GENETIC COMPARISONS OF SEED BANK AND SEEDLING POPULATIONS OF A PERENNIAL DESERT MUSTARD,LESQUERELLA FENDLERI

Soil seed banks may accumulate and store seed genotypes produced over many seasons. If germination and establishment of these soil seeds are influenced by seed genotypes, then seed bank and seedling populations may differ genetically. I compared the genetic structure of dormant but viable soil seeds of the desert mustard Lesquerella fendleri with the genetic structure of Lesquerella seedlings at the Sevilleta Long-Term Ecological Research Site. In 1991 and 1992, soil seeds and seedlings were mapped and genetically analyzed using starch gel electrophoresis. When data from all loci were lumped, there were highly significant differences in allele frequencies between soil seeds and seedlings at the population level (all plots) in both years, in all subpopulation (adjacent plots) comparisons in 1991, and three of five subpopulations in 1992. Differences at some individual loci were also detected in one or both years. Analysis of data pooled across both years revealed highly significant differences in the distribution of multilocus soil seed and seedling heterozygosity, but no significant differences in mean heterozygosity. F_st values showed small but statistically significant genetic differentiation within soil seeds and seedlings in both years. F_st values also showed significant genetic differentiation between these two groups at three of seven loci in 1991, and at one locus in 1992. Soil seeds and seedlings showed a general pattern of decreasing genetic relationship with distance, as estimated by the coefficient of coancestry analyses. In 1991, seedlings were roughly twice as genetically related to each other than were soil seeds at fine spatial scales (0–0.25 and 0.25–0.50 m). This study suggests that Lesquerella seedlings in this system represent a nonrandom genetic subset of the underlying Lesquerella seed bank. Such temporal genetic change may be an important yet frequently overlooked mechanism for generating population genetic structure.