Pollination, breeding system, and genetic structure in two sympatric Delphinium (Ranunculaceae) species

Two sympatric Delphinium species, D. barbeyi and D. nuttallianum, are ecologically and morphologically similar. However, D. barbeyi has multiple, large inflorescences while D. nuttallianum has a single, small inflorescence. These differences in floral display should result in greater intraplant pollen transfer in D. barbeyi, leading to higher rates of self-pollination through geitonogamy. Reduced gene flow by pollen should in turn produce greater population differentiation among populations of D. barbeyi relative to D. nuttallianum. We tested these predictions by comparing pollinator behavior, breeding systems, outcrossing rates, and population genetic structure of sympatric populations of the two species in Colorado. Bumble bee and hummingbird pollinators visit more flowers and inflorescences per foraging bout in D. barbeyi than in D. nuttallianum. The species differed in breeding system; D. barbeyi produced more seeds by autogamy (9 vs. 2%) than D. nuttallianum and suffered no reduction in seed set in hand-self vs. outcross pollinations, in contrast to a 41% decline in D. nuttallianum. The outcrossing rate in one D. barbeyi population was 55%, but ranged from 87 to 97% in four D. nuttallianum populations. Genetic differentiation among population subdivisions estimated by hierarchical F statistics was >10 times greater in D. barbeyi ( = 0.055-0.126) than D. nuttallianum ( = 0.004-0.009) at spatial scales ranging from metres to 3.5 km. Spatial autocorrelation analysis also indicated more pronounced local genetic structure in D. barbeyi than D. nuttallianum populations. Fixation indices (F(IS)) of D. barbeyi adults were much lower than expected based on mating system equilibrium and suggest that differences in the degree of self-compatibility and/or the timing of postpollination selection/inbreeding depression between the two species further contribute to the genetic differences between them.