The Mexican Balsam, Impatiens mexicana Rydb: A Redescription Based on Morphological and Phylogenetic Studies, with an Update of the Current Geographical Range of the Species

Impatiens mexicana is a native balsam restricted to the cloud forests of central and southwest Mexico, which is currently known to exist in four states (Zacatecas, Veracruz, Puebla, and Oaxaca), and is probably threatened by geographic restriction. Morphological and distributional information about this species is still scarce, with only a couple of investigations since its first description in 1910, which means the phylogenetic position of this species has yet to be placed. In order to contribute to knowledge of I. mexicana, morphological and molecular studies were carried out with material collected in the localities of Hidalgo, Veracruz, and Oaxaca, during expeditions from July to October 2021 and in 2022. The specimens selected were measured, photographed, and compared with morphological information from the six American balsams. At the same time, phylogenetic studies were carried out by using two regions of the cpDNA and one of the nrDNA. We report I. mexicana for the first time in Hidalgo state, and identify new registers in the Veracruz and Oaxaca localities, thus increasing the knowledge in its geographic distribution. We also redescribe the Mexican balsam by using new and complementary traits: we note that while the species is morphologically similar to I. capensis, it is distinguished by differences in flower color, the distribution of spot patterns on the upper petal, lower sepal, and lateral petals, and geographic information such as endemic geographic distribution in Mexico’s Tropical Montane Cloud Forest (TMCF). We also carried out phylogenetic analyses by using ITS and ITS + atpb-rbcL, which showed the Mexican balsam was independent from its Asian-American congeners. Interestingly, our genetic distance analyses reveal differences of 0.01–0.16% between I. mexicana and the other North American congeners that use ITS, atpb-rbcL and trnL-F, both independently and as concatenated genes: this low divergence most likely occurred because of the recent diversification of the group. However, both future redescriptions and phylogenetic studies in American species are imperative, as this will enable better discrimination.