The role of developmental constraint in mating-system evolution in Leavenworthia: A quantitative genetic analysis

The shift to a self-fertilizing mating system from an outcrossing ancestral state is one of the most common evolutionary transitions in flowering plants. It is striking that several morphological and developmental changes, such as reduction in flower size and earlier timing of reproduction, are consistently observed in the selfing taxa relative to outcrossing taxa. Why is this? If a developmental connection exists between earlier time-to-flowering and reduction in flower size in selfing plant species relative to their outcrossing ancestors, direct selection for earlier time-to-flowering could cause an increase in selfing rate due to decreased spatial separation of male and female parts within the flower. This indirect selection on selfing rate via direct selection on a developmental parameter is an example of a developmental constraint and can be estimated by measuring the genetic correlation between these two traits. I used two approaches to measure the consequences of developmental constraints in related outcrossing and selfing species in the genus Leavenworthia . First, measurements of phenotypic correlation patterns in three species of Leavenworthia were carried out to test predictions of functional and developmental constraints among floral traits in taxa that differ in mating system. Second, measurement of responses and correlated responses in lines artificially selected for high and low autonomous selfing rate and flowering time were undertaken to estimate the realized genetic correlation between these traits. Patterns of floral-trait correlations suggested that relaxed pollinator-mediated selection leads to weaker integration in floral traits in selfing taxa of Leavenworthia; and other observed patterns supported the hypothesis that underlying developmental relationships among floral traits can also increase phenotypic integration and constrain evolutionary change. Responses and correlated responses in the selection experiment indicate a positive genetic correlation between selfing rate and time-to-flowering, and correlated responses were also observed in a suite of floral and developmental traits. The results of the artificial selection experiment also support the role of bud development, rather than flowering time, in indirect selection for selfing rate in Leavenworthia torulosa. In general, my results suggest that autonomous selfing rate can be decoupled from whole plant development, but may be associated with floral development and morphology.