Variance-based selection may explain general mating patterns in social insects

Colony fitness increases in the honey bee at queen mating frequencies higher than genetic diversity asymptote

Abstract

Across the eusocial Hymenoptera, a queen’s mating frequency is positively associated with her workers’ genetic diversity and colony’s fitness. Over 90% of a colony’s diversity potential is achieved by its mother’s tenth effective mating (me); however, many females mate at levels of me > 10, a zone we here call hyperpolyandry. We compared honey bee colony fitness at mating levels near and above this genetic diversity asymptote. We were interested in how hyperpolyandry affects colony phenotypes arising from both common tasks (brood care) and rare specialized tasks (parasite resistance). We used an unselected wild line of bees and a Varroa Sensitive Hygiene (VSH) line selected to resist the parasite Varroa destructor. Virgin queens were instrumentally inseminated to replicate the following queen/colony conditions: (1) VSH semen/low polyandry (observed mating number = mo = 9), (2) VSH semen/high polyandry (mo = 54), (3) wild type semen/low polyandry, or (4) wild semen/high polyandry. There was a positive effect of polyandry on brood survival, an outcome of common tasks, with highest values at mo = 54. There was an interaction between polyandry and genetics such that differences between genetic lines expressed only at mo = 54, with fewer mites in VSH colonies. These results are consistent with two hypotheses for the evolution of mating levels in excess of the genetic diversity asymptote: hyperpolyandry improves colony fitness by (1) optimizing genotype compositions for common tasks and (2) by capturing rare specialist allele combinations, resisting cliff-edge ecological catastrophes.

Significance statement

Polyandry is a female’s practice of mating with several males, storing their sperm, and using it to produce one or more clutches of genetically diverse offspring. In the social Hymenoptera, polyandry increases the genetic diversity and task efficiency of workers, leading to improved colony fitness. Over 90% of the increase in a colony’s diversity potential is achieved by its mother’s tenth mating; however, many females practice hyperpolyandry, a term we reserve here for mating levels above this genetic diversity asymptote. We show that a token of colony fitness arising from common tasks, brood survival, improves universally as one moves from sub- to hyperpolyandrous mating levels. However, a colony phenotype arising from a rare parasite resistance task is only expressed in the presence of the controlling alleles and under conditions of hyperpolyandry. These results suggest adaptive mechanisms by which hyperpolyandry could evolve.

Queen Quality and the Impact of Honey Bee Diseases on Queen Health: Potential for Interactions between Two Major Threats to Colony Health

Western honey bees, Apis mellifera, live in highly eusocial colonies that are each typically headed by a single queen. The queen is the sole reproductive female in a healthy colony, and because long-term colony survival depends on her ability to produce a large number of offspring, queen health is essential for colony success. Honey bees have recently been experiencing considerable declines in colony health. Among a number of biotic and abiotic factors known to impact colony health, disease and queen failure are repeatedly reported as important factors underlying colony losses. Surprisingly, there are relatively few studies on the relationship and interaction between honey bee diseases and queen quality. It is critical to understand the negative impacts of pests and pathogens on queen health, how queen problems might enable disease, and how both factors influence colony health. Here, we review the current literature on queen reproductive potential and the impacts of honey bee parasites and pathogens on queens. We conclude by highlighting gaps in our knowledge on the combination of disease and queen failure to provide a perspective and prioritize further research to mitigate disease, improve queen quality, and ensure colony health.

Convergent Reversion to Single Mating in a Wasp Social Parasite

While eusociality arose in species with single-mating females, multiple mating by queens has evolved repeatedly across the social ants, bees, and wasps. Understanding the benefits and costs of multiple mating of queens is important because polyandry results in reduced relatedness between siblings, reducing kin-selected benefits of helping while also selecting for secondary social traits that reduce intracolony conflict. The leading hypothesis for the benefits of polyandry in social insects emphasizes advantages of a genetically diverse workforce. Workerless social parasite species (inquilines) provide a unique opportunity to test this hypothesis, since they are derived from social ancestors but do not produce workers of their own. Such parasites are thus predicted to evolve single mating because they would experience the costs of multiple mating but not the benefits if such benefits accrue through the production of a genetically diverse group of workers. Here we show that the workerless social parasite Dolichovespula arctica, a derived parasite of wasps, has reverted to obligate single mating from a facultatively polyandrous ancestor, mirroring a similar reversion from obligate polyandry to approximate monandry in a social parasite of fungus-farming ants. This finding and a comparison with two other cases where inquilinism did not induce reversal to monandry support the hypothesis that facultative polyandry can be costly and may be maintained by benefits of a genetically diverse workforce.

Genetic diversity confers colony-level benefits due to individual immunity

Several costs and benefits arise as a consequence of eusociality and group-living. With increasing group size, spread of disease among nest-mates poses selective pressure on both individual immunity and group-level mechanisms of disease resistance (social immunity). Another factor known to influence colony-level expression of disease is intracolony genetic diversity, which in honeybees (Apis mellifera) is a direct function of the number of mates of the queen. Colonies headed by queens with higher mating numbers have less variable infections of decreased intensity, though the underlying mechanisms remain unclear. By pathogen-challenging larvae in vitro, we decoupled larval immune response from mechanisms of social immunity. Our results show that baseline immunity and degree of immune response do not vary with genetic diversity. However, intracolony variance in antimicrobial peptide production after pathogen challenge decreases with increasing genetic diversity. This reduction in variability of the larval immune response could drive the mitigation of disease observed in genetically diverse colonies.

Geographic variation in polyandry of the Eastern Honey Bee, Apis cerana, in Thailand

The repeated evolution of extreme polyandry in advanced social insects is exceptional and its explanation has attracted significant attention. However, most reported estimates of the number of matings are derived from limited sampling. Temporal and geographic variation in mating behavior of social insects has not been sufficiently studied. Worker offspring of 18 Eastern Honey Bee (Apis cerana Fabr.) queens from three populations across Thailand were genotyped at five microsatellite markers to test for population differences of mating behavior across three different ecosystems. The number of matings decreased from a northern, more seasonal environment to a southern tropical population and was lowest in a tropical island population. Our study confirms earlier findings that social insect mating behavior shows biogeographic variation and highlights that data from several populations are needed for reliable species-specific estimates of the number of matings. Detailed studies of populations that show significant differentiation in the number of matings may be able to discriminate effectively among the different hypotheses that have been proposed to explain the evolution of polyandry in honey bees and other advanced social insects.

Multiple mating but not recombination causes quantitative increase in offspring genetic diversity for varying genetic architectures

Explaining the evolution of sex and recombination is particularly intriguing for some species of eusocial insects because they display exceptionally high mating frequencies and genomic recombination rates. Explanations for both phenomena are based on the notion that both increase colony genetic diversity, with demonstrated benefits for colony disease resistance and division of labor. However, the relative contributions of mating number and recombination rate to colony genetic diversity have never been simultaneously assessed. Our study simulates colonies, assuming different mating numbers, recombination rates, and genetic architectures, to assess their worker genotypic diversity. The number of loci has a strong negative effect on genotypic diversity when the allelic effects are inversely scaled to locus number. In contrast, dominance, epistasis, lethal effects, or limiting the allelic diversity at each locus does not significantly affect the model outcomes. Mating number increases colony genotypic variance and lowers variation among colonies with quickly diminishing returns. Genomic recombination rate does not affect intra- and inter-colonial genotypic variance, regardless of mating frequency and genetic architecture. Recombination slightly increases the genotypic range of colonies and more strongly the number of workers with unique allele combinations across all loci. Overall, our study contradicts the argument that the exceptionally high recombination rates cause a quantitative increase in offspring genotypic diversity across one generation. Alternative explanations for the evolution of high recombination rates in social insects are therefore needed. Short-term benefits are central to most explanations of the evolution of multiple mating and high recombination rates in social insects but our results also apply to other species.

Evolution of self-organized division of labor in a response threshold model

Division of labor in social insects is determinant to their ecological success. Recent models emphasize that division of labor is an emergent property of the interactions among nestmates obeying to simple behavioral rules. However, the role of evolution in shaping these rules has been largely neglected. Here, we investigate a model that integrates the perspectives of self-organization and evolution. Our point of departure is the response threshold model, where we allow thresholds to evolve. We ask whether the thresholds will evolve to a state where division of labor emerges in a form that fits the needs of the colony. We find that division of labor can indeed evolve through the evolutionary branching of thresholds, leading to workers that differ in their tendency to take on a given task. However, the conditions under which division of labor evolves depend on the strength of selection on the two fitness components considered: amount of work performed and on worker distribution over tasks. When selection is strongest on the amount of work performed, division of labor evolves if switching tasks is costly. When selection is strongest on worker distribution, division of labor is less likely to evolve. Furthermore, we show that a biased distribution (like 3:1) of workers over tasks is not easily achievable by a threshold mechanism, even under strong selection. Contrary to expectation, multiple matings of colony foundresses impede the evolution of specialization. Overall, our model sheds light on the importance of considering the interaction between specific mechanisms and ecological requirements to better understand the evolutionary scenarios that lead to division of labor in complex systems.

Within- and across-colony effects of hyperpolyandry on immune function and body condition in honey bees (Apis mellifera)

Honey bees (Apis mellifera) have become a model system for studies on the influence of genetic diversity on disease. Honey bee queens mate with a remarkably high number of males-up to 29 in the current study-from which they produce a colony of genetically diverse daughter workers. Recent evidence suggests a significant benefit of intracolony genetic diversity on disease resistance. Here, we explored the relationship between the level of genetic diversity and multiple physiological mechanisms of cellular and humoral immune defense (encapsulation response and phenoloxidase activity). We also investigated an effect of genetic diversity on a measure of body condition (fat body mass). While we predicted that mean colony phenoloxidase activity, encapsulation response, and fat body mass would show a positive relationship with increased intracolonial genetic diversity, we found no significant relationship between genetic diversity and these immune measures, and found no consistent effect on body condition. These results suggest that high genetic diversity as a result of extreme polyandry may have little bearing on the physiological mechanisms of immune function at naturally occurring mating levels in honey bees.

Strict monandry in the ponerine army ant genus Simopelta suggests that colony size and complexity drive mating system evolution in social insects

Altruism in social insects has evolved between closely related full-siblings. It is therefore of considerable interest why some groups have secondarily evolved low within-colony relatedness, which in turn affects the relatedness incentives of within-colony cooperation and conflict. The highest queen mating frequencies, and therefore among the lowest degrees of colony relatedness, occur in Apis honeybees and army ants of the subfamilies Aenictinae, Ecitoninae, and Dorylinae, suggesting that common life history features such as reproduction by colony fission and male biased numerical sex-ratios have convergently shaped these mating systems. Here we show that ponerine army ants of the genus Simopelta, which are distantly related but similar in general biology to other army ants, have strictly monandrous queens. Preliminary data suggest that workers reproduce in queenright colonies, which is in sharp contrast to other army ants. We hypothesize that differences in mature colony size and social complexity may explain these striking discrepancies.

Comparative linkage mapping suggests a high recombination rate in all honeybees

Meiotic recombination is required for proper chromosome assortment, and accordingly, 1-2 chiasmata per chromosome are found in most species. However, observed recombination rates deviate in some cases from neutral expectations between and within genomes and may play an important role in adaptive evolution. One potentially important argument for an adaptive evolution of recombination rates is the exceptionally high genome-wide recombination rates of social Hymenoptera, in particular the Western honeybee, Apis mellifera. It has the highest metazoan recombination rate reported so far. Proximate or ultimate causes for this elevated recombination rate have not yet been resolved. In a comparative study, we investigated meiotic recombination in the red dwarf honeybee Apis florea. Microsatellite markers developed for A. mellifera were genotyped in a natural mapping population of A. florea. From these genotypes, we calculated local recombination rates, using the physical distances from A. mellifera. In addition to a few comparisons of intervals across the genome, we particularly focused on chromosomes 3 and 12. Confirming marker synteny, we found that recombination rates in A. florea are as high as or higher than those in A. mellifera. Our results are limited to select genomic regions but suggest that A. florea also exhibits an exceptionally high genome-wide recombination rate. This trait may thus occur genus wide. Although our study cannot identify a single explanation for the high rates of recombination in Apis, it favors hypotheses that apply to the entire genus. Furthermore, we conclude that the genome structure of the 2 species has been largely conserved, at least in the parts we investigated.