Large proportions of nonhybrid progeny result from controlled crosses between Palmer amaranth and common waterhemp, both dioecious weeds. Agamospermy was proposed as an explanation for this phenomenon, and here we provide evidence in support of this hypothesis. We evaluated 60 nonhybrid offspring from two interspecific crosses, and all plants were females and had DNA content values similar to the female (Palmer amaranth) parent. Among nine hybrids resulting from these crosses, eight were nonviable (lethal or neuter), and only one hybrid allowed for continued gene movement. Cytogenetic evaluation of this hybrid revealed triploidy, further supporting the occurrence of unreduced gametes in these species. In light of this new evidence, we examine earlier data regarding Palmer amaranth by common waterhemp hybridization and suggest some prior conclusions may be premature.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPAcommon waterhemp, Amaranthus tuberculatus (Moq.) Sauer var. rudis (Sauer) Costea & Tardif AMATA