The evolution of body size in termites.

Nobuaki Mizumoto,Thomas Bourguignon

doi:10.1098/rspb.2021.1458

Abstract

Termites are social cockroaches. Because non-termite cockroaches are larger than basal termite lineages, which themselves include large termite species, it has been proposed that termites experienced a unidirectional body size reduction since they evolved eusociality. However, the validity of this hypothesis remains untested in a phylogenetic framework. Here, we reconstructed termite body size evolution using head width measurements of 1638 modern and fossil termite species. We found that the unidirectional body size reduction model was only supported by analyses excluding fossil species. Analyses including fossil species suggested that body size diversified along with speciation events and estimated that the size of the common ancestor of modern termites was comparable to that of modern species. Our analyses further revealed that body size variability among species, but not body size reduction, is associated with features attributed to advanced termite societies. Our results suggest that miniaturization took place at the origin of termites, while subsequent complexification of termite societies did not lead to further body size reduction.

Highlights

Body size diversification is an indicator of ecological diversification [1,2,3]
By fitting various evolutionary models for termite body size evolution, we found that the inclusion of fossil species had large effects on the results
After accounting for the phylogenetic relationship among taxa, we found that the mean head width disparity between imagoes and soldiers was larger in genera with true workers than in genera with pseudegates (PGLS, λ, κ, δ transformation, F2 = 11.32, p = 0.001; figure 4c), and that soldiers were on average larger than imagoes in genera with pseudergates, but not in genera with true workers (PGLS, intercept, with pseudergates: t = −4.790, p < 0.001, with true workers: t = −0.255, p = 0.799; figure 4c)

Summary

Introduction

Body size diversification is an indicator of ecological diversification [1,2,3]. Diversification occurs when new resources or niches become available [4,5], often leading to the evolution of new phenotypes (i.e. key innovations [6,7]). Species with a forked developmental pathway possess a caste of ‘true workers’ that irreversibly deviates from the imaginal developmental line at an early developmental stage and cannot moult into alate imagoes, they are still able to reproduce in most lower termite species and some higher termite species [27] Owing to this additional caste, species with true workers have increased phenotypic and behavioural plasticity [28], potentially allowing for the evolution of more complex social systems. We examined whether characteristics traditionally attributed to complex termite societies, including separate-piece nesting strategy, the presence of a true worker caste and large colony size, are linked to body size evolution. We investigated how termite social evolution has shaped body size variation among castes, including body size variation among alate imagoes, soldiers and working castes (pseudergates or true workers)

Methods

Findings

Conclusion