Understanding the Genetic Diversity of the Invasive Callery pear, Pyrus calleryana Decne. in Native and Introduced Ranges of the U.S. using Microsatellite Loci

Pyrus calleryana Decne. (Callery pear) is a deciduous tree native to China, Japan, Korea, and Taiwan. In the early 1900s, this species was initially brought in the U.S. to assist with disease resistance to fire blight-causing bacteria Erwinia amylovora Burrill. via hybridization with P. communis L. Since then, many popular ornamental hybrid cultivars of P. calleryana have been developed. ‘Bradford’ is the most well-known P. calleryana cultivar in the U.S. Today, P. calleryana has become an extremely common invasive tree species that is naturalized across the eastern U.S. Knowledge on genetic diversity and population structure of P. calleryana is very limited. In this study, we estimated the genetic diversity and population structure of P. calleryana across its native and introduced ranges. As the species is highly invasive in its introduced range and presents evidence of variation in morphological traits, we hypothesized that high genetic diversity, and the presence of population structure would be evident among P. calleryana collection distributed across its native and introduced ranges. For the first study, we developed and used 18 microsatellite loci that were used to analyze 147 Pyrus source samples and to articulate the status of genetic diversity within Asian P. calleryana and specimens from U.S. cultivars. For the second study, we used 15 microsatellite loci to determine the genetic diversity and population structure of 180 wild P. calleryana individuals collected across six naturally occurring sites in Tennessee, Georgia, and South Carolina. In both studies, our data revealed high genetic diversity (H_e of 0.81 and H_e of 0.74, respectively), high genetic differentiation (D_est of 0.42 and D_est of 0.21, respectively), high gene flow (N_m of 1.79 and N_m of 3.94, respectively), and presence of population structure in P. calleryana. Both of our studies supported China as the source of origin for P. calleryana cultivars of the U.S. These findings indicate the highly invasive capability of P. calleryana across its introduced range. Based on these results, we suggest taking management actions to control invasive P. calleryana. We suggest homeowners consider planting native flowering deciduous tree species, including serviceberry (Amelanchier spp.), Cornus florida, or Cercis canadensis, as alternatives for P. calleryana.