Several populations of different Amaranthus species have been reported resistant to single or multiple herbicides. Interspecific hybridization among amaranths is hypothesized to contribute to the evolution of herbicide resistance. Although other studies have shown the occurrence of interspecific Amaranthus hybrids, little has been done to establish the likelihood of hybridization under field conditions. The main objective of this study was to determine potential field frequencies of hybridization between tall waterhemp females and smooth pigweed. Field hybridization plots were established during each of two growing seasons. Individuals of the two species were transplanted to field plots and arranged at varying distances from each other. Hybrid progeny were detected using the acetolactate synthase (ALS) gene as a marker. Smooth pigweed parents were homozygous for a herbicide-resistance ALS allele, whereas maternal tall waterhemps were homozygous for a herbicide-sensitive ALS form. Heterozygous interspecific progeny were thus detected by means of herbicide selection. Molecular and cytogenetic tools were used to verify the validity of the data obtained. Averaged among female waterhemp plants and across the two field seasons, hybridization occurred at a frequency of 33%. A single tall waterhemp plant was capable of producing more than 200,000 hybrids, suggesting little if any gametic incompatibility. All flowering hybrids obtained from tall waterhemp females were of dioecious condition and female sex. Observed sexual segregation was consistent with previously postulated chromosomal XY-type system in tall waterhemp sex determination, where males are the heterogametic sex.

Nomenclature: Smooth pigweed, Amaranthus hybridus L. AMACH; tall waterhemp, Amaranthus tuberculatus (Moq.) Sauer AMATU.