The mating system of invasive plants can play an important role in determining the success of these species in colonizing new habitats and rapidly expanding their range. Self-fertilization provides reproductive assurance in the absence of conspecifics and/or pollinators. However, inbreeding can result in inbreeding depression and lower fitness of inbred individuals as compared to their non-inbred counterparts. Additionally, invasive species are frequently founded by few individuals and are isolated from other populations. These populations may go through significant genetic bottlenecks and be subjected to genetic drift as a result of founder effects. The object of this study was to examine the effects of inbreeding on the highly invasive biennial, Alliaria petiolata (garlic mustard) by examining the relative quality of progeny produced by self- and cross-fertilization. A controlled pollination experiment was applied to Central Illinois populations of A. petiolata to produce self- and cross-fertilized seeds. Cross-fertilization treatments included two between population outcrosses and one within population outcross.

Results/Conclusions

Overall, seed and seedling traits were significantly affected by cross-type. No significant differences for seed or seedling traits were detected between progeny from within-population crosses and self-fertilization. However, there were significant differences among progeny from between-population outcrosses when compared to self-fertilized progeny and within population outcross progeny. Seed mass and cotyledon length were greater and leaves emerged earlier for progeny derived from between population outcrosses compared to progeny resulting from.self-fertilization and within population crosses. Low levels of inbreeding depression were expressed for early traits (Seed mass δ = 0.11, Cotyledon length δ = 0.05) when progeny derived from between population-outcrosses and self-fertilization were compared. Within population outcrosses and selfing produced progeny of the same relative quality for seed and seedling traits (δ = 0.01), indicating that there is genetic drift load within populations consistent with founder effects and continued population genetic structure. Although expression of inbreeding depression in early traits of A. petiolata is relatively low, deleterious genetic effects of inbreeding on early life cycle traits, such as seed mass and seedling growth, are often masked by maternal effects. Consequently, we are currently quantifying the effects of inbreeding depression across the life cycle of A. petiolata and the effect of intraspecific competition on the expression of inbreeding depression in this species.