Population Genetics and Mating System of the Rare Polyploid Lepidium Papilliferum (Brassicaceae), a Southwestern Idaho Endemic

Rare plant species typically exhibit low levels of genetic diversity within populations and high levels of genetic differentiation among populations. Several mechanisms may counter the genetic consequences of rarity such as polyploidy or an outcrossing mating system. However, for rare species whose size and distribution may already be limited, habitat destruction and fragmentation can alter historic patterns of gene flow and increase the risk of genetic erosion. Lepidium papilliferum, slickspot peppergrass, is a rare polyploid, insect pollinated, herbaceous plant endemic to sagebrush steppe communities of southwestern Idaho. Plants exhibit a naturally patchy distribution, occurring in specialized microhabitats called slick spots. Habitat destruction and fragmentation due to overgrazing by livestock, invasive plant species, urban and agricultural development, and off-road vehicle use has lead to a decline in this species. Enzyme electrophoresis was used to determine the amount and distribution of genetic diversity within and among 25 populations of L. papilliferum, including 18 populations from the Snake River Plain and 7 populations from the Jarbidge region. Enzyme banding patterns and a gene duplication at TPI were consistent with polyploid gene expression. Across all populations, 10 of 11 loci (91%) were polymorphic, with an average of 4.27 alleles per locus. Thus, compared with endemic diploid plant species, L. papilliferum exhibits high levels of genetic diversity. On average, the disjunct Jarbidge populations exhibited higher levels of polymorphic loci (P = 80.1%) compared with the Snake River Plain populations (P = 58.1%) and similar number of alleles per locus, A = 2.16 and 2.09, respectively. Measures of genetic diversity were positively correlated with population size for all populations, however this trend was most pronounced in Snake River populations. Genetic differentiation (GST) among all populations was 0.109. Among populations of each region, GST was 0.042, indicating that the majority of genetic differentiation among all populations is due to genetic differences between the two regions. Management of L. papilliferum should include maintaining large population that still harbor high amounts of genetic diversity, conserving small populations where rare alleles are known to occur, and the conservation of insect pollinators and their habitat in this primarily outcrossing species.