The genetic architecture of reproductive isolation in Louisiana irises
Bouck, Amy Christine
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This dissertation was motivated by an interest in how the genetic basis of species boundaries and traits involved in reproductive isolation influence both the process of speciation and the evolutionary dynamics of quantitative trait evolution in hybrid zones. This work specifically involves the species Iris fulva and I. brevicaulis, wild irises native to the Mississippi river drainage which are known to form hybrid zones in the southernmost extent of their range. Genetic mapping was used to investigate 1.) the genomic distribution of genetic incompatibilities between these species, and 2.) the genetic basis of adaptations to different pollinators. Genetic maps of I. fulva and I. brevicaulis were constructed with retrotransposon-based genetic markers segregating in interspecific backcross (BC1) hybrids. Linkage analyses detected very little genetic incompatibility between I. fulva and I. brevicaulis in the form of segregation distortion: each map shows only a single region in which introgression of heterospecific alleles is reduced. Hybrid breakdown in pollen fertility is fairly severe in (I. fulva x I. brevicaulis) x I. brevicaulis BC1 hybrids, but the genetic basis of this is simple, involving two QTL with large phenotypic effects. Two QTL were also found to affect pollen fertility of (I. fulva x I. brevicaulis) x I. fulva BC1 hybrids, but these QTL are of small phenotypic effect. Taken together, the few genetic incompatibilities between I. fulva and I. brevicaulis and the simple genetic basis of hybrid breakdown in pollen fertility indicate that the majority of these species' genomes are highly permeable to gene flow and introgression from one another. Speciation in flowering plants may often involve the evolution of divergent pollination syndromes -- suites of characters that typify flowers specialized to particular pollinators. Because pollinators serve as agents of gene flow, floral adaptations conferring pollinator specialization also serve as mechanisms of reproductive isolation, and are therefore also speciation traits. Iris fulva is pollinated primarily by hummingbirds, while I. brevicaulis is pollinated by bumblebees, and both species demonstrate a suite of traits characteristic of hummingbird and bumblebee pollination syndromes, respectively. The genetic basis of these different pollination syndromes is likely to affect the rapidity and ease with which one pollination syndrome evolves from another, and the dynamics of floral trait evolution in hybrid zones. Quantitative trait loci (QTL) mapping was used to assess the genetic basis of the floral phenotypes of I. fulva and I. brevicaulis. Our results show that many floral trait QTL have large effects relative to the magnitude of the phenotypic difference between these species, indicating that few genetic steps, strictly in terms of QTL substitutions, may separate them.