Pollination biology in hybridizing Baptisia (Fabaceae) populations.

In their classic study, Alston and Turner (American Journal of Botany, vol. 50, 159-173, 1963) documented extensive hybridization among four morphologically distinct Baptisia species native to East Texas. While Alston and Turner found putative F1 hybrids in great numbers, they found no evidence of backcrossing. In this study prezygotic and postzygotic reproductive barriers between two of these species, B. leucophaea and B. sphaerocarpa, were investigated and found to be quite weak. Flowering times overlap and bumble bees were observed visiting both species and intermediate hybrids. While pollinator constancy in flights between B. leucophaea and B. sphaerocarpa was moderately strong, significant levels of constancy were not observed in flights involving hybrids and either parental species. Thus, backcrossing was not impeded by pollinator behavior. Further, hybrid pollen was highly stainable (93.5%) and able to effectively set seed in crossing experiments with both parental species. Pollinator behavior was compared in experimental populations with and without hybrid ramets and found to differ between these two treatments. Hybrids were found to facilitate pollinator movement between species. In total, these results suggest that reproductive isolation is not responsible for the rarity of backcrossing in naturally hybridizing B. leucophaea and B. sphaerocarpa populations.