Multidirectional hybridization challenges the species barriers in North American Arbutus (Ericaceae)

Multispecies networks or syngameons engage in multidirectional hybridization and are more prevalent than previously thought. They are part of the evolutionary dynamics playing a major role in speciation events. However, allopatric or sympatric species distribution as well as reproductive isolating mechanisms largely impact the syngameonic structure and dynamics. We addressed how species maintain intraspecific cohesiveness in the presence of extensive interspecific hybridization in the genus Arbutus , which has been suggested to have widespread hybridization in North America based on morphological studies. Our study aimed to 1) determine if distribution patterns (allopatric vs sympatric) influence levels of interspecific gene flow and elucidate species boundaries, 2) identify the structure of the syngameon, and 3) determine whether pre- or post- zygotic reproductive isolating mechanisms are present that may deter advanced hybrid generations in the genus Arbutus . Our genomic data confirmed widespread hybridization in sympatric species within mainland Mexico, with multidirectional gene flow and at least eight species combinations were determined at the genetic level. Based on flowering times and hybridization analyses, we found that there was a lack of or weak pre and post-zygotic reproductive isolating mechanisms among sympatric species. Our study is the first of its kind to apply a genomic approach to addressing both the species boundaries and syngameon structure in Arbutus . These findings are relevant as habitat destruction and severe droughts are impacting Arbutus within mainland Mexico.