Parental Conflict, Parent of Origin Effects, and the Evolution of Hybrid Seed Failure in Mimulus
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The earth is home to roughly 9 million eukaryotic species. The formation and maintenance of this diversity requires the accumulation of barriers to reproduction. One of the most common post-zygotic barriers in plants is hybrid seed inviability (HSI). Despite its commonality, we know relatively little about the genetic mechanisms and evolutionary forces which are responsible for this barrier, particularly in naturally co-occurring species. Here I tested the role of parental conflict in HSI between co-occurring monkeyflowers in the M. guttatus species complex. I assessed the strength and directionality of HSI within and between phenotypically described perennial variants of the M. guttatus species complex. I find substantial HSI between two morphologically described variants, M. guttatus and M. decorus, amounting to ~30-50% reproductive isolation. Genetically distinct clades of M. decorus vary in their ability to cross with M. guttatus in both magnitude and direction of RI. Intriguingly, northern and southern clades of M. decorus are also incompatible with one another, showing strong, symmetric hybrid seed inviability. In all cases, HSI is accompanied by parent of origin effects on F1 seed size, consistent with a role of resource allocation to hybrid inviability. Parent of origin effects on reciprocal F1s were not limited to final size. I characterize the developmental trajectory of seeds between M. guttatus and both the northern and southern clades of M. decorus. Hybrid seeds show similar developmental trajectories depending on their end fate (i.e. viable vs inviable), despite differing in the maternal and paternal contributors in these crosses. Inviable seeds are characterized by patterns of paternal excess; chaotic and malformed endosperm. Viable hybrid seeds are characterized by maternal excess; limited, and precocious endosperm development. Relatively normal embryo development early on suggests that hybrid seed inviability stems, in part, from malformed endosperm. Lastly, I use a combination of modeling, next generation sequencing, and genotyping approaches to determine the genetic architecture and basis of HSI. I find that the genetic basis of HSI is complex, wherein several maternally and paternally inherited nuclear loci interact to cause HSI. In total, I show the presence of multiple perennial species in the M. guttatus species complex. These species show little phenotypic or obvious ecological differentiation, but are genetically unique and have substantial post-zygotic reproductive isolation, namely through the formation of inviable seeds. Patterns of seed development and final size are indicative of parent of origin effects on resource allocation to endosperm. Despite relatively low genetic differentiation for the group, the genetic basis of HSI is quite complex, involving multiple maternally and paternally interacting alleles. Future work will be needed to determine the identities of these genes, as well as patterns of repeatability across the complex.
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