Biogeographic transition zones are promising areas to study processes of biogeographic evolution and its influence on biological groups. The Mexican transition zone originated due to the overlap of Nearctic and Neotropical biota, which promoted great biological diversification. However, since most previous studies in this area were focused on revealing the phylogeography of Nearctic plants, how historical biogeographic configuration influenced the expansion and diversification of the Neotropical flora remains almost unknown. Using the cycad genus Dioon (Zamiaceae), this study aimed to test whether the biogeographic provinciality of the Mexican transition zone reflects the history of diversification of Neotropical plants. Two chloroplast DNA (cpDNA) regions were analysed from 101 specimens of 15 Dioon species to reveal the distribution of haplogroups. In addition, genome-wide single nucleotide polymorphisms (SNPs) from 84 specimens were used to test the concordance between phylogenetic clusters and the biogeographic provinces. An ultrametric tree was constructed from the sequences containing SNPs to reconstruct the biogeographic events of vicariance and dispersal of Dioon across the Neotropical biogeographic provinces. Four Dioon lineages with strong phylogeographic structures were recognized using both cpDNA and SNP data. The lineages correspond to two clades that originated from a common ancestor in Eastern Mexico. One clade expanded and diversified in South-east Mexico and Central America. Another clade diversified into three lineages that dispersed to North-east, South and North-west Mexico. Each lineage was biogeographically delimitated. Biogeographic provinces might have provided disparate ecological conditions that facilitated speciation in Dioon since the Miocene. The current genetic structure and species diversity of Dioon depict the history of expansion and diversification of the northernmost Neotropical provinces. Past biogeographic connectivities were favoured by elevated topographies, since mountain systems served as corridors for the migration of Dioon and as refugia of tropical communities that diversified during the formation of modern Neotropical forests.

Aridification is considered a selective pressure that might have influenced plant diversification. It is suggested that plants adapted to aridity diversified during the Miocene, an epoch of global aridification (≈15 million years ago). However, evidence supporting diversification being a direct response to aridity is scarce, and multidisciplinary evidence, besides just phylogenetic estimations, is necessary to support the idea that aridification has driven diversification. The cycad genus Dioon (Zamiaceae), a tropical group including species occurring from humid forests to arid zones, was investigated as a promising study system to understand the associations among habitat shifts, diversification times, the evolution of leaf epidermal adaptations, and aridification of Mexico. A phylogenetic tree was constructed from seven chloroplast DNA sequences and the ITS2 spacer to reveal the relationships among 14 Dioon species from habitats ranging from humid forests to deserts. Divergence times were estimated and the habitat shifts throughout Dioon phylogeny were detected. The epidermal anatomy among Dioon species was compared and correlation tests were performed to associate the epidermal variations with habitat parameters. Events of habitat shifts towards arid zones happened exclusively in one of the two main clades of Dioon. Such habitat shifts happened during the species diversification of Dioon, mainly during the Miocene. Comparative anatomy showed epidermal differences between species from arid and mesic habitats. The variation of epidermal structures was found to be correlated with habitat parameters. Also, most of the analysed epidermal traits showed significant phylogenetic signals. The diversification of Dioon has been driven by the aridification of Mexico. The Miocene timing corresponds to the expansion of arid zones that embedded the ancestral Dioon populations. As response, species in arid zones evolved epidermal traits to counteract aridity stress. This case study provides a robust body of evidence supporting the idea that aridification is an important driver of biodiversity.

Antecedentes: La Sierra Madre Oriental es una Provincia Fisiográfica ubicada en el noreste de México con características geológicas, climáticas y edáficas distintivas. La flora de esta región aún no ha sido inventariada en conjunto. Preguntas: ¿Cuál es la diversidad florística de la Provincia Fisiográfica de la Sierra Madre Oriental? ¿Cuál es la afinidad geográfica de sus géneros? Sitio y años de estudio: Provincia Fisiográfica de la Sierra Madre Oriental. El trabajo se desarrolló desde agosto del 2012 a diciembre del 2020. Métodos: Se hizo una búsqueda exhaustiva de información en diversas fuentes como herbarios, bases de datos, trabajos florísticos y monografías. Para cada especie, se registró la vegetación, afinidad geográfica del género y endemismo. Resultados: Se registran 6,981 especies de las cuales 1,542 son endémicas. Las familias más ricas en especies son Asteraceae (1,000 especies), Fabaceae (495), Cactaceae (365) y Poaceae (328). Los bosques templados registran el mayor número de especies (2,906). Querétaro fue el estado con más especies (2,803), seguido de Coahuila (2,710) y Nuevo León (2,406). La Sierra Madre Oriental comprende una mayor cantidad de especies con géneros de afinidad geográfica boreal (2,742), seguidas por especies de afinidad tropical (2,020), especies endémicas de México (1,227), cosmopolitas (803) y de los desiertos del mundo (189). Conclusiones: La Sierra Madre Oriental alberga más de la cuarta parte de la flora vascular y un 13 % de las plantas endémicas del país. Es un área que reúne géneros de diferentes afinidades geográficas en su mayoría boreales, seguidos de tropicales y endémicos.