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search.filters.applied.f.advisor: Goodisman, Michael × search.filters.applied.f.resourcesubtype: Dissertation ×

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Population genetics and genomics of eusocial animals

2017-11-09 , Chau, Linh M.

Major evolutionary transitions have been associated with increases in organismal complexity. One of the latest evolutionary transitions is from solitary life to eusociality. This transition led to a reproductive division of labor in which individuals are divided into castes. Reproductive castes are responsible for reproduction, while nonreproductive castes take part in colony maintenance and brood care. This division of labor represents a challenge to selection and has long been of curiosity to researchers. My dissertation research examined the population genetics and genomics of eusociality in a spectrum of eusocial species. First, I examined the population structure and genetic diversity of Vespula pensylvanica, a wasp native to North America that has invaded the Hawaiian archipelago. I found a lack of population structure in V. pensylvanica’s native range and determined how the population structure of invasive social insects can be shaped by geography. I also examined the population genetics of captive naked mole rats, one of the only known eusocial mammals. I sought to understand how captivity can shape the population structure of a eusocial animal. Interestingly, there was evidence that naked mole rat populations are not as inbred as previously theorized and that sex ratios are equal within captive colonies. Finally, I examined how the phenomenon of gene duplication can affect the evolution of castes in eusocial species. I uncovered a relationship between duplication rate and level of sociality across the bees. Also, I saw that duplicates were differently expressed across phenotypes compared to single copy genes. These studies provide insight on an array of population genetic and genomic questions concerning the evolution of eusociality. Therefore, this research furthers our understanding of the rare distribution of this social system across the tree of life.

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Queen-specific selective pressures and caste dimorphism in the social wasp Vespula maculifrons

2009-08-19 , Kovacs, Jennifer L.

Within social insect colonies, sterile workers are responsible for brood care, while queens are the primary egg-layers. These reproductive caste differences are often accompanied by pronounced morphological distinctions. Queen and worker phenotypic differences are particularly remarkable considering caste is environmentally, rather than genetically, determined. Environmental caste determination can produce intralocus genetic conflict between castes, particularly when homologous traits are highly dimorphic. Therefore, when studying the evolution of social insect caste dimorphism, one must consider the genetic architecture underlying phenotypic expression as well as the selective pressures that have shaped caste morphology. This dissertation presents the results of four studies that investigated factors affecting caste morphology in the social wasp Vespula maculifrons. The first two studies focused on identifying queen morphological traits that were positively associated with queen fitness and would therefore be subject to selection. Queen length, specifically gaster length, was positively associated with overwintering survival and was consistently associated with mating success. Both of these findings suggest that queen gaster length is under selection during two life-history events, mating and overwintering, in which workers do not participate. These findings provide empirical support for the adaptive evolution of a caste dimorphic trait. The third and fourth studies used classical quantitative genetic and morphological analyses to examine the genetic architecture underlying caste dimorphism in V. maculifrons. I determined which traits were under caste-specific selection by analyzing trait allometries and the levels of genetic control, variation, and dimorphism of traits between castes. Little genetic variation for morphological trait size was detected for most worker and queen traits, suggesting a strong influence of environment on phenotypic variation. Additionally, analyses of trait allometries indicated that several queen traits (mass, thorax width and length) were under queen-specific selection. The relationship between thorax length, gaster length, and overall body size is further evidence of selection on length in queens. Overall, these studies provide evidence for the importance of queen-specific selection in the evolution of caste dimorphism. When placed in the broader context of caste evolution, they point to the importance of life-history in shaping the genetic architecture underlying caste dimorphism.

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Epigenetics in social insects

2016-01-14 , Glastad, Karl M.

Virtually all multicellular organisms are capable of developing differently in response to environmental variation. At the molecular level, such developmental plasticity requires interpretation and perpetuation of environmental signals without changing the underlying genotype. Such non-genetic, heritable information is known as epigenetic information. This dissertation examines epigenetic information among social insects, and how differences in such information relate to phenotypic caste differences. The studies included herein primarily focus on one form of epigenetic information: DNA methylation. In particular, these studies explore DNA methylation as it relates to and impacts (i) alternative phenotype and particular gene expression differences in two social insect species, (ii) histone modifications, another important form of epigenetic information, in insect genomes, and (iii) molecular evolutionary rate of underlying actively transcribed gene sequences. We find that DNA methylation exhibits marked epigenetic and evolutionary associations, and is associated with alternative phenotype in multiple insect species. Thus, DNA methylation is emerging as one important epigenetic mediator of phenotypic plasticity in social insects.

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Molecular evolution in the social insects

2011-04-01 , Hunt, Brendan G.

Social insects are ecologically dominant because of their specialized, cooperative castes. Reproductive queens lay eggs, while workers take part in brood rearing, nest defense, and foraging. These cooperative castes are a prime example of phenotypic plasticity, whereby a single genetic code gives rise to variation in form and function based on environmental differences. Thus, social insects are well suited for studying mechanisms which give rise to and maintain phenotypic plasticity. At the molecular level, phenotypic plasticity coincides with the differential expression of genes. This dissertation examines the molecular evolution of genes with differential expression between discrete phenotypic or environmental contexts, represented chiefly by female queen and worker castes in social insects. The studies included herein examine evolution at three important levels of biological information: (i) gene expression, (ii) modifications to DNA in the form of methylation, and (iii) protein-coding sequence. From these analyses, a common theme has emerged: genes with differential expression among castes frequently exhibit signatures of relaxed selective constraint relative to ubiquitously expressed genes. Thus, genes associated with phenotypic plasticity paradoxically exhibit modest importance to overall fitness but exceptional evolutionary potential, as illustrated by the success of the social insects.

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