Out of the swamp: unidirectional hybridization with weedy species may explain the prevalence of Amaranthus tuberculatus as a weed.

*Amaranthus tuberculatus represents one of the most dramatic cases of weed invasion documented in the midwestern USA. The species is infamous for evolving resistance to multiple herbicides, and predicting whether these resistances may be transferred to widespread weeds of the Amaranthus hybridus aggregate is a matter of epidemiological concern. Here, we explore the patterns of genetic exchange between Amaranthus tuberculatus and A. hybridus in an effort to understand whether allele introgression occurs throughout the genome and if fecundity penalties are associated with genetic exchange. *We evaluated 192 homoploid BC(1)s at 197 amplified fragment length polymorphism (AFLP) loci, as well as two loci associated with herbicide resistance: ALS and PPO. We also assessed the fecundity of each genotype by evaluation of seed production or pollen development. *It was discovered that genetic exchange between the species is unidirectional. Whereas A. hybridus alleles transfer with little or no penalty to A. tuberculatus, the reciprocal exchange is significantly distorted and potentially of limited evolutionary consequence. *Our previous hypothesis suggesting unidirectional introgression at ALS owing to circumstantial linkage is now modified to account for the more generalized distortion of genetic exchange observed in this study.