Reproductive Performance as a Function of Outcrossing Distance in \u3cem\u3eLepidium Papilliferum\u3c/em\u3e (Brassicaceae), a Rare Plant Endemic to Southwest Idaho

Plant populations are often spatially structured such that individuals closer to one another are more genetically related than those farther apart. Individuals that reproduce with closely related individuals may experience reduced fruit production and/or reduced offspring performance owing to partial or full self-incompatibility, inbreeding depression, or both. Individuals that reproduce with distantly related individuals may also experience a reduction in fruit production and offspring fitness owing to outcrossing depression. By crossing plants that grow at different distances from each other and examining the resulting fruit and seed production, as well as the germination ability and growth rates of the offspring, it is possible to infer the spatial genetic structure within populations. Small populations may be expected to have less spatial structure than large populations because of reduced overall genetic variability and/or higher levels of inbreeding, particularly if they have become genetically isolated from other populations. I examined fruit production and offspring vigor as a function of outcrossing distance in 10 small populations and one large population of slickspot peppergrass, Lepidium papilliferum (Brassicaceae), a rare mustard endemic to southwest Jdaho. Long distance crosses were also conducted to determine if L. papilliferum experiences outcrossing depression. A self-compatibility experiment was conducted to determine if fruit set in self-pollination treatments in a previous experiment was due to insect contamination. Fruit production increased significantly with increasing distance between parents up to approximately 3 m, and then declined slightly at greater outcrossing distances, suggesting that populations are spatially structured. Percent fruit set for each treatment was not significantly different between the one large population and the 10 small populations, indicating that population structure, at least as it relates to percent fruit set, is similar between differently sized populations. Log transformed leaflength and plant width measurements of offspring increased with increasing outcrossing distance, suggesting that L. papilliferum experiences inbreeding depression. The decline in percent fruit set at distances greater than 3 m was not statistically significant, although offspring resulting from these long distance crosses were significantly larger than closer crosses. Given these contrasting results, more research is necessary to determine whether L. papilliferum suffers from outcrossing depression. Results of the self-compatibility study indicate that L. papilliferum has some self-fertilization ability, contrary to assertions in some earlier field studies, but consistent with the results of a recent genetics study