Systematics of Harrisia (Cactaceae)

The genus The genus Harrisia Britton (Cactaceae) comprises species of columnar cacti that are united by a unique seed morphology. The species range in form from prostrate shrubs to large trees and are native to South America and the Caribbean region. Harrisia is placed in an unresolved position within subtribe Trichocereinae of tribe Cereeae of subfamily Cactoideae. Relationships among the species within Harrisia are also poorly understood. In this study, several species of Harrisia were sequenced for as many as seven different regions of nuclear and plastid DNA. Species in the Caribbean were also examined with amplified fragment length polymorphisms. The morphology of Harrisia was characterized from herbarium specimens, live plants, and original descriptions. A biogeographic scenario was extrapolated from the molecular and morphological data. The flower morphology suggests a relationship between Harrisia and some species of Echinopsis s. 1. However, DNA sequence analyses in this study do not clearly resolve generic relationships with Harrisia. Molecular and morphological data support recognition of two subgenera, four sections, and two series within Harrisia. It is proposed that Harrisia originated in the west-central Andes, ∼3.5-6.5 Ma ago. Subgenus Eriocereus is composed of the species in the east Andes of Bolivia and the nearby species radiation in the Gran Chaco. Subgenus Harrisia originated by an early dispersal event into Brazil with subsequent dispersal into the Caribbean. In the last 500 Ka, Harrisia, colonized west Cuba and further diversified into other areas of the Caribbean. Harrisia is revised to contain 18 species.