BARRIERS TO GENE EXCHANGE IN A FIELD CRICKET HYBRID ZONE
Understanding how speciation influences patterns of molecular evolution and how molecular changes drive speciation are central questions in evolutionary biology. In this dissertation I address questions of behavioral barriers to gene exchange, cytoplasmic incompatibility, molecular evolution, population history, gene flow and species boundaries in the hybridizing field crickets Gryllus firmus and G. pennsylvanicus. I examine the role of behavioral barriers to gene exchange in the context of previous studies that documented temporal and ecological isolation and a one-way post-mating incompatibility. My results reveal strong behavioral premating barriers, but no apparent fecundity or fertility costs for G. firmus females when they mate with conspecific and heterospecific males. I also document a failure of heterospecific males to induce normal oviposition in G. firmus females, a previously unknown post-mating, pre-zygotic barrier. Gryllus firmus and G. pennsylvanicus exhibit a very clear unidirectional incompatibility and have been cited as a possible example of Wolbachia-induced cytoplasmic incompatibility. Wolbachia are cytoplasmically inherited alpha-proteobacteria that can cause cytoplasmic incompatibility in insects. I conduct curing experiments, intra- and interspecific crosses, cytological examination of Wolbachia in testes, and Wolbachia quantifications via Real-Time PCR. All of the data strongly suggest that Wolbachia are not involved in the reproductive incompatibility between G. firmus and G. pennsylvanicus. Finally I analyze DNA sequence divergence for seminal protein loci, housekeeping loci, and mtDNA, using a combination of analytical approaches and extensive sampling. In recently diverged species, such as G. firmus and G. pennsylvanicus, ancestral polymorphism and introgression can cause incongruence between gene trees and species trees. In the face of hybridization only genomic regions that cannot cross the species boundaries will show reciprocal monophyly. These regions, usually evolving rapidly under selection, are essential for the maintenance of species identity. I report discordant genealogical patterns and differential introgression rates across the genome. The most dramatic outliers, showing near zero introgression and more structured species trees, are also the only two seminal protein loci under selection. These are candidate barrier genes with possible reproductive functions. I also use the genealogical data to examine the demographic history and the current structure of the hybrid zone.
speciation; hybrid zone; barriers to gene exchange; evolution
dissertation or thesis