Date of Award

Spring 1-1-2012

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ecology & Evolutionary Biology

First Advisor

Pamela K. Diggle

Second Advisor

William Friedman

Third Advisor

David Stock


Modern studies of evolutionary developmental (evo-devo) biology explore the molecular and developmental basis of morphological novelty, diversification, and deep homologies. Such studies primarily have focused on macroevolutionary problems by comparing developmental patterns across broad phylogenetic distances. However, natural selection and other evolutionary processes occur at the level of populations. Whether the key genes and genetic networks identified by evo-devo studies in model organisms also are responsible for morphological variation at the population level, where evolutionary processes such as selection act, is less clear. I address this problem by examining the morphological and molecular developmental basis for intraspecific variation of shoot architecture in plants from two locally adapted populations (DUN and IM) of Mimulus guttatus (monkeyflower) with contrasting flowering times, life histories, and branch numbers. I grew second-generation seed in growth chambers to control for maternal and environmental effects. At the third and more distal nodes of the main axis, heterochronic processes contributed to flowering time and branch number differences between populations. Most branches, however, occurred at nodes one and two. At these nodes, branching occurred earlier and with greater frequency in DUN compared to IM plants. Plants from both populations initiate axillary meristems at the same time. At node 1, axillary meristems from both populations are vegetative; branch number at node 1 is determined by the frequency of meristem outgrowth. At node 2, axillary meristems of DUN plants are vegetative whereas meristems of IM plants are either vegetative or floral. Differences in branch number at node 2 are caused by differences in both axillary meristem fate and the frequency of vegetative axillary meristems outgrowth. I provide some of the first quantitative molecular genetic data from plants that associates developmental genes with intraspecific, natural variation in a functionally and evolutionarily critical aspect of morphology over the course of development. I identified four putative M. guttatus MORE AXILLARY GROWTH (MAX) orthologs. In Arabidopsis (and other model species), MAX genes and their orthologs negatively regulate branch outgrowth. Consistent with this function, MgMAX1, 2, 3, and 4 are all expressed at significantly higher levels in unbranched IM plants than highly branched DUN plants.