Evolution and genetics of floral form in Mimulus

Debate over the pace of adaptive evolution has been ongoing for over 200 years. This debate is exemplified in research on the mechanisms by which new floral forms give rise to new species of angiosperms. The monkeyflowers Mimulus lewisii and M. cardinalis have been used to study the genetics of adaptation for decades. The work presented here addresses the issue of pacing in floral evolution in three ways: (1) Using chemical mutagenesis to induce single locus mutations in M. lewisii, (2) Examining the ecological consequences of changes to floral form by observing pollinator preference, and (3) Developing genetic maps in which these mutations can be mapped and identified. The large-scale mutant screen yielded mutant plants with phenotypes that mimic traits seen in its sister taxon, M. cardinalis, as well as other, more distantly related species of Mimulus, suggesting that the rapid radiation of floral form in this genus may have occurred by single locus mutations. The overall reduction in bumblebee visitation experienced by three of these mutants suggests that novel forms that arise in nature would likely see a similar reduction. Exclusion of the dominant pollinator, however, may be the first step in the process of a pollinator shift as pollinator exclusion may allow for a greater reward due to the accumulation of nectar, and greater sensitivity to selective pressures by new or previously minor pollinators. Using new gene-based markers, we created two genetic maps, one within M. lewisii and one in a hybrid backcross of M. lewisii and M. cardinalis. These maps revealed two instances of pseudolinkage, as well as significant compression in nearly every linkage group and may explain previous observations of correlation between unrelated traits in these species. This work provides a framework for further investigation into the adaptive evolution of floral form in Mimulus through mutant characterization, pollinator observation and genetic mapping.