Laboratory of Todd Vision
Department of Biology
University of North Carolina at Chapel Hill

Genomic Analysis of Speciation in Mimulus

The plant genus Mimulus contains ~160 species, many of which are found in western North America and Australia. It has been a model for evolutionary ecology for over fifty years. The genus hosts a diverse array of mating systems, pollinator syndromes, habitat preferences and other adaptations, often between closely related species. The short generation time, small size, compact genome, self-compatibility, high fecundity, and ease of cultivation and crossability allow investigators to easily dissect the genetic basis for these traits. As part of a large collaborative effort, we are developing genomic resources for Mimulus and applying these same tools to study the molecular basis of reproductive isolation and species differences between several closely related members of the genus.

M. guttatus is a widespread outcrossing species, from which several selfing species have been derived. Interspecific crosses made by John Willis and colleagues at Duke University with one of these offshoorts, M. nasutus, have shown the presence of several hybrid incompatibility systems. These include a pair of interacting nuclear loci that cause sterility in F2 and later generation hyrbids, male sterilizing cytoplasm in guttatus that is only revealed when the guttatus cytoplasm is expressed on an nasutus nuclear background, and pervavise segregation distortion that predominantly favors the gutattus alleles. The genetic mechanisms of postzygotic isolation in this species pair has been influenced by the difference in mating systems in the two species and by the secondary effect this difference has had on the dynamics of intragenomic conflict.

Another species pair of interest is M. lewisii and M. cardinalis, which has been studied for a number of years by Doug Schemske (Michigan State University), Toby Bradshaw (University of Washington) and colleagues. There is a striking difference in pollination syndrome between the species: lewisii has the wide corolla, pale coloration, and small nectaries characteristic of a bee-pollinated flower, while cardinalis has the narrow, vertically-oriented, bright red flowers with nectar spurs that are characteristic of hummingbird pollination. Due to pollinator fidelity, along with altitudinal segregation, hybrids between these closely related species are extremely rare in the wild. Quantitative trait locus (QTL) mapping studies have identified the small number of genes responsible for these dramatic phenotypic differences. Field studies with artificial hybrids have shown which QTL are most important from the pollinator's perspective.

The genomic resources that we have been or are developing in Mimulus include expressed sequence tags, gene-based markers, saturated genetic maps, integrated physical maps based on large-insert clones that have been fingerprinted, end-sequenced and probed with markers on the genetic map, and permanent QTL mapping populations. Using these resources, we are investigating the molecular basis of the hybrid incompatibility systems in the guttatus x nasutus cross, and the adaptive differentiation in pollination syndrome seen in the lewisii x cardinalis cross. We are undertaking studies of the molecular evolution and population genetics of the loci contributing to reproductive isolation and adaptively important species differences.

Funded by the National Science Foundation Frontiers in Biological Research. For more information see the Mimulus FIBR homepage.

Latest news: The Department of Energy's Joint Genome Institute to sequence Mimulus! JGI news release