The proposed research will examine the processes contributing to the spread of recombinant genotypes after hybridization between morphologically and ecologically distinct lineages. The mixing of divergent taxa via natural hybridization is considered to have been important in the history of a variety of plants and animals. Several studies have analyzed the factors responsible for the maintenance and structuring of narrow hybrid zones; however, we know much less about the dynamics of hybrid zones that extend over broad geographic regions. The process of spread of hybrid genotypes and expansion of the hybrid zone will be complicated by hybrid breakdown (loss of viability and fertility) in the early generations following hybridization. Here I propose to examine patterns of hybrid breakdown and its demographic and genetic consequences. Specifically I will examine the patterns of fitness loss that accompanies hybridization and it’s impact on the process of hybrid zone expansion.

I will use closely-related taxa in the Piriqueta caroliniana complex (morphotypes "caroliniana" and "viridis") and their natural hybrids as a model system to examine factors that contribute to the success and spread of hybrid genotypes. Analyses of historical relationships in this group and the geographic distribution of taxa indicate that the caroliniana morphotype has probably been present in North America since the early Pleistocene, while viridis immigrated to south Florida from the northern Bahamian Islands much more recently (within the last 7000 years). The patterns of morphological and genetic variation across the Florida peninsula are consistent with the northward dispersal of viridis genes into populations of caroliniana. The hybrid zone in central Florida was formed when viridis expanded its range northward into regions in central Florida that were occupied by populations of caroliniana. The resulting broad hybrid zone in central Florida provides a unique opportunity to examine the consequences of hybridization between closely-related taxa.

The proposed work will address the factors affecting the spread of advantageous genotypes within and among populations. The Piriqueta hybrid zone in central Florida is unusual among documented cases because it appears to have expanded as a result of selection favoring certain genetic combinations and may represent an intermediate stage of melding of two previously isolated taxa. Such a pattern of temporary isolation followed by the fusion of divergent lineages as distributions shift in response to climatic changes has been prevalent in the evolutionary history of a variety of species, but opportunities to study these processes have been limited.

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