Morphologic and Taxonomic Analysis of the Weedy and Cultivated Amaranthus hybridus Species Complex

The hybridus species complex of the genus Amaranthus is a group of weedy and cultivated plants from the New World that are considered difficult to identify. Classification schemes have varied between a single species approach, Amaranthus hybridus s.l., and a five species approach that recognizes the widespread weedy A. hybridus s.s., the South American endemic A. quitensis, and the three cultivated taxa (A. hypochondriacus, A. cruentus, and A. caudatus) as distinct species. The goals of this study were to analyze patterns of floral variation within the species complex and to determine distinguishing morphological features of the species. Twenty-one pistillate and twelve staminate floral characters from 41 specimens representing all five species were analyzed morphologically. Results indicate that morphological characters split the hybridus complex into two larger groups; that the widespread weedy A. hybridus divides into two morphologically distinct groups, each associated with different cultivated taxa; and that staminate morphological variation may be more taxonomically informative than previously assumed.