Temperate-zone socially polymorphic sweat bees (Hymenoptera: Halictidae) are ideal model systems for elucidating the origins of eusociality, a major evolutionary transition. Bees express either social or solitary behaviour in different parts of their range, and social phenotype typically correlates with season length. Despite their obvious utility, however, socially polymorphic sweat bees have received relatively little attention with respect to understanding the origins of eusociality. Lasioglossum (Evylaeus) calceatum is a widespread sweat bee that is thought to be socially polymorphic, with important potential as an experimental model species. We first determined the social phenotype of L. calceatum at three sites located at different latitudes within the UK. We then investigated sociality in detail across two years at the southernmost site. We found that L. calceatum exhibits latitudinal social polymorphism within the UK; bees were solitary at our two northern sites but the majority of nests were social at our southern site. Sociality in the south was characterised by a relatively small mean of two and 3.5 workers per nest in each year, respectively, and a small to medium mean caste-size dimorphism of 6.6 %. Foundresses were smaller in our more northern and high altitude populations. Sociality is clearly less specialised than in some closely related obligately social species but probably more specialied than other polymorphic sweat bees. Our research provides a starting point for future experimental work to investigate mechanisms underlying social polymorphism in L. calceatum.

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Eusociality is characterised by a reproductive division of labour, where some individuals forgo direct reproduction to instead help raise kin. Socially polymorphic sweat bees are ideal models for addressing the mechanisms underlying the transition from solitary living to eusociality, because different individuals in the same species can express either eusocial or solitary behaviour. A key question is whether alternative social phenotypes represent environmentally induced plasticity or predominantly genetic differentiation between populations. In this paper, we focus on the sweat bee Lasioglossum calceatum, in which northern or high-altitude populations are solitary, whereas more southern or low-altitude populations are typically eusocial. To test whether social phenotype responds to local environmental cues, we transplanted adult females from a solitary, northern population, to a southern site where native bees are typically eusocial. Nearly all native nests were eusocial, with foundresses producing small first brood (B1) females that became workers. In contrast, nine out of ten nests initiated by transplanted bees were solitary, producing female offspring that were the same size as the foundress and entered directly into hibernation. Only one of these ten nests became eusocial. Social phenotype was unlikely to be related to temperature experienced by nest foundresses when provisioning B1 offspring, or by B1 emergence time, both previously implicated in social plasticity seen in two other socially polymorphic sweat bees. Our results suggest that social polymorphism in L. calceatum predominantly reflects genetic differentiation between populations, and that plasticity is in the process of being lost by bees in northern populations.Significance statementPhenotypic plasticity is thought to play a key role in the early stages of the transition from solitary to eusocial behaviour, but may then be lost if environmental conditions become less variable. Socially polymorphic sweat bees exhibit either solitary or eusocial behaviour in different geographic populations, depending on the length of the nesting season. We tested for plasticity in the socially polymorphic sweat bee Lasioglossum calceatum by transplanting nest foundresses from a northern, non-eusocial population to a southern, eusocial population. Plasticity would be detected if transplanted bees exhibited eusocial behaviour. We found that while native bees were eusocial, 90% of transplanted bees and their offspring did not exhibit traits associated with eusociality. Environmental variables such as time of offspring emergence or temperatures experienced by foundresses during provisioning could not explain these differences. Our results suggest that the ability of transplanted bees to express eusociality is being lost, and that social polymorphism predominantly reflects genetic differences between populations.Electronic supplementary materialThe online version of this article (10.1007/s00265-018-2475-9) contains supplementary material, which is available t...

Abstract. 1. Annual insects are predicted to grow larger where the growing season is longer. However, transitions from one to two generations per year can occur when the season becomes sufficiently long, and are predicted to result in a sharp decrease in body size because available development time is halved. The potential for resulting saw-tooth clines has been investigated only in solitary taxa with free-living larvae.2. Size clines were investigated in two socially polymorphic sweat bees (Halictidae): transitions between solitary and social nesting occur along gradients of increasing season length, characterised by the absence or presence of workers and offspring that are individually mass provisioned by adults. How the body size changes with season length was examined, and whether transitions in social phenotype generate saw-tooth size clines. We measured Lasioglossum calceatum and Halictus rubicundus nest foundresses originating from more than 1000 km of latitude, encompassing the transition between social and solitary nesting.3. Using satellite-collected temperature data to estimate season length, it was shown that both species were largest where the season was longest. Body size increased linearly with season length in L. calceatum and non-linearly in H. rubicundus but the existence of saw-tooth clines was not supported.4. The present results suggest that because the amount of food consumed by offspring during development is determined by adults, environmental and social influences on the provisioning strategies of adult bees may be more important factors than available feeding time in determining offspring body size in socially polymorphic sweat bees.

Understanding the ecological and environmental contexts in which eusociality can evolve is fundamental to elucidating its evolutionary origins. A sufficiently long active season is postulated to have been a key factor facilitating the transition to eusociality. Many primitively eusocial species exhibit an annual life cycle, which is thought to preclude the expression of eusociality where the active season is too short to produce successive worker and reproductive broods. However, few studies have attempted to test this idea experimentally. We investigated environmental constraints on the expression of eusociality in the obligate primitively eusocial sweat bee Lasioglossum malachurum, by transplanting nest foundresses from the south to the far north of the United Kingdom, far beyond the natural range of L. malachurum. We show that transplanted bees can exhibit eusociality, but that the short length of the season and harsher environmental conditions could preclude its successful expression. In one year, when foundresses were transplanted only after provisioning first brood (B1) offspring, workers emerged in the north and provisioned a second brood (B2) of reproductives. In another year, when foundresses were transplanted prior to B1 being provisioned, they were just as likely to initiate nesting and provisioned just as many B1 cells as foundresses in the south. However, the life cycle was delayed by approximately 7 weeks and nests suffered 100% B1 mortality. Our results suggest that short season length together with poor weather conditions represent an environmental barrier to the evolution and expression of eusociality in sweat bees.Electronic supplementary materialThe online version of this article (10.1007/s00040-018-0642-7) contains supplementary material, which is available to authorized users.

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