Pollinator behaviour and the evolution of Louisiana iris hybrid zones

Abstract

Pollinator preference may influence the origin and dynamics of plant hybrid zones. Natural hybrid populations between the red-flowered Iris fulva and the blue-flowered Iris brevicaulis are found in southern Louisiana. The genetic structure of these populations reflects a lack of intermediate genotypes. We observed pollinator behaviour in an experimental array with five plants each of I. fulva, I. brevicaulis, their F1, and the first backcross generation in each direction, to obtain data on flower type preferences and transitions between flower types. The most abundant visitors were Ruby-throated Hummingbirds (Archilochus colubris) and workers of the bumblebee Bombus pennsylvanicus. Hummingbirds visited I. fulva twice as often as I. brevicaulis and visited hybrids at intermediate frequencies. Bumblebee workers preferred the purple-flowered F1s and visited plants of I. fulva and the backcross to I. fulva more often than I. brevicaulis and its backcross. Overall, F1 flowers were visited most frequently. Both hummingbirds and bumblebees visited nearest neighbours in almost 80% of the interplant movements. This meant that a majority of movements were between different flower types, rather than between plants of the same type. Findings from the present study suggest that pollinator preference is not a major causal factor for the lack of intermediate genotypes in natural iris hybrid populations. Instead, pollinator behaviour in our array promoted mixed mating between flower types belonging to different pollination syndromes. However, owing to predominant nearest-neighbour visitation, the spatial distribution of parental and hybrid genotypes (in concert with pollinator behaviour) will have a strong influence on mating patterns and thus the genotypic structure and evolution of Louisiana iris hybrid zones.