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Interspecific gene flow between more than two species is a common phenomenon in oaks, which can occur simultaneously among different species, promoting the transfer of genetic material across species boundaries. However, the hybridization dynamics in multispecies hybrid zones remain unknown. In this study, we provide genetic evidence of hybridization and introgression of Quercus castanea across a natural gradient of red oak species richness. We analyzed five populations recognized morphologically as ''pure'' Q. castanea, one allopatric and four sympatric populations, where the number of red oak species associated with Q. castanea ranged from one to four. Also, one allopatric population of each red oak species that occurs in sympatry with Q. castanea was chosen as reference population (Q. crassipes, Q. laurina, Q. mexicana and Q. crassifolia). In total, six nSSRs were used in 10 and 20 individuals from each allopatric and sympatric populations, respectively. Our results showed that allopatric populations formed completely distinct genetic clusters.In sympatric populations, we found evidence of hybridization and introgression among Q. castanea and three of its associated red oak species. However, the occurrence and frequency of hybrids between Q. castanea and these species varied among stands. Our analyses provide evidence and new insights into hybridization and introgression dynamics within a Mexican red oak species complex, through a focal species, Q. castanea.

Natural hybridization has significant ecological, genetic and evolutionary consequences altering morphological and chemical characters of individuals. Quercus glabrescens, Q. rugosa and Q. obtusata are white oak species well separated by their morphological characters when they occur in allopatry in Mexican temperate forests. However, in sympatry, individuals with atypical morphology have been observed, suggesting hybridization events. In this study, we determined with microsatellites and secondary metabolites if interspecific gene flow occurs when these three-oak species coexist in sympatry. In total, 180 individuals belonging to seven populations [three allopatric (one for each parental species) and four sympatric sites] were analyzed. Allopatric populations represent well defined genetic groups and the sympatric populations showed genetic evidence of hybridization between Q. glabrescens × Q. rugosa and Q. glabrescens × Q. obtusata. The hybridization percentage varied between sites and combination of involved species. We registered the presence of unique flavonoid compounds for Q. glabrescens (caffeic acid and flavonol 2), Q. rugosa (flavonol 5), Q. obtusata (flavonol 1). Three compounds (quercetin rhamnoside, flavonol 3 and alkyl coumarate) were expressed in all taxa. Finally, the hybrid genotypes identified in this study (Q. glabrescens × Q. rugosa and Q. glabrescens × Q. obtusata) showed particular chemical profiles, resulting from combination of those of their parental species. These results show that hybridization events between these oak species alter chemical expression of secondary metabolites, creating a mosaic of resources and conditions that provide the substrate for different combinations of foliar-associated species as herbivores, endophytic fungi or epiphyte plants. This article is protected by copyright. All rights reserved.

Canopy of forest ecosystems has been recognized as a habitat that supports a wide variety of plants, vertebrates, invertebrates, and microbes. Within the invertebrate group, arthropods are characterized by their great abundance, diversity, and functional importance. Particularly in temperate forests, species of the genus Quercus (oaks) are one of the most important tree canopy groups, for its diversity and dominance. Different studies have shown that the oak canopy contains a high diversity of arthropods suggesting their importance as habitat for this group of organisms. In this review, we investigated the factors that determine the establishment, organization, and maintenance of arthropod communities in the oak canopy. In general, it was found that there is a lack of literature that addresses the study of oak canopy arthropod communities. Also, the following patterns were found: (a) the research has covered a wide variety of topics; however, there are differences in the depth to which each topic has been analyzed, (b) there are ambiguous criteria to define the structure of the canopy, (c) groups with different habitat preferences belonging to different guilds and uneven development stages have been studied, avoiding generalizations about patterns found, (d) the standardization in sampling techniques and collection has been difficult, (e) bias exists towards the study of phytophagous insects belonging to the Coleoptera, Hymenoptera, and Lepidoptera orders, and (f) there are few studies in other groups of arthropods, for example, acorn borers, whose activity has an impact on the fitness and dispersion of the host plants. Finally, we propose that the detection and study of patterns in oak canopy communities can be of great value to propose management and conservation strategies in these forests.