aspirants."…”

Caste differentiation pathways in the Neotropical termite Silvestritermes euamignathus (Isoptera: Termitidae)

“…Bourguignon et al . () tested this hypothesis, and observed that while the hypothesis was held for many lower termite species, multiple species of Termitidae did not exhibit SSD but presented a highly skewed soldier sex ratio. This is the case for S. euamignathus .…”

Building a new research framework for social evolution: intralocus caste antagonism

“…In Reticulitermes speratus a sex-linked locus with two alleles is responsible for caste determination, where the same allele that causes males to develop into reproductives also causes females to develop into workers; whereas the other allele causes males to develop into workers and females to develop into reproductives (Hayashi et al , 2007). In a different study, it was suggested that termite species with greater levels of sexual size dimorphism tend to have workers of a single sex that are more specialised (Bourguignon, Yoshinobu & Miura, 2012). These examples suggest that sexual dimorphism might have enabled functional specialisation of worker castes, as predicted if mechanisms to resolve one conflict act to mitigate the other.…”