POPULATION STRUCTURE HAS LIMITED FITNESS CONSEQUENCES IN THE HIGHLY SELFING PLANT LEAVENWORTHIA UNIFLORA (BRASSICACEAE)

Selfing populations are often genetically depauperate and substantially differentiated. These genetic consequences of selfing are thought to contribute to the reduced longevity of natural populations. We analyzed multilocus microsatellite variation and population structure to examine the likelihood of high levels of selfing in Leavenworthia uniflora. We also conducted crosses between two populations with the greatest degree of microsatellite divergence to test the hypothesis that these populations have substantial mutation loads or express epistasis for fitness indicative of local adaptation or intrinsic reproductive isolation. Populations were inbred (F-IS = 0.80) and highly selfing (S = 0.89) and maintained limited allelic diversity (A = 1.42). Genetic differentiation in L. uniflora reflects high levels of selfing and spatial structure (F-ST = 0.66), with two recent seed dispersal events having strong effects on the distribution of genetic diversity. Reciprocal crosses between populations (Missouri and Indiana) revealed divergent germination and flowering rates, yet dominance and epistasis were not inferred for any trait. The Indiana cytoplasm lowered germination and flowering rates and fruit number in the F-1, but these effects diminished in the F-2, being caused by maternal environmental effects. Although L. uniflora reproduces predominantly through selfing in nature, the lack of support for heterosis or hybrid breakdown is likely explained by the recent divergence of populations in this species.