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Duplicate genes cause hybrid lethality between sympatric species of Mimulus


As a common cause of reproductive isolation in diverse taxa, hybrid incompatibilities are fundamentally important to speciation. Recent work has provided insight into the molecular functions of genes involved in hybrid dysfunction, but we still know little about how such genes initially evolve within species. Here we describe a simple genetic incompatibility that causes lethality in hybrids between two closely related species of yellow monkeyflower ( Mimulus guttatus and M. nasutus). This hybrid incompatibility, which causes a fraction of F2 hybrid seedlings to lack chlorophyll and die shortly after germination, occurs between sympatric populations that are connected by ongoing interspecific gene flow.
We genetically mapped this hybrid lethality to two small regions that contain duplicated copies of the Mimulus homolog of pTAC14, an essential gene for chloroplast development. Furthermore, in a series of genetic crosses between plants collected from throughout the species’ ranges, we have discovered that both hybrid lethality alleles are widely distributed and rarely fixed within populations. Polymorphism at one of the loci appears to be driven by introgression between the two Mimulus species. This hybrid lethality system offers a unique chance to directly investigate the evolutionary dynamics of incompatibilities in nature.