Population biology of mitochondrial DNA in the crickets, Gryllus pennsylvanicus and Gryllus firmus

This dissertation describes the biology of cricket mitochondrial DNA (mtDNA) on four different organizational levels: (1) the sequence of repetitive DNA in a size-variable region of the genome, (2) the inheritance of mtDNA size variants through the female cytoplasm, (3) the distribution and hierarchical structure of mtDNA size variation in natural populations and (4) the ecological genetics of mtDNA in a hybrid zone between the two cricket species. The repetitive sequences of DNA in the mitochondrial genome of Gryllus firmus lie near the control region of the molecule and contain the dyad symmetric sequence GGGGGCATGCCCCC. These sequences define the boundaries of a 220 based pair (bp) repeat which describe precisely the molecular basis of mtDNA size variation in this species. Mechanisms by which new size classes of cricket mtDNA might be generated are discussed in reference to these sequences. Crickets possessing two or more different-sized mtDNA types in their cells (heteroplasmic crickets) are used to study the cytoplasmic inheritance of mtDNA. The frequencies of different-sized mtDNAs in heteroplasmic females and samples of their offspring are quantified by densitometry of autoradiographs. The data indicate that several hundred animal generations would be required for the heteroplasmic condition to drift to fixation/loss (all descendants homoplasmic) and suggest that smaller mtDNA molecules have an advantage over larger mtDNAs in the transmission from mother to offspring. Densitometry of autoradiographs is also used to quantify the frequency of heteroplasmic individuals and the frequency distributions of mtDNA size classes in nature populations of the two cricket species. Heteroplasmy is very frequent (...