Homoploid Hybrid Speciation in Louisiana Iris
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As hybridization necessarily occurs in sympatry, hybrid species may represent the most frequent mechanism of sympatric speciation, and, as such, hybrid speciation events present the opportunity to study the evolution of reproductive isolation in sympatry. A number of purported cases of homoploid hybrid speciation in both plants and animals have recently been reported, suggesting that this process may be more common than once believed. The limited number of well-documented studies of reproductive isolation between a homoploid hybrid species and its progenitors yield varying patterns of reproductive isolation. In some taxa, strong fertility selection in early generation hybrids may yield a hybrid lineage that is isolated from its progenitors by strong postzygotic isolation. Alternatively, ecological barriers may result in reproductive isolation between a hybrid lineage and its progenitors. Here, I examined homoploid hybrid speciation and reproductive isolation between a purported homoploid hybrid species, Iris nelsonii, and one of its progenitors, Iris hexagona. In order to investigate homoploid hybrid speciation in this group, I used population genetic surveys to estimate the genomic makeup of Iris nelsonii, developed a genetic linkage map from a cross between I. nelsonii and one of its progenitor species, I. hexagona to investigate genomic collinearity, determined the response of each species to abiotic habitat conditions, described the pollination biology of I. nelsonii, and investigated the genetic architecture of floral differences that may contribute to pollinator isolation in this system. Iris nelsonii appears to be genotypically intermediate between at least I. fulva and I. brevicaulis, but a larger number of molecular markers are needed in order to thoroughly investigate the hypothesized hybrid origin of I. nelsonii. The genome of I. nelsonii appears to be roughly collinear with the other species of Louisiana Iris, suggesting that reproductive isolation is not due to chromosomal rearrangements. Ecological isolation is largely important in this system as the species of Louisiana Iris appear to respond to abiotic habitat conditions and I. nelsonii and I. fulva are visited by different pollinators than I. hexagona and I. brevicaulis. Despite the originally strict early verbal models of hybrid speciation, reproductive isolation between a hybrid lineage and progenitors can be a result of barriers other than chromosomal rearrangements and is likely a function of a number of ecological, genic, and/or chromosomal barriers.