Description
Evolutionary processes such as reinforcement strengthen diverging species boundaries by producing differences at the temporal, demographic, and reproductive levels. As these boundaries begin to weaken however, species may be capable of interbreeding with one another, producing hybrid individuals. Long term, hybridization can alter the evolutionary trajectories of both the parental species and hybrid individuals with a range of effects from the fusion of two parental species back into one, the coexistence of parental species and their hybrids, or new species arising via hybrid speciation. Arbutus (Ericaceae) is a genus of trees and shrubs with ten species distributed throughout North America. Within mainland Mexico eight of the ten North American species can be found with varying numbers of species found in sympatry, often including hybrid individuals. To date, many of these species have been taxonomically described based of morphological descriptions, however, no previous studies have attempted to assess the validity of these descriptions using genetics. Using next-generation sequencing technology and a population genomics approach, our study aimed to (1) determine whether distribution patterns influence levels of gene flow interspecifically and elucidate species boundaries in eight of the North American Arbutus species, (2) identify how patterns of hybridization vary among species and (3) determine whether pre- or post- zygotic reproductive isolating mechanisms are present that may deter advanced hybrid generations in this system. Results of our study were able to confirm that of the eight described North American species collected, seven genomic clusters were present within our dataset corresponding to each of the described species except for A. mollis and A. occidentalis forming one genomic cluster, reaffirming the previous description of A. mollis as a subspecies of the latter. Additionally, we were able to detect high levels of interspecific gene flow among sympatric occurring species, revealing multidirectional hybridization within our system as well as the presence of advanced generation hybrids. We hope that the results of our study provide some insight as to species designations among the Arbutus complex as well as shed light onto the need for further research to be conducted on this system.
