Genetic diversity and population structure of Chionanthus virginicus L. throughout the eastern and southern range within the United States

Authors: Wadl, Phillip; Rinehart, Timothy - Tim; Olsen, Richard; Waldo, Benjamin; KIRKBRIDE, JOSEPH - Smithsonian Institute

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2021
Publication Date: 12/23/2021
Citation: Wadl, P.A., Rinehart, T.A., Olsen, R.T., Waldo, B.D., Kirkbride, J.H. 2021. Genetic diversity and population structure of Chionanthus virginicus L. throughout the eastern and southern range within the United States. Journal of the American Society for Horticultural Science. https://doi.org/10.21273/JASHS05095-21.
DOI: https://doi.org/10.21273/JASHS05095-21

Interpretive Summary: The genus Chionanthus L., known as fringetrees, is a member of the olive family. Two species are temperate, deciduous trees and shrubs, C. pygmaeus (pygmy fringetree) and C. virginicus (American fringetree), from southeastern North America. Chionanthus virginicus is often used as a woody landscape or ornamental plant and is known to be free of insects and disease in the wild. The species is tolerant of a wide range of environmental conditions, and there is interest in developing new C. virginicus cultivars with improved horticultural traits such as tree form or upright growth habit and superior flowering display that are widely adapted. The distribution of C. virginicus and variation in morphological traits suggests it may be the progenitor of C. pygmaeus, which is an endangered species that has a greatly restricted distribution, presumably due to isolating barriers. Prior taxonomic studies have identified populations in Florida with traits characteristic of both species, suggesting that hybridization might occur. Therefore the goal of the study was to estimate the genetic diversity and population structure to assess the distribution and life history of C. virginicus as a precursor to further studies on native Chionanthus species. Knowledge of gene flow and genetic diversity within C. virginicus across the species distribution would be beneficial as diagnostic characters for resolving taxonomic questions about Chionanthus species. Genetic diversity and population structure for about 275 individuals from 4 states were estimated using molecular markers. High genetic diversity and low population differences were observed, and cluster analysis identified six genetic groups that match the geographic distribution of collection sites. The C. virginicus samples analyzed in this study indicate there is sufficient diversity for breeding purposes. Given the relatively low genetic differentiation, there are not likely to be unique islands of genetic diversity that may be missed when gathering parental materials for a breeding program. This information will be useful to ornamental plant breeders, population geneticists, taxonomists, and biologists working with Chionanthus species.

Technical Abstract: The genus Chionanthus L., known as fringe trees, is a member of the olive family. C. virginicus is an understory tree or shrub with a wide range (cold hardiness zones 3 to 9) in forests of eastern United States and is often used as a woody landscape or ornamental plant that is known to be free of insects and disease in the wild. The species is tolerant of a wide range of environmental conditions, and there is interest in developing new C. virginicus cultivars with improved horticultural traits such as tree form or upright growth habit and superior flowering display that are widely adapted. Genetic diversity and population structure were assessed for 274 individuals from 12 locations (Florida, Maryland, North Carolina, and Texas) using 26 simple sequence repeats (SSRs). A total of 312 alleles were detected and the number of alleles varied considerably between loci, from 3 to 32, with 12.54 alleles/locus detected. Allelic richness ranged from 1.46 to 4.39 and averaged 2.80. Observed and expected heterozygosity also varied considerably between loci but the average for all loci was 0.49 and 0.55, respectively. The mean inbreeding coefficient (FIS) was 0.09 and indicated a slight excess of homozygotes within individuals. Genetic differentiation (FST) was 0.11 indicating moderate differentiation among subpopulations. Despite the high genetic diversity and low populations differentiations, Bayesian clustering analysis identified six genetic groups that match the geographic distribution of collection sites. Analysis of molecular variance (AMOVA) indicated that most (82%) of the variation is explained within individuals, and 11% and 7% of the variation is due to differences among individuals within populations and among populations. Analysis of isolation by distance across all samples showed a positive relationship between geographic distance (km) and genetic distance. Genetic diversity within C. virginicus across the species distribution would be beneficial as diagnostic characters for resolving taxonomic questions about Chionanthus species. The C. virginicus samples analyzed in this study indicate there is sufficient diversity for breeding purposes. Given the relatively low genetic differentiation, there are not likely to be unique islands of genetic diversity that may be missed when gathering parental materials for a breeding program.