Biology subjects

Koch, J. L.

Publications and source records attributed to Koch, J. L..

3 recordsLinked to original sources

bioRxiv2023

Gene flow from Fraxinus cultivars into natural stands of Fraxinus pennsylvanica occurs range-wide, is regionally extensive, and is associated with a loss of allele richness

In North America, the ubiquitous planting of a comparatively small number of Fraxinus (ash) cultivars in both urban and rural environments over the last 70-80 years may have permitted extensive gene flow into naturally regenerated stands. In the light of multiple biotic threats to the North American Fraxinus, an assessment of the extent of gene flow from ash cultivars and the current state of genetic diversity in F. pennsylvanica (green ash), one of the most widely distributed species, is needed to inform seed collection strategies for the preservation of genetic diversity range-wide. We used 16 EST-SSR markers to genotype 1291 trees from 48 naturally regenerated populations of green ash across the native range, 19 F. pennsylvanica cultivars and one F. americana (white ash) cultivar. We detected first generation cultivar parentage with high confidence in 172 individuals in 34 of the 48 populations and extensive cultivar parentage (23-50%) in eight populations. The incidence of cultivar parentage was negatively associated with allele richness (R2 = 0.151, p = 0.006). The high frequency of cultivar propagule dispersal in our study suggests that a significant proportion of the standing genetic variation in local populations may not be of local origin, a result that has serious implications for the study of adaptive variation and the conservation of the Fraxinus gene pool.

ecology↗

bioRxiv2022

Profiles of secoiridoids and alkaloids in tissue of susceptible and resistant green ash progeny reveal patterns of induced responses to emerald ash borer in Fraxinus pennsylvanica

The emerald ash borer (Agrilus planipennis, EAB) invasion in North America threatens most North American Fraxinus species, including green ash (F. pennsylvanica), the mostly widely distributed species (1, 2). A small number of green ash ("lingering ash", 0.1-1%) survive years of heavy EAB attack (3) and kill more EAB larvae when challenged in greenhouse studies than susceptible controls (4). We combined untargeted metabolomics with intensive phenotyping of segregating F1 progeny from susceptible or lingering ash parents to detect chemotypes associated with defensive responses to EAB. We examined three contrasting groups: low larval kill (0-25% of larvae killed), high larval kill (55-95% of larvae killed) and uninfested. Contrasting the chemotypes of these groups revealed evidence of an induced response to EAB. Infested trees deployed significantly higher levels of select secoiridoids than uninfested trees. Within the infested group, the low larval kill (LLK) individuals deployed significantly higher levels of select secoiridoids than the high larval kill (HLK) individuals. The HLK individuals deployed significantly higher concentrations of three metabolites annotated as aromatic alkaloids compared to the LLK and uninfested individuals. We propose a two-part model for the North American Fraxinus response to EAB wherein every individual has the capacity to detect and respond to EAB, but only certain trees mount an effective defense, killing enough EAB larvae to prevent or minimize lethal damage to the vascular system. Integration of intensive phenotyping of structured populations with metabolomics reveals the multi-faceted nature of the defenses deployed in naive host populations against invasive species. SignificanceLong-lived forest trees employ evolutionarily conserved templates to synthesize an array of defensive metabolites. The regulation of these metabolites, honed against native pests and pathogens, may be ineffective against novel species, as illustrated by the high mortality (>99%) in green ash infested by the invasive emerald ash borer (EAB). However, high standing genetic variation may produce a few individuals capable of an effective defense, as seen in the rare surviving green ash. In an investigation of this plant-insect interaction, we annotated metabolites associated with generalized but ineffective responses to EAB, and others associated with successful defensive responses. Untargeted metabolomics combined with intensive phenotyping of structured populations provides a framework for understanding resistance to invasive species in naive host populations.

plant biology↗

bioRxiv2019

Genes for ash tree resistance to an insect pest identified via comparative genomics

Genome-wide discovery of candidate genes for functional traits within a species typically involves the sequencing of large samples of phenotyped individuals1, or linkage analysis through multiple generations2. When a trait occurs repeatedly among phylogenetically independent lineages within a genus, a more efficient approach may be to identify genes via detection of amino acid residues shared by species possessing that trait3,4. Here, by taking this approach, we identify candidate loci in the genus Fraxinus (ash trees) for resistance to the emerald ash borer beetle (EAB; Agrilus planipennis), a pest species that appears innocuous to otherwise healthy ash in its native East Asian range5 but is highly destructive in North America6 and poses a threat to ash trees in Europe7. Assembling whole genome sequences for 24 diploid species and subspecies of ash, and estimating resistance to EAB for 26 taxa from egg bioassays, we find 53 genes containing amino acid variants shared between two or more independent Fraxinus lineages with EAB-resistant species, that are unlikely to be due to chance or undetected paralogy. Of these, seven genes have putative roles relating to the phenylpropanoid biosynthesis pathway and 17 are potentially connected to herbivore recognition, defence signalling or programmed cell death. We also find that possible loss-of-function mutations among our 53 candidate genes are more frequent in susceptible species, than in resistant ones. Patterns of polymorphism for the EAB-associated amino acid variants in ash trees representing different European populations suggest that selection may be able to enhance their resistance to EAB.

evolutionary biology↗