| Summary: | The Caribbean corals, Acropora palmata and A. cervicornis, are abundant in fossil records but have recently undergone drastic declines primarily as a result of disease. Acropora prolifera, a hybrid of these species, has no fossil record and was previously considered rare and to occupy nonparental habitats. Now, hybrids have equivalent or greater abundance than the parental species and have expanded into the parental habitat at some sites. Previous molecular studies have demonstrated regional variability in unidirectional introgression of A. palmata genes into A. cervicornis. The goals of this dissertation are (1) to determine the strength of prezygotic mechanisms and to establish the likelihood of density dependent reproductive isolation, (2) to determine the strength of intrinsic and extrinsic postzygotic barriers, and (3) to ascertain if hybrid populations are composed of rare hybridization events that have asexually fragmented, or if colonies are genotypically distinct suggesting separate hybrid events. Overall barriers to hybridization in this genus are weak, and the efficacy of these semipermeable isolating mechanisms may depend on density. In addition, hybrids are as viable as the parental species at a variety of life history stages and are less or equally susceptible to the typical afflictions that have lead to their decline. Most hybrid populations do not seem to be composed of a single hybridization event that has asexually propagated, but rather the genotypic diversity varies across sites with up to 17 different distinct genets in one population. Taken together, it appears that hybridization in a threatened Caribbean genus is evolutionarily significant with a range of possible outcomes from the benefit of novel alleles to the swamping of A. cervicornis' genome. These outcomes may hinge on the ability of the Caribbean acroporids to withstand the onslaught of threats that currently faces this genus (i.e. Allee Effect, disease, predation, increased sea temperature, ocean acidification, and increased ...
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