Individual and social heterosis act independently in honey bee (Apis mellifera) colonies

Abstract

Heterosis occurs in individuals when genetic diversity, e.g., heterozygosity, increases fitness. Many advanced eusocial insects evolved mating behaviors, including polyandry and polygyny, which increase inter-individual genetic diversity within colonies. The possibility of this structure of diversity to improve group fitness has been termed social heterosis. Neither the independence of individual and social heterosis nor their relative effect sizes have been explicitly measured. Through controlled breeding between pairs of Western honey bee queens (Apis mellifera L.; n = 3 pairs) from two distinct populations, we created inbred colonies with low genetic diversity, hybrid colonies with high heterozygosity, and mixed colonies (combining inbred workers from each population) with low heterozygosity and high social diversity. We then quantified two independent traits in colonies: survival against bacterial challenge and maintenance of brood nest temperature. For both traits, we found hybrid and mixed colonies outperformed inbred colonies but did not perform differently from each other. During immune challenge assays, hybrid and mixed colonies experienced hazard ratios of 0.49 (95% CI [0.37, 0.65]) and 0.69 (95% CI [0.50, 0.96]) compared to inbred colonies. For nest temperatures, hybrid and mixed colonies experienced 1.94 ± 0.97 °C and 2.82 ± 2.46 °C less thermal error and 0.14 ± 0.11 °C² and 0.16 ± 0.06 °C² less thermal variance per hour than inbred lines. This suggests social and individual heterosis operate independently and may have similar effect sizes. These results highlight the importance of both inter- and intra-individual diversity to fitness, which may help explain the emergence of polyandry/polygyny in eusocial insects and inform breeding efforts in these systems.

Introduction

Across living organisms, genetic diversity is important in maintaining fitness. This has been invoked to explain the ubiquity and success of sexual reproduction (Haldane 1949; Bell 1982). Despite the potential costs of sex (Lehtonen et al. 2012), the increased genetic diversity it generates helps individuals resist parasites, pathogens, and environmental change (Hamilton et al. 1990; Schmid-Hempel and Schmid-Hempel 1998; Hughes et al. 2008). Social organisms are no exception, and “advanced” eusocial species with distinct sterile worker and reproductive queen castes (Bourke 2011) may exhibit additional strategies to increase genetic diversity. Through polygyny, where nests maintain multiple queens (Hughes et al. 2008), polyandry, where queens mate with multiple males (Frumhoff and Schneider 1987; Kraus and Moritz 2010), and drift, where workers join non-natal nests (Nonacs 2023), colonies can increase genetic diversity beyond that achieved via sexual reproduction. There are several hypotheses as to why these behaviors benefit colony fitness (Crozier and Page 1985; Sherman et al. 1988; Oldroyd et al. 1992; Page et al. 1995; Nonacs et al. 2017), but increased intra-colonial genetic diversity has become a central argument (Palmer and Oldroyd 2000). For example, diverse colonies represent large genetic samples of the population gene pool and should therefore display lower variation in phenotypic expression compared to homogeneous colonies. In terms of additive genetic variation, the impact on colony fitness of a queen mating with a male of low breeding value is diminished if there are more mates or more queens to compensate. Experimental evidence comparing monandrous/monogynous to polyandrous/polygynous colonies supports this hypothesis in honey bees (Tarpy and Seeley 2006; Mattila and Seeley 2007; Oldroyd and Fewell 2007; Tarpy et al. 2013) as well as leaf-cutting, harvester, and desert ants (Hughes and Boomsma 2004; Wiernasz et al. 2008; Baudier et al. 2022; Eyer et al. 2023). However, many of these experiments also show increased mean performance in diverse colonies (except Eyer et al. 2023). This cannot be explained by additive genetic variation alone.

The non-additive benefits of genetic diversity have been known to animal and crop breeders for centuries. One common case is heterosis, where the offspring of two (often inbred) individuals perform better than the parental mean (Birchler et al. 2010). Directional dominance acting across polygenic loci has been proposed as a genetic mechanism explaining heterosis. If different dominant alleles at different loci are fixed in parents, offspring may outperform the average of their parents by inheriting beneficial alleles from both (Mackay et al. 2021). As such, increased heterozygosity in individuals can be beneficial for their fitness. In social organisms, genetic diversity can also exist between individuals. In a socially diverse group, genetically distinct individuals may specialize within their collective niche space (avoiding conflict) or specialize in distinct “tasks” that contribute to a collective goal or phenotype (increasing efficiency). The potential increase in fitness due to social diversity has been termed social heterosis (Nonacs and Kapheim 2007). This may play a larger or smaller role in each phenotype depending on how efficiently labor can be divided between individuals.

While the fitness benefits of genetic diversity in social insects have been observed in the context of polyandry/polygyny (above) and line crosses (Cale and Gowen 1956), these experiments do not differentiate between diversity generated at the individual and social scales. In fact, there is little work in social organisms able to assess the independence or relative effect size of individual and social heterosis. One such study in pharaoh ants failed to show heterosis at either scale. However, populations of pharaoh ants undergo regular inbreeding which could purge alleles responsible for inbreeding depression—perhaps masking heterosis (Schmidt et al. 2011).

We conducted an experiment using the Western honey bee (Apis mellifera L.) to explicitly test the independence and relative contribution to fitness of individual and social heterosis. Several life history characteristics make this organism an ideal choice. Honey bees are polyandrous and haplodiploid, with diploid queens embarking on a single set of mating flights far from their natal colony to mate with an expected 15 haploid males known as drones (Polhemus et al. 1950; Tarpy et al. 2013). This mating system expressly avoids inbreeding, suggesting honey bees may be a good model for exploring heterosis. Mated queens produce sterile diploid offspring known as workers which perform all non-reproductive colony tasks (Winston 1987). While workers initially recognize and repel foreign queens and worker groups, they eventually habituate, allowing the creation of functional colonies with multiple genetic lines using a technique known as cross-fostering (Calderone and Page 1992; Arathi and Spivak 2001). This allows for controlled manipulation of the social environment. Colonies can be kept in small “nucleus” hive boxes rather than full-sized “deep” boxes to economize resources. Finally, there are standard methods for rearing and breeding queens (Büchler et al. 2024), allowing for control over worker heredity.

Using controlled breeding methods and two genetically distinct populations, we partitioned genetic diversity either within or between workers in separate colonies (Fig. 1). By segregating alleles responsible for heterosis between separate individuals within a colony, as well as between separate chromosomes within an individual, we could directly compare the effects of social and individual heterosis. We created inbred colonies with low genetic diversity, hybrid colonies with high heterozygosity, and mixed colonies (combining inbred workers from each population) with low heterozygosity but high social diversity. This allowed us to explicitly test the independence and relative effect size of heterosis at individual and social scales. To measure fitness, we assessed colonies on survival against bacterial challenge and thermoregulation ability. Individual worker immune response is complex, involving multiple processes and benefiting from social interaction (Simone-Finstrom 2017). Immune function has been linked to social diversity in polyandrous eusocial insects in some (Liersch and Schmid-Hempel 1998; Hughes and Boomsma 2004; Eyer et al. 2023) but not all (Schmidt et al. 2011; Wilson-Rich et al. 2012) cases. Thermoregulation is a complex phenotype where workers maintain an optimal nest temperature (around 34.5°C) by performing multiple heating and cooling tasks (Johnson 2002; Stabentheiner et al. 2010). Since both hot and cold nest temperatures damage immature and adult physiology (Wang et al. 2016; Abou-Shaara et al. 2017; Zhao et al. 2021), accurate and even thermoregulation is linked with higher fitness. It has also been correlated with high levels of polyandry (Jones et al. 2004; Cook and Breed 2013).

Fig. 1.

A single replicate of our honey bee breeding design. Daughter queens and drones were produced from a pair of mated founder queens (*) from distinct populations A and B. Both inbred crosses and reciprocal out-crosses were performed, using single-drone insemination in all cases. The inbred (AA and BB; outer blue sqares) and hybrid (AB and BA; inner orange squares) offspring of these crosses are shown as boxes. Inbred offspring from populations A and B were mixed in a single colony to create the third experimental group (AA + BB; lower red rectangle). Dotted lines denote worker mixing rather than breeding. This design was replicated three times with three separate pairs of founder queens.

Due to their individual and social components, we hypothesized thermoregulation ability and survival against immune challenges are both subject to individual and social heterosis. Accordingly, we predicted both types of diverse colonies would achieve better thermoregulation (i.e. experience less departure from optimal temperature and lower temperature variance) and better survival against bacterial challenge (i.e. experience lower hazard) than inbred colonies. In addition, we hypothesized thermoregulation, being a colony-scale trait, is more influenced by social heterosis while immune function, having a more individual basis, would be more influenced by individual heterosis. Therefore, we predicted hybrid colonies would outperform mixed colonies in the immune challenge assay and vice versa in the thermoregulation assay.

Materials and methods

Breeding design

Mated honey bee “founder” queens were sourced from the US states of Indiana (population A) and Texas (population B). Current analyses indicate these populations represent the maximum possible genetic variance in naturalized honey bee populations in the United States (Whitfield et al. 2006; Ryals & Fikere et al. 2024, manuscript in preparation). All full-size colonies were kept at the Purdue University Research Apiary in West Lafayette, Indiana. We produced daughter queens and drones from a pair of founder queens A and B, one from each population. Using instrumental insemination, inbred crosses were performed for both A and B by mating daughter queens to a single drone from their own mother (AA and BB crosses). Accounting for haplodiploidy, their offspring have an expected inbreeding coefficient of 0.25. Reciprocal outbred crosses were also performed by mating a daughter queen of A to a single drone of B and vice versa (AB and BA crosses; Fig. 1). Semen from a different, single drone was used in each cross to decrease unwanted genetic variation within worker groups and control for effects of multiple mating.

This design partitions the average genetic divergence existing between the chosen founder pair within individuals in the hybrid offspring of AB or BA and between individuals when combining inbred offspring of AA and BB in a single mixed colony. These groups were compared to the mean performance of inbred colonies to measure the effect of individual and social heterosis respectively. Hybrid, mixed, and inbred workers represent our three experimental groups. Because there is natural phenotypic variation in each population, the performance of all experimental groups and corresponding effect sizes of heterosis likely depend on the choice of founder queens. To achieve independent measurements of heterosis, the breeding design was repeated using three separate pairs of founder queens, resulting in 12 instrumentally inseminated daughter queens and 15 worker groups.

Apiculture

To produce drones, uniquely paint-marked founder queens were introduced into separate queenless colonies and presented with a frame of wide-diameter “drone comb,” inducing them to lay unfertilized eggs which develop into drones. Each drone comb was cleared of bees and placed in a cage prior to drone eclosion. Newly emerged, caged drones were paint-marked according to parentage and returned to natal colonies. Daughter queens were produced following standard methods (Büchler et al. 2024). The presence of each founder queen was verified before grafting their larvae into paint-marked JZBZ cups (Mann Lake Ltd. Z350). Grafting was scheduled so queens and drones would be reproductively mature around the same date. All grafted larvae were raised into queens in a single queenless “cell-building” colony. Prior to eclosion, all daughter queens were introduced into separate queenless, nucleus colonies sized for five standard Langstroth frames. Queen excluders were placed over entrances to prevent mating flights.

Daughter queens in each replicate were instrumentally inseminated on the same day, 5 to 10 days after their emergence, and 10 to 20 days after drone emergence, following standard procedures (Laidlaw 1977; Büchler et al. 2024). Daughter queens were captured from their nucleus colonies in small, marked cages and anesthetized with CO₂ gas. Marked drones were captured from natal colonies into large cages. Each queen was inseminated immediately after semen collection from the intended drone. All queens were marked with numbered disks. Queens were reintroduced to respective nucleus colonies (with excluders in place) and treated with CO₂ the following day to induce oviposition. All nucleus colonies were then fed a one-gallon sucrose solution to stimulate growth. After verifying the presence of a capped worker brood (indicating successful insemination), colonies were expanded into 10-frame deep boxes and fed an additional gallon. Colonies were routinely inspected to remove “supersedure” queen cells and prevent the replacement of daughter queens by workers.

Immune challenge assay

The immune challenge assay was performed in “micro-colonies,” each made from ~1.5 L plastic cups with mesh-covered windows for ventilation, a ~20 cm² piece of comb suspended from the top, absorptive paper towel covering the bottom, and two feeding tubes (15 mL Falcon tubes with 1 mm holes in the lids) entering from the top and in contact with the comb. Each micro-colony was filled with 20 marked, day-old worker offspring from a single experimental queen in hybrid and inbred trials and 10 from each queen in mixed trials. Because these groups are much smaller than natural colonies, more replication is practically possible and bees likely experience increased stress leading to greater hazard. Taken together, this likely results in greater sensitivity to differences between treatments. A culture of Serratia marcences (Thomas Scientific laboratory stock) was combined with equal parts Pro-Sweet feed (Mann Lake Ltd. Z320) and dispersed to micro-colonies in the treatment group. The bacterial stock was brought to a standard viscosity by achieving a reading at A600 nm of 0.11 on a NanoDrop spectrophotometer (Thermo Scientific ND-2000), corresponding to roughly 3 × 10⁸ cells/mL (estimated via hemocytometer), after incubating for 3 days at 23°C in LB broth (Thermo Scientific H26760.36). The control group was fed equal parts Pro-Sweet and filtered water. Micro-colonies were kept in a dark incubator (Percival I-36NL) at 34°C and checked at ~12-h intervals to remove and record dead workers until all workers died. Within each replicate, mixed groups of all possible pairs of crosses were created, but for direct comparison to thermoregulation results, only the AA + BB mixed group was included in the analysis (see Supplementary Fig. 3 for a summary of all mixed groups).

Results from three micro-colonies were removed from analysis due to holes in cages allowing workers to escape or enter, potentially compromising their results. In total, 22 trials and 13 controls were included for analysis. The survival of each micro-colony was estimated using the Kaplan–Meier survivor function (Kaplan and Meier 1958) considering replicate as a stratification variable to allow for differences in baseline hazard function between family groups. Because stress likely increases as individuals are removed from trials, differences between treatments were tested using log-rank tests (Peto and Peto 1972) and estimated using a Cox proportional hazard model (Cox 1972), neither of which assumes a constant hazard function.

Thermoregulation assay

The thermoregulation assay was performed in the field, following Jones et al. (2004), using newly-made queenless nucleus colonies sized for three Langstroth frames. Within each replicate, five separate colonies were created using the offspring of experimental crosses: one for each inbred and outbred cross and one for the mixed group. Due to the premature death of an outbred queen (replicate 2), this gave a total of 14 colonies. After allowing inseminated queens to produce workers for at least 51 days to replace any unrelated workers in their (full-sized) home colonies, a total of 500 mL of workers were moved at night into each empty nucleus colony. For mixed nucleus colonies, one volume of 250 mL was added from each source, spraying bees with sugar water to minimize potential fighting during introduction. Each colony was provided a full frame of honey, a full frame of unrelated larvae, TempQueen pheromone (to simulate the presence of a queen; Mann Lake Ltd. Z133), and a frame of empty comb. All nucleus colonies were moved to Purdue’s Lee Apiary in Americus, Indiana, roughly 20 km from natal colonies. They were placed randomly in full sun roughly 5 m apart with entrance directions varying randomly between SSE and SSW and allowed to freely forage for 7 days before temperature recordings began.

To measure thermoregulation ability, iButton (Thermocron DS1921H) temperature sensors were placed in the brood nest recording every 5 minutes for 4 days between August 30th and September 3rd, 2023 (Fig. 3A). An additional sensor was placed in a nucleus box without bees to record ambient temperature. At the end of the period, all sensors and data were recovered. For each colony, we subset data into one-hour windows (12 readings per window, 96 total windows) and calculated two response variables: thermal variance as the variance in temperature readings and thermal error as the absolute difference between recorded and optimal nest temperature (Supplementary Fig. 1). To determine significance of individual and social heterosis, hybrid and mixed treatments were compared to the average of inbred controls within each replicate using a paired t-test for each thermal variable (for explicit paired data, see Supplementary Fig. 2). All tests were single tailed because the directions of inequalities were explicitly predicted (see “Introduction” section).

Fig. 3.

A) Each solid line shows the raw temperature readings within a nucleus colony over 4 days with colors representing experimental groups. Optimal (34.5 °C; dashed), and ambient (dot-dashed) temperatures are shown in black. Ambient temperatures are low-gated at the sensors’ thermal minimum. B) Colony performance in terms of thermal error, calculated as the sum of absolute differences between recorded and optimal temperatures within each 1-h window (12 records per colony). Each point represents the mean performance of a single colony, with vertical lines depicting the standard error of all 96 windows. Colored bars represent the mean performance within experimental groups and point shapes represent independent replicates. Significance values for differences between groups are calculated from t-tests paired by replicate. C) Same as B) but in terms of thermal variance, calculated as the variance in temperature recordings within each 1-h window. For B) and C), lower values correspond to higher fitness.

Results

Immune challenge

All control micro-colonies experienced significantly lower hazard (i.e. higher survival) than the inoculated group (P < 0.001). We only observed three total deaths over 16 days across all control trials and therefore detected no significant differences between genotypes in the control group. Within the treated group we observed 302 total deaths over 22 days. Both the hybrid and mixed trials experienced significantly lower hazards than inbred trials (P < 0.001; P < 0.001). The hybrid micro-colonies experienced significantly less hazard than the mixed colonies (P = 0.025; Fig. 2). Hazard ratios for hybrid and mixed groups relative to the inbred group were estimated at 0.49, 95% CI [0.37, 0.65] and 0.69, 95% CI [0.50, 0.96] using a Cox proportional hazard model stratified by replicate.

Fig. 2.

Kaplan–Meier survival curves depicting the attrition of micro-colonies for 20 days after bacterial inoculation. Colors represent experimental groups, with solid lines representing mean survival and surrounding fill representing standard error. The table displays P values of log-rank tests between groups.

Thermoregulation

Hybrid and mixed nucleus colonies experienced 1.94 ± 0.97 °C and 2.82 ± 2.46 °C less thermal error each hour than inbred lines respectively (t = 5.87, P = 0.014; t = 3.35, P = 0.039), but the 0.87 ± 2.75 °C difference between mixed and hybrid colonies was not significant (t = 0.93, P = 0.226; Fig. 3B). Similarly, hybrid and mixed colonies experienced 0.14 ± 0.11 °C² and 0.16 ± 0.06 °C² less thermal variance each hour than inbred colonies, but the 0.01 ± 0.08 °C² difference between them was not significant (t = 3.78, P = 0.031; t = 8.20, P = 0.007; t = 0.52, P = 0.329; Fig. 3C). All intervals represent 95% confidence and all tests used 2 degrees of freedom. Across all colonies, thermal error and thermal variance were highly correlated (r² = 0.78).

Discussion

For honey bee survival against bacterial challenge and thermoregulation ability, both hybrid and mixed groups performed significantly better than inbred workers alone. Not only did hybrid colonies with a single family of genetically diverse individuals experience higher fitness, but also colonies with a socially diverse mixture of two inbred families. This supports our hypothesis that individual and social heterosis can occur independently in social organisms. Furthermore, given the mixed group outperformed the mean of inbred groups in the immune response assay, it seems social heterosis can even impact individual performance. However, we failed to find a difference between hybrid and mixed groups in the thermoregulation assay, and while log-rank tests show differences in their survival against bacterial challenge, the 95% confidence intervals of their estimated hazard in the Cox proportional hazard model are overlapping. Taken together, this evidence does not support our hypothesis that social and individual heterosis affect traits differently. It may be possible to detect a difference by increasing the number of replicates, but our results suggest any difference in mean expression is likely to be small compared to the difference between diverse and inbred colonies.

In both our assays, inbred workers from two distinct populations perform better when mixed together than when acting alone—suggesting inbreeding depression at the colony level can be rescued through social interaction. Division of labor and indirect genetic effects could contribute to this increase in phenotypic expression. Due to the use of single-insemination, our colonies likely do not contain the most optimal genotypes available in each population for each task contributing to our observed phenotypes—as hyperpolyandry may have evolved to achieve (Delaplane et al. 2024). However, given both thermoregulation (Stabentheiner et al. 2010) and immune function (Cremer et al. 2007; Wilson-Rich et al. 2012) are complex traits requiring workers to perform multiple discrete tasks, and alleles influencing tasks are likely distributed unevenly between populations due to differences in drift and selective pressures, each inbred line may be predisposed to a different set of tasks. This can lead to task partitioning between worker groups (Calderone and Page 1992; Page and Erber 2002; Oldroyd and Fewell 2007). The mixed group in our experiment may achieve a more efficient division of labor between tasks compared to inbred groups, increasing colony-level performance (Nonacs and Kapheim 2007; Johnson and Linksvayer 2010). A genetic bias in task allocation has been observed in honey bee defense response (Breed and Rogers 1991), foraging decisions (Calderone and Page 1992; Mattila and Seeley 2011), hygienic behavior (Masterman et al. 2001), and thermoregulation (Jones et al. 2004), as well as harvester ant foraging effort (Wiernasz et al. 2008). In addition, an individual with a given genotype may alter its task expression when interacting with others. This is known as an indirect genetic effect (Moore et al. 1997). While one of our inbred lines may have low expression alone, social interaction with the other line could cause it to increase expression or shift to other tasks—increasing division of labor. This too could explain the observed increase in colony-level expression. For honey bee defense response and hygienic behavior, indirect genetic effects have been shown to alter individual behavior (Guzmán-Novoa and Page 1994; Arathi and Spivak 2001) and brain gene expression (Alaux et al. 2009; Gempe et al. 2012), both of which impacted colony-scale phenotypes.

Hybrid bees outperforming inbred lines is consistent with the understood quantitative genetics of polygenic inheritance and directional dominance, given beneficial alleles are likely to be distributed unevenly between the two populations (Mackay et al. 2021). The impact of indirect genetic effects in the mixed group could be analogous to directional dominance in the hybrid group. In both cases, the presence of a beneficial allele masks the expression of an alternative allele—but acting within individuals in the hybrid group and between individuals in the mixed group. This has been termed “social dominance” when observed in cross-fostering experiments studying multiple colony-level honey bee behaviors (Hillesheim et al. 1989; Guzmán-Novoa and Page 1994). While the proposed mechanisms for individual and social heterosis are different, this analogous action could explain why our hybrid and mixed groups did not differ significantly in effect size for either trait.

There is potential to improve this experimental design. If thermoregulation colonies were housed in a controlled environment, extreme temperatures could be maintained, perhaps maximizing observed differences between treatment groups. While genetic variation between workers in hybrid colonies was reduced by single-drone insemination, some variation still exists due to the segregation of the daughter queen’s alleles. If founder queens were inbred and singly mated, social diversity within hybrid colonies could be further reduced. Under such a breeding scheme, differences between social and individual heterosis may be easier to detect. Furthermore, given honey bee colonies are clearly adapted to hyperpolyandry (Tarpy et al. 2013), it is likely the benefits of social heterosis are more pronounced with higher mate numbers. While the direct comparison to individual heterosis would no longer be possible, a similar experiment using larger numbers of patrilnes may observe a more accurate and likely larger effect size for social heterosis. Additionally, observational data on individual behavior could provide stronger support for differences in task allocation between genotypes leading to increased colony phenotypes. It should be tested whether the division of labor depends on the social setting. For example, if task specialization is important for optimizing colony fitness, we would expect a particular genotype alone to perform all colony tasks but specialize in a few tasks when placed in a socially diverse setting. Focal honey bees in highly polyandrous colonies were observed to participate in fewer total tasks than those in less polyandrous colonies (Delaplane et al. 2024), and an emergence of division of labor under experimentally increased social diversity has been observed in honey bee hygiene (Arathi and Spivak 2001). Behavioral data could detect differences in the division of labor between hybrid and mixed groups to investigate the extent to which heterosis is driven by increases in net expression (due to directional dominance or indirect genetic effects) or by more efficient division of labor.

Understanding the roles of social and individual heterosis will likely find application in honey bee breeding. Optimal breeding decisions, particularly the selection of multiple mates, depend on accurately modeling the contribution of colony-level genotypes to colony-level phenotypes (Brascamp and Bijma 2014; Andonov et al. 2019; Hoppe et al. 2020; Bernstein et al. 2023). Given the polyandrous mating system, interactions between genetically distinct workers within a colony are likely to be highly influential in these models. Maintaining genetic diversity is important for population health and long-term progress in most breeding systems, but must be weighed against short-term progress gained through intensive selection (Notter 1999; Litrico and Violle 2015; Allier et al. 2019). For social organisms, genetic diversity may be especially important, even in short timescales, due to the observed benefits of social heterosis. This work provides evidence that both heterozygosity and social diversity independently contribute to heterosis in colony phenotypes. Breeding programs could achieve phenotypic improvement not only through selection and avoiding inbreeding but by actively maintaining heterogeneous populations to promote social diversity within colonies.

Supplementary Material

Supplementary material is available at Journal of Heredity online.

Funding

This work was supported by the Purdue Ross Fellowship and grants from the United States Department of Agriculture’s Agriculture and Food Research Initiative.

Conflict of interest statement. None declared.

Author contributions

Dylan Ryals (Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Supervision, Visualization, Writing – original draft, Writing – review & editing), Amos Buschkoetter (Data curation, Investigation, Writing – review & editing), J. Krispn Given (Conceptualization, Methodology, Supervision, Writing – review & editing), and Brock Harpur (Conceptualization, Funding acquisition, Methodology, Resources, Supervision, Validation, Writing – review & editing)

Data availability

Raw assay data and code required to reproduce analyses are found at https://github.com/dryals/social-heterosis. This directory is also hosted on DataDryad at https://doi.org/10.5061/dryad.cc2fqz6g1.