Abstract
The Mexican highlands are areas of high biological complexity where taxa of Nearctic and Neotropical origin and different population histories are found. To gain a more detailed view of the evolution of the biota in these regions, it is necessary to evaluate the effects of historical tectonic and climate events on species. Here, we analyzed the phylogeographic structure, historical demographic processes, and the contemporary period, Last Glacial Maximum (LGM) and Last Interglacial (LIG) ecological niche models of Quercus castanea, to infer the historical population dynamics of this oak distributed in the Mexican highlands. A total of 36 populations of Q. castanea were genotyped with seven chloroplast microsatellite loci in four recognized biogeographic provinces of Mexico: the Sierra Madre Occidental (western mountain range), the Central Plateau, the Trans‐Mexican Volcanic Belt (TMVB, mountain range crossing central Mexico from west to east) and the Sierra Madre del Sur (SMS, southern mountain range). We obtained standard statistics of genetic diversity and structure and tested for signals of historical demographic expansions. A total of 90 haplotypes were identified, and 29 of these haplotypes were restricted to single populations. The within‐population genetic diversity was high (mean h S = 0.72), and among‐population genetic differentiation showed a strong phylogeographic structure (N ST = 0.630 > G ST = 0.266; p < .001). Signals of demographic expansion were identified in the TMVB and the SMS. The ecological niche models suggested a considerable percentage of stable distribution area for the species during the LGM and connectivity between the TMVB and the SMS. High genetic diversity, strong phylogeographic structure, and ecological niche models suggest in situ permanence of Q. castanea populations with large effective population sizes. The complex geological and climatic histories of the TMVB help to explain the origin and maintenance of a large proportion of the genetic diversity in this oak species.
Highlights
The Mexican Highlands encompass the main mountain systems of Mexico (i.e., Sierra Madre Occidental, Sierra Madre Oriental, TransMexican Volcanic Belt, Sierra Madre del Sur and Altos de Chiapas) and are located between the Nearctic and Neotropical regions
The objectives of the present study were to (a) determine the patterns of genetic diversity and differentiation of populations of Q. castanea across its entire geographical range using simple sequence repeats of the chloroplast DNA, (b) test for signals of historical demographic expansions and to determine the timing of the population expansion, (c) contrast the conclusions derived from genetic data with models of the contemporary period, Last Glacial Maximum (LGM) and Last Interglacial (LIG) ecological niche of Q. castanea, and (d) contrast the phylogeographic patterns revealed in this study with the regionalization of the biogeographic provinces based on the physiographic features
Our results indicate a similar pattern to previous studies in the Mexican highlands that have reported pre-Quaternary divergence times (Bryson et al, 2017, 2018; Bryson & Riddle, 2012; Ornelas & González, 2014) and to patterns of pre-Quaternary diversification for Neotropical species, characterized by complex mixtures of taxa that radiated and spread over a wide range of timescales (Bennett et al, 2012). These findings indicate that the origin and maintenance of a large proportion of the genetic diversity in Q. castanea have occurred within the Trans-Mexican Volcanic Belt, which is a hotspot for oak species diversity and has acted as a natural bridge between other biogeographic provinces (Bryson et al, 2018)
Summary
The Mexican Highlands encompass the main mountain systems of Mexico (i.e., Sierra Madre Occidental, Sierra Madre Oriental, TransMexican Volcanic Belt, Sierra Madre del Sur and Altos de Chiapas) and are located between the Nearctic and Neotropical regions. In the Mexican Transition zone, key evolutionary processes have been studied such as species diversification and historical demography including the role of paleoclimatic and geological events in the evolutionary history of the species (Mastretta-Yanes, MorenoLetelier, Piñero, Jorgensen, & Emerson, 2015). Even though initial syntheses concerning multitaxa phylogeographic patterns in the Mexican Transition Zone suggest some common responses of species to geographic barriers and historical climate changes (Ramírez-Barahona, & Eguiarte, 2013; Ornelas et al, 2013), we are still far from attaining a detailed view of the processes that have shaped the diversity and evolution of the species in the region
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