Subspecies classification of Hydrilla verticillata (Hydrocharitaceae)

Abstract

Hydrilla verticillata is a globally distributed aquatic plant and one of the most noxious aquatic weeds in the U.S.A. Phylogenetic evidence has shown that plants in different geographic regions are largely isolated, and there are morphological features that can distinguish the five major phylogenetic groups. Hydrilla plants in the U.S.A. have been identified as monoecious or dioecious biotypes, with each biotype having distinctive morphology and molecular sequence data. However, the recent discovery of a third invasive biotype in the northeastern U.S.A. has prompted the need for precise nomenclature that can refer to Hydrilla plants in the native and non-native range of this widespread species. New subspecific names and combinations are proposed in order to enable effective communication about the five major phylogenetic variants of Hydrilla verticillata. Two subspecies names are made as new combinations of existing species names: Hydrilla verticillata subsp. angustifolia and Hydrilla verticillata subsp. lithuanica, and two subspecies names are newly established: Hydrilla verticillata subsp. australis and Hydrilla verticillata subsp. peregrina.