Limber pine (Pinus flexilis James) co-evolved with the mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) and is now also challenged by the non-native pathogen Cronartium ribicola (J.C. Fisch.) that causes the lethal disease white pine blister rust (WPBR). Previous research suggests that trees infected with WPBR can be preferred hosts for MPB. Using resin duct traits associated with MPB resistance, we tested for a relationship between resistance to MPB and WPBR in limber pine, in the absence of either biological agent. These analyses will help evaluate if MPB historically may have contributed to natural selection for WPBR resistance in advance of WPBR invasion, and could help explain the unusually high frequency of the dominant Cr4 allele for complete resistance to WPBR in limber pine populations of the Southern Rocky Mountains. Resin duct production, density and relative duct area did not differ between healthy trees previously inferred to carry the dominant Cr4 allele and trees that lack it at 22 sites, though some duct traits varied with elevation. MPB resistance does not appear to have played an evolutionary role in contributing to the high frequency of Cr4 in naïve populations, however, MPB may affect the future evolution of resistance to WPBR in the pines where the two pests coincide and WPBR will affect forest recovery after MPB epidemics. MPB-WPBR interactions in a changing climate will affect the future trajectory of limber pine.
Fungal endophytic communities in needles of field-grown Pinus flexilis previously inferred to carry major gene resistance (R) to white pine blister rust (WPBR) or to lack it (S) were surveyed to identify unique microbes that may be recruited by WPBR-resistant genotypes. Resistant and susceptible trees were sampled in each of 11 P. flexilis populations for a total of 50 trees sampled. Through next-generation sequencing, this study showed a diverse needle mycobiota in P. flexilis, of which many remain unknown, regardless of the presence or absence of the WPBR resistance gene, Cr4. Ascomycota dominated the mycobiota (88.9%) followed by Basidiomycota (4.4%) and Chytridiomycota (0.03%), and the remaining 6.7% were unclassified. Shared ( n = 105) and unique ( n = 48 in R and n = 49 in S) fungal taxa, including differentially abundant operational taxonomic units, were identified that could provide insights into core mycobiota and host genotype-specific fungal groups. Marginal variation of the fungal diversity and structure was observed between host genotypes, which indicates that neither Cr4 nor the physiological differences associated with the presence or absence of the gene affects mycobiota recruitment. Instead, other parameters, including host size (diameter at breast height) and site elevation, significantly influenced the variability of the composition and structure of the fungal endophytic community. Further investigations are needed to understand the relationship of unique or differentially abundant taxa with one genotype or the other, and to determine the role of the needle mycobiota in WPBR disease development in natural stands of P. flexilis.
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