Evolution of sociality in insects: insights from the only known social colletid bee Amphylaeus morosus

Author: Lucas Hearn

Hearn, Lucas, 2022 Evolution of sociality in insects: insights from the only known social colletid bee Amphylaeus morosus, Flinders University, College of Science and Engineering

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Understanding how social behaviour first evolves requires that we can identify species that have only recently evolved social traits. I investigated the transition from solitary to social living using the only unambiguously known social species in the bee family Colletidae, Amphylaeus morosus (Smith, 1879).

Ecological factors such as parasitism and predation have been emphasised as important facilitators promoting cooperative nesting. Here I detail the host-parasite associations of A. morosus with eight different parasitoid species in the Dandenong Ranges, Victoria, and how these parasitoids may have influenced social nesting dynamics of their host. Targeted sampling of A. morosus nests across the reproductive season revealed that these eight parasitoid species have staggered timings of attack. I found that as the reproductive season progressed, the number of host adults in a nest declined, often to zero, but the presence of even one adult host female during late brood-rearing stages appeared to offer substantial brood protection against mutillid wasps. These temporal patterns in parasitism and host colony size mean that the benefits of initial colony size may not be evident until much later in the reproductive season as adult colony members gradually die, leading to a temporal dissonance between early and late stages of the reproductive season. Understanding these relationships may provide insights into social evolution that have not been previously explored in studies that take temporal ‘snapshot’ measures of how group size affects colony productivity.

Using genome-wide SNP genotyping, I inferred robust pedigree relationships to identify maternity of brood and intracolony relatedness for colonies of A. morosus at the end of the reproductive season. I show that social behaviour in this species involves the formation of both matrifilial and full-sibling colonies with either complete or near-complete monopolization of reproduction in absence of morphological hierarchies. My results suggest that secondary females gain large indirect fitness benefits from any defensive outcomes and these benefits satisfy the conditions of kin selection. These results suggest an avenue to eusociality that involves high relatedness and, very surprisingly, extreme reproductive skew in its earliest stages.

Finally, I show that sex allocation in this species is defined by split sex ratios between solitary and social nests. Socially nesting mothers produced more sons when a non-reproductive nest guard remained in the natal nest whilst solitary foundresses biased their investment towards daughters. These results provide evidence that population sex ratios are indirectly influenced by decisions by some females to become guards although guards remaining in their natal nest are not able to actively manipulate sex ratios in their favour. This pattern of male biased allocation in social nests could raise the threshold for social nesting to spread throughout the population, which contrasts previous views of split sex ratios.

This thesis demonstrates that sociality in A. morosus is strongly driven by defence against parasitism. Social behaviour in this species is consistent with kin selection theory, but this form of social nesting does not appear to promote a high frequency of social colonies. These findings challenge previous notions of the major transition from solitary to social living in insects.

Keywords: Social evolution, Inclusive fitness theory, Eusociality, Bee, Hymenoptera, Parasitism

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2022
School: College of Science and Engineering
Supervisor: Michael Schwarz