Abstract
The cycad genus Dioon (Zamiaceae) was described by J. Lindley in the 19th century, but most species were discovered during the late 1970s through the mid-1980s. Despite recent efforts to clarify the actual species diversity within the genus, the definition of some species is still problematic. The northernmost populations of the cycad genus Dioon in northwestern Mexico are currently recognised as Dioon sonorense, but important variation among populations has been suggested by multiple authors, and the definition of what is D. sonorense remains unclear. Documented evidence on leaf and leaflet morphology, cuticular and epidermal anatomy, and population genetics across the populations of D. sonorense, as currently circumscribed, suggest that the taxon contains two biological units that should be taxonomically distinguished, given their importance for systematics and conservation. Here we highlight that 1) D. sonorense is defined by the variation exhibited only in populations from southern Sonora and northern Sinaloa, Mexico; 2) the northernmost populations of Dioon in northwestern Mexico represent a new taxon that we describe as Dioon vovidesii. We also discuss the implications of this reassessment for cycad systematics and conservation.