Organisms exhibit characteristics that can change depending on the environment and vary across a species, especially those with expansive ranges. Due to eusociality, which has led to proliferation throughout nearly every terrestrial ecosystem, many social insects make ideal subjects for studying variation in how a species interacts with the environment across its range. Soil-dwelling ants create and modify habitat as ecosystem engineers while building nests that act as an extended phenotype, buffering the colony against climate. Understanding how ants can persist in a wide range of environments and developing a predictive understanding of their soil impacts across environmental gradients will advance our knowledge of soil ecosystem function. Through soil sampling in and around nests and a controlled laboratory experiment, I investigated the extent to which climate influences nest architecture and the effects of nests on soil properties. Additionally, I conducted genomic analyses to identify markers associated with climatic heterogeneity in a widespread species, Formica podzolica. I found that nests differentially affect soil chemistry across elevational gradients; at lower elevations, nest soil had lower amounts of carbon and nitrogen than control soil, but at higher elevations, the opposite pattern was present. Nest architecture is shaped by local adaptation and a plastic response to temperature; while workers experiencing a high temperature excavated deeper nests than those experiencing a cooler temperature, I observed a significant interaction effect of natal elevation and temperature treatment on nest size and complexity. While these traits are plastic, genomic underpinnings may also influence ants’ fitness and their impact on soil in different climates; genomic signatures of adaptation to temperature, precipitation, and seasonality were present across F. podzolica’s range, with one locus exhibiting a precipitation-associated alternative allele exclusively at the northern edge of the range. Combined, these studies suggest that Formica ants likely modulate soil properties differently across environmental gradients, their nests are shaped by a combination of plastic and locally adapted behaviors, and genomic variation may be a factor in adaptive potential, especially at range margins.

Task partitioning allows for coordination of behavior in animal societies, potentially enhancing task efficiency. Many task allocation studies focus on social insects with discrete morphological worker subcastes, such as those possessing major and minor workers with strongly differentiated body plans. Much less is known about task partitioning among size-variable workers lacking discrete morphological subcastes, like in Formica ants. Through a large-scale mark-recapture study and a controlled laboratory experiment, we investigated how worker size affects task fidelity and proficiency across Formica species with differing degrees of body size variation. Additionally, we carried out genomic analyses to identify any genetic underpinnings of size-based task partitioning in these species. In species with high levels of worker size variation, a worker’s body size is strongly correlated with the tasks it performs. Specifically, large workers specialize in nest building or protein foraging, while small workers specialize in honeydew collection. This size-task correlation is weaker, but still present, in species with less size variation among workers. Interestingly, our laboratory experiments suggest that, in Formica species with substantial intracolony size variation, large workers outperform small workers at both nest building and sugar-water collection. It is unclear whether small workers’ relatively poor performance at a task they typically perform in nature is due to limitations of our experimental design, or if small workers make other important contributions to colony efficiency. Genomic analyses reveal that both worker size and task may be under genetic control, although this is variable across species. The two phenotypes are not always genetically linked, although they appear to share some genetic associations in the most size-variable species analyzed. Combined, these studies suggest that Formica ants utilize a size-based task partitioning strategy, but the reliance on, benefits of, and genetic underpinnings of this strategy vary considerably across species. We expect social insects with varying degrees of morphological task specialization to differ in ontogeny, evolutionary history, and behavioral flexibility. Additional comparative studies will help us understand the potential costs and benefits of alternative strategies.

Supergenes are chromosomal regions of tightly linked loci experiencing reduced recombination that are inherited as a single unit when heterozygous. Polygenic morphological or behavioral traits can be maintained as distinct morphs, polymorphisms, within a population due to alternative supergene haplotypes that rarely engage in recombination with one another. Supergenes have been found in a variety of organisms resulting in various types of polymorphisms including mimicry complexes in butterflies, mating morphs in birds, and alternative social organization in ants. Formica is a genus of ants known for having alternative supergene haplotypes on chromosome 3, resulting in either monogyne or polygyne colonies. Monogyne colonies are single family units headed by one queen, with the associated haplotype being Sm. Polygyne colonies can have multiple queens, with the alternative supergene haplotype, Sp, being associated with this form of social organization. This system, first described in Formica selysi, is now known to be present in multiple other Formica species. As we continue to expand our knowledge of the Formica supergene, two questions emerge: is the function of the supergene the same across the genus and what genomic events contributed to the evolution of the supergene? To answer the first question, I utilized SNP data from 280 workers of Formica neoclara, sampled from 32 colonies and 8 transect locations spanning from California to British Columbia. I determined that F. neoclara is socially polymorphic, with monogyne and polygyne colony assignment stemming from three metrics: inferred queen number, average colony relatedness, and opposing homozygosity. This social polymorphism is due to the supergene found in other Formica species through principal component analysis as well as a genome wide association study. Finally, I analyzed the population structure of F. neoclara, showing that populations from all sampled regions are experiencing limited gene flow with one another; a finding supported by expected heterozygosity values, principal component analysis, and isolation by distance analysis. Unlike F. selysi, in which all individuals within polygyne colonies must have an Sp haplotype, polygyne colonies of F. neoclara can consist of workers with all three genotypes (Sm/Sm, Sm/Sp, Sp/Sp), showing that the mode of action of the supergene is not identical across the genus. To answer the second question, regarding the evolution of the supergene, I performed comparative linkage mapping with F. selysi and an outgroup genus Polyergus. I constructed a linkage map for Polyergus using RADseq data from 83 workers from a single colony, with F. selysi serving as a reference. While Polyergus has synteny with F. selysi in many regions of the genome, including a large portion of chromosome 3, there are also many regions with inversions. By performing comparative linkage mapping between genera, we can better understand the timing and conditions surrounding the evolution of the supergene. This thesis, in exploring the elements of the supergene both within a genus and between genera, provides a glimpse into the function and evolution of this social supergene.

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