Unravelling the Mexican Magnolia dealbata (Magnoliaceae) species complex

In the last two decades, approximately 80 new Magnolia species have been described from the Neotropics; thus this region now hosts almost half of the world’s known Magnolia diversity. Many of these likely are not segregate taxa but rather separate populations or groups of populations of the previously broadly circumscribed, widespread species. Such is possibly the case of the Magnolia dealbata species complex (belonging to Magnolia sect. Macrophylla), distributed throughout the Sierra Madre Oriental mountain range in Eastern Mexico. This species complex has been divided into six morphospecies based on morphological criteria only. However, recent microsatellite markers have suggested that these may be a single entity. Considering geographical data and the isolation of populations, we hypothesised that the different morphospecies could form two entities, corresponding to the north and centre of the Sierra Madre Oriental. This hypothesis was tested by morphological observations, chloroplast comparisons and phylogenetic analyses of plastomes, angiosperm DNA plastid barcodes and Magnolia-specific plastid DNA barcodes from hypervariable regions. Phylogenetic results from plastomes and angiosperm DNA plastid barcodes refute the multispecies hypothesis and show that the six morphospecies of this complex inhabiting the Sierra Madre Oriental form a single entity. Evidence is also provided that the morphological characters used to delimit the morphospecies of the complex, mainly numbers of carpels and the absence-presence and colour of a spot in the petals, are, in fact, phenotypic variation and have no taxonomic significance. Therefore, the taxa M. alejandrae, M. nuevoleonensis, M. rzedowskiana, M. vovidesii and M. zotictla are synonymised here under M. dealbata. However, the possibility remains of including the latter as a variety of M. macrophylla, based on the results of the Magnolia-specific plastid DNA barcodes. Furthermore, this study proposes an updated conservation status for M. dealbata, highlighting the urgent need for effective conservation measures. The taxonomic clarification presented here is essential to properly target such efforts, especially in the face of threats such as indiscriminate collection and vulnerability to environmental disturbance.