Variation of Self‐Incompatibility within Invasive Populations of Purple Loosestrife (Lythrum salicariaL.) from Eastern North America

Colonization may favor self‐compatibility (SC) in invasive plants, a process consistent with Baker’s law. We investigated this hypothesis in invasive eastern North American populations of tristylous Lythrum salicaria L. (purple loosestrife) by controlled self‐ and cross‐pollinations of 124 plants sampled from 12 populations grown under uniform glasshouse conditions. We evaluated whether populations at the northern front of the invasion exhibited higher levels of SC than southern populations, which are closer to source populations for the North American invasion. We also sought evidence for morph‐specific differences in the strength of trimorphic incompatibility. We used the ASTER statistical program to jointly model three measures of SC: (1) the probability of producing at least one seed following self‐pollination, (2) the number of fruits produced by self‐pollination, and (3) the average number of seeds per fruit resulting from self‐pollination. Just over one‐quarter of plants set at least one fruit after self‐pollination, and there was a significant effect of style morph on variation in SC, with the mid‐styled morph most compatible. Although variation for SC was detected in 11 of the 12 populations, there was no evidence that SC increased toward the northern range limit as a result of the invasion process.