Tuesday, June 19, 2007 - 4:05 PM
Ponderosa Pines (Boise Centre on the Grove)

Insect-mediated pollination and spatial structuring in Lepidium papilliferum populations

Ian C. Robertson, Amy C. Ulappa, and Stephanie Billinge. Boise State University, Boise, ID

Spatial genetic structuring in plant populations is expected to occur when the populations are fragmented and pollen movement and seed dispersal are limited. Lepidium papilliferum, a rare mustard endemic to southwest Idaho, fits this description in at least two ways: L. papilliferum populations are fragmented at several spatial scales, and the plant's fruit and seed lack obvious structures for long-distance dispersal. Here we examine whether the third condition, limited pollen movement, holds and whether there is evidence of genetic structuring in L. papilliferum populations. We show that L. papilliferum relies primarily on outcrossed pollination mediated by insects, including several families of Hymenoptera (Apidae, Halictidae, Sphecidae, Vespidae), and Diptera (Bombyliidae, Calliphoridae, Tachinidae). Although these insects may be expected to serve as long distance pollen dispersers, evidence was to the contrary. Insects visiting L. papilliferum flowers tended to linger on individual plants or among plants that were in close proximity to one another. More importantly, outcrossing experiments revealed that fruit production in L. papilliferum increased as a function of outcrossing distance, indicating that populations are spatially structured. Structuring was evident even at relatively small spatial scales (e.g., < 3 m), suggesting either that genetic relatedness among individuals declines sharply as a function of distance, or that L. papilliferum, while intolerant of high levels of inbreeding, is tolerant of low levels of inbreeding. The consequences of habitat fragmentation for gene flow in L. papilliferum will be discussed.