Genetic structure of green ash (Fraxinus pennsylvanica): implications for the establishment ofex situconservation protocols in light of the invasion of the emerald ash borer

Abstract

The USA is experiencing a prolific invasion of the wood-boring emerald ash borer, Agrilus planipennis. Native to Asia, this beetle completes its life cycle on ash trees and results in nearly complete mortality of all infested trees. In the present study, we examined the levels of genetic diversity and differentiation among eight populations of Fraxinus pennsylvanica (green ash) using five polymorphic microsatellite loci. Genetic information was used to design guidelines for the establishment of a seed collection sampling strategy to conserve the genetic diversity of ash trees. We found high levels of genetic diversity, as indicated by the allelic richness, both across the populations (16.4^5.18 alleles per locus) and within them (8.03^1.21 alleles per locus). The expected and observed heterozygosity was also high (0.805^0.38 and 0.908^0.04, respectively), and there was moderate genetic differentiation among the populations (F ST ¼ 0.083) with members of these eight populations grouped into three distinct clusters. We examined the relationship between the number of individuals sampled and the number of alleles captured in a random sample taken from a population of 10,000 individuals. Only sample sizes of 100 individuals captured most of the alleles (average ¼ 78.74 alleles), but only seven of 50 samples effectively captured all the 82 alleles. Smaller samples did not capture all alleles. A probabilistic model was used to determine an optimal sampling strategy, and it was concluded that a collection of 200 seeds from each of five mother trees would have the highest likelihood of capturing all alleles in a population.

Virulence of Hymenoscyphus albidus and H. fraxineus on Fraxinus excelsior and F. pennsylvanica

“…In addition, a relatively high proportion (16% out of 75) of natural stem infections was observed on plants delivered from a nursery in Germany [ 21 ]. As a consequence, we cannot discount the importance of environment-related predisposition with regards to this host’s susceptibility [ 47 ] or the possibility that particularly susceptible provenances exist [ 48 ]. The native range of green ash is vast and the species comprises at least three ecotypes [ 49 ].…”

Virulence of Hymenoscyphus albidus and native and introduced Hymenoscyphus fraxineus on Fraxinus excelsior and Fraxinus pennsylvanica

“…Furthermore, it is likely that natural hybrids between F. pennsylvanica and Fraxinus americana exist, which sometimes live in sympatry and cross-pollinate (Wallander, 2008). Unfortunately, a comprehensive population study across the whole distribution range of F. pennsylvanica is lacking (Hausman et al, 2014).…”

“…White ash (Fraxinus pennsylvanica) is the most broadly distributed Fraxinus species in North America and is a major species along stream networks in the central United States (Hanberry, 2014;Hausman et al, 2014). In recent decades, F. pennsylvanica has even expanded its range and density and is often planted to achieve rapid reforestation of abandoned agricultural fields and floodplain forests (Groninger et al, 2004).…”

Ash dieback due toHymenoscyphus fraxineus: what can be learnt from evolutionary ecology?

“…This is the case not just for European ash, but also for susceptible North American Fraxinus species should the fungus manage to cross the Atlantic (or the Pacific). This type of interaction between invasive species is another reminder of the need to maintain the genetic diversity of ash and other tree species as an evolutionary insurance against further unexpected threats (Hausman et al ., ; Strandberg, ; Telford et al ., ).…”