Background: Shrubs recognized as I. hartmanii were known from the northern Mexican deserts, whereas I. cassiniiformis is considered to occur throughout the Mexican highlands. Morphologically similar individuals were reported under both names also south beyond the Isthmus of Tehuantepec to Guatemala.
Question: Are I. hartmanii and I. cassiniiformis distinct species, or do they represent widespread taxa exhibiting clinal variation due to adaptation along a gradient spanning desert to evergreen tropical forest?
Taxon: Amaranthaceae, Iresine.
Study site: Mexico and Central America.
Method: Field and herbarium work, molecular phylogenetics, morphology, palynology, distribution mapping.
Results: Plastid and nuclear phylogenetic trees suggest a complex speciation scenario in Iresine, revealing new lineages in the mountains of Chiapas, Oaxaca and Guatemala that also differ by morphological characters. Iresine cassiniiformis appears monophyletic and occurs in the Trans-Mexican Volcanic Belt whereas I. hartmanii comprises a core clade of individuals confined to the northern deserts. Specimens morphologically intermediate to I. cassiniiformis, and part of the I. hartmanii species clade in nrITS, occur directly south of the desert zone, either representing ancestral types in the speciation of I. hartmanii or introgression from I. cassiniiformis.
Conclusion: Our results underscore the importance of an integrative taxonomy approach including molecular phylogenetics and morphology with a dense sampling of putative species from throughout the geographical range in order to arrive at explicit hypotheses on species limits. Here this supports the description and naming of two new species.
