Date of Award
Doctor of Philosophy (PhD)
Interspecific hybridization is a relatively common occurrence within all animal groups. Two main factors make hybridization act differently in ants than in other species: eusociality and haplodiploidy. These factors serve to reduce the costs of interspecific hybridization in ants while simultaneously allowing them to take advantage of certain benefits. Eusociality may mitigate the effects of hybridization by allowing hybrids to be shunted into the worker caste, potentially reducing the effects of hybrid sterility. In haplodiploid species, males do not have a father. They instead develop from unfertilized eggs as haploid clones of their mother. This means that interspecifically mated queens do not completely sacrifice reproductive potential even if all hybrids are sterile because they can still produce fertile males. These factors in turn suggest that hybridization should be more common among the social Hymenoptera than other animal groups. Nevertheless, current data suggest that ants hybridize at rates similar to other animal groups, although these data are limited. Furthermore, there is a large amount of overlap between cases of interspecific hybridization and cases of genetic caste determination. A majority of the cases in ants where caste is determined primarily by genotype are associated with hybridization. However, it is not clear how these two phenomena are related, and more research is needed to answer this question. As a first step in answering these questions, I designed a set of microsatellite markers for use in African driver ants in the genus Dorylus. Additionally, to facilitate population genetics research in all ant species I aimed to develop a set of primers that are broadly applicable to most ant species, since PCR primers for microsatellite loci are often not useful outside the species for which they were designed. I identified 45 conserved microsatellite loci based on the eight ant genomes that were available at the time and designed primers for PCR amplification. Among these loci, I chose 24 for in-depth study in six species covering six different ant subfamilies. On average, 11.16 of these 24 loci were polymorphic and in Hardy-Weinberg equilibrium in any given species. The average number of alleles for these polymorphic loci within single populations of the different species was 4.59. This set of genetic markers will thus be useful for population genetic and colony pedigree studies across a wide range of ant species, supplementing the markers available for previously studied species and greatly facilitating the study of the many ant species lacking genetic markers. This work shows that it is possible to develop microsatellite loci that are both conserved over a broad range of taxa, yet polymorphic within species, and should encourage researchers to develop similar tools for other large taxonomic groups. After the development of these microsatellites, I used them to investigate a system of hybridization between two species of African driver ants. All driver ants belong to the subgenus (Anomma) in the genus Dorylus. They are swarm-raiding army ants with colonies that can have as many as 12 million individual ants. Colonies frequently migrate to new nest sites and conduct daily swarm-raids, capturing and eating any invertebrates or even small vertebrates in their path. Colonies are monogynous, and the queens are highly multiply mated, mating with as many as 20 males. A previous study suggested that hybridization occurs between Dorylus molestus and Dorylus wilverthi at a site in western Kenya. However, the extent and exact pattern of hybridization have remained unclear, and its possible effect on caste determination has not been investigated. I aimed to determine the extent and direction of hybridization by measuring how frequently hybrids occur in colonies of both species, and to investigate the possibility of genetic caste determination. I show that hybridization is bidirectional and occurs at equal rates in both species. Hybrid workers make up only 1–2% of the population, and successful interspecific matings represent approximately 2% of all matings in both species. This shows that, although interspecific matings that give rise to worker offspring occur regularly, they are much rarer than intraspecific matings. Finally, I find no evidence of an association between hybridization and genetic caste determination in this population. Genetic caste determination may be associated with hybridization, but it is not a necessary outcome of it in ants. Although there was no evidence of genetic caste determination, studying this Dorylus system has uncovered the potential for a novel project. After viewing collection data from a collaborator, Caspar Schöning, I hypothesized that Dorylus ants in the subgenus Anomma would constitute a good system for addressing an unanswered question in evolutionary biology: what is the relationship between the permissibility of the genome to introgression between two species and divergence time? Dorylus (Anomma) is a good system for this study because it has multiple species with different areas of allopatry and areas of sympatry with other species in the group. This project would involve sequencing multiple samples of each species from both allopatric and sympatric areas and comparing the genomes of samples from areas of allopatry to those from areas of sympatry to measure the amount of introgression between multiple species pairs. A model is then fit to a plot of the amount of introgression versus divergence time to determine the shape of the relationship.
Butler, Ian, "Hybridization in Ants" (2020). Student Theses and Dissertations. 597.