Evolution and genetics of hybrid lethality in mimulus
Zuellig, Matthew Paul
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Understanding the evolutionary forces and genetic mechanisms that give rise to new species is a major goal among biologists. The evolution of reproductive barriers is a key component of speciation, as these barriers prevent interspecific gene flow, allowing for species to diverge along distinct evolutionary trajectories. While numerous studies have inferred the importance of particular reproductive barriers in the evolution of species, no study has identified the genetic basis and evolutionary dynamics of reproductive barriers in species that co-occur and hybridize in nature. My dissertation is the first to resolve this problem, by studying the evolutionary genetics of hybrid lethality that occurs between two closely related species of yellow monkeyflower, Mimulus guttatus and M. nasutus, which co-occur throughout western North America and exhibit patterns of ongoing gene flow. In the first chapter I take a genetic mapping approach to describe the genetic basis of hybrid lethality in Mimulus, demonstrating that hybrid lethality is caused by two recessive nuclear loci (hl13 and hl14) consistent with the Dobzhansky-Muller incompatibility model. In chapter two, I perform a large crossing experiment, to determine the distribution, frequency, and origin of hybrid lethality alleles in M. guttatus and M. nasutus populations throughout their common range. I demonstrate that hl13 and hl14 are widespread and polymorphic throughout the Mimulus range, where polymorphism is likely caused by interspecific gene flow and incompatibility alleles may be maintained in migration-selection balance. Finally, chapter three describes a series of transcriptomic experiments that were used to identify the genes that cause hybrid lethality. Our data demonstrate that the gene plastid transcriptionally active chromosome 14 (pTAC14) was duplicated in M. guttatus followed by nonfunctionalization of the ancestral copy, whereas no duplication occurred in M. nasutus. Hybrid lethality occurs when the chromosomal region associated with the ancestral copy of pTAC14 is homozygous for M. guttatus alleles and the region where the functional pTAC14 copy transposed, but is lacking in M. nasutus, is homozygous for M. nasutus alleles. This work addresses long-standing questions regarding the origin of reproductive barriers and the mechanisms that allow them to persist in systems where gene flow is ongoing.