Abstract
Although indirect selection through relatives (kin selection) can explain the evolution of effectively sterile offspring that act as helpers at the nest (eusociality) in the ants, bees, and stinging wasps (aculeate Hymenoptera), the genetic, ecological, and life history conditions that favor transitions to eusociality are poorly understood. In this study, ancestral state reconstruction on recently published phylogenies was used to identify the independent transitions to eusociality in each of the taxonomic families that exhibit eusociality. Semisociality, in which a single nest co-foundress monopolizes reproduction, often precedes eusociality outside the vespid wasps. Such a route to eusociality, which is consistent with groups consisting of a mother and her daughters (subsocial) at some stage and ancestral monogamy, is favored by the haplodiploid genetic sex determination of the Hymenoptera (diploid females and haploid males) and thus may explain why eusociality is common in the Hymenoptera. Ancestral states were also reconstructed for life history characters that have been implicated in the origins of eusociality. A loss of larval diapause during unfavorable seasons or conditions precedes, or coincides with, all but one transition to eusociality. This pattern is confirmed using phylogenetic tests of associations between state transition rates for sweat bees and apid bees. A loss of larval diapause may simply reflect the subsocial route to eusociality since subsociality is defined as females interacting with their adult daughters. A loss of larval diapause and a gain of subsociality may be associated with an extended breeding season that permits the production of at least two broods, which is necessary for helpers to evolve. Adult diapause may also lower the selective barrier to a first-brood daughter becoming a helper. Obligate eusociality meets the definition of a major evolutionary transition, and such transitions have occurred five times in the Hymenoptera.
Highlights
IntroductionThe evolution of an effectively sterile worker caste, known as eusociality (see Table 1 for definitions of social structures), has occurred many times in the aculeate Hymenoptera (ants, bees, and stinging wasps), but explaining these transitions remains a challenge
The evolution of an effectively sterile worker caste, known as eusociality, has occurred many times in the aculeate Hymenoptera, but explaining these transitions remains a challenge
A semisocial route to eusociality is consistent with a subsocial route and with ancestral functional monogyny and, because semisociality is favored by haplodiploidy, may explain why eusociality is common in the Hymenoptera
Summary
The evolution of an effectively sterile worker caste, known as eusociality (see Table 1 for definitions of social structures), has occurred many times in the aculeate Hymenoptera (ants, bees, and stinging wasps), but explaining these transitions remains a challenge. Sterile workers are understood to evolve through kin selection (Hamilton, 1964a,b; see Table 2 for definitions of key life history terms), the genetic, ecological, and life history conditions favoring reproductive altruism remain poorly understood (Bourke, 2019; Field and Toyoizumi, 2020). Parasocial — Females of the same generation interact on the same nest. Eusocial — Multiple females cooperate in nesting and exhibit reproductive division of labor and there is an overlap of generations so that adult offspring assist their mother. Solitary — Females nest alone and mass provision their nests. They do not interact with their developing young
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