Ruehssia magalloniae (Apocynaceae; Asclepiadoideae), una especie nueva del bosque mesófilo de montaña de la Sierra Norte de Oaxaca, México

Background: As part of an expedition to checklist the Flora of Oaxaca, a fieldtrip was carried out in the cloud mountain forest of the ‘Sierra Norte’, during which there was a climbing plant found of the genus Ruehssia that differs of any other species in the genus. Questions: The morphology of the corolla, the gynostegial corona and the fruit, as well as the geographical distribution are all elements that provide a basis for distinguishing between species of Ruehssia. Here we propose a new species on the grounds of the cohesive concept of species. Studied species: Ruehssia sp. nov. Study site and dates: Sierra Norte de Oaxaca, México; 1989-2022. Methods: An analysis of the morphology and distribution of the populations of the species in question and related taxa was conducted using the collected specimens and herbarium material. In respect to the IUCN criteria, a vulnerability category was established for the species. Results: The species is similar to Ruehssia rzedowskiana and R. calichicola, but there is a morphological difference regarding the corolla, the gynostegial corona, and its distribution. A morphological description of the new species is provided, in addition to images of the revised taxa, and a map of its geographical distribution and state of conservation. A key to the 18 species of Ruehssia in Oaxaca is also included. Conclusions: Ruehssia magalloniae is recognised as a new species in science. This discovery increases the number of species of Ruehssia listed in Mexico to 39 with a 58 % of endemism.