Measuring the Impact of P. lateralis Infection on Genetic Variation of Diseased and Disease-Free Port Orford-Cedar Stands

Due to the low frequency of resistant Port-Orford-cedar (POC) in natural stands, Phytophthora  lateralis-mediated mortality in infested areas is causing severe reductions in the census size of POC, and it may contribute to a loss of genetic diversity and increased inbreeding in surviving stands. The impact of P. lateralis on standing genetic variation of POC is unknown, and knowledge of stand impacts could identify regions with reduced genetic diversity that might benefit from gene conservation and restoration efforts. Twenty populations were identified where trees could be collected from adjacent diseased (D) and ‘clean’ (disease-free; C) stands, and these trees were genotyped with 11 microsatellite markers. Our analysis shows that that trees from D stands showed reduced allelic richness and heterozygosity, and unique clustering behavior relative to C stands. Examination of allele frequency spectra shows that eight of the eleven loci tested exhibit evidence of allelic drift, with shifts in the major allele frequencies as high as 27%. Range-wide diversity and differentiation was also assessed for a larger sample of C populations (N=36). This analysis shows that POC exhibits significant spatial variation (R_ST = 8.5%), and that population genetic structure is correlated with longitude. In total, these results show that P. lateralis–induced mortality of POC is reducing the frequency and abundance of rare genetic variation, and some of this variation may be regionally distinctive. The potential for ex-situ germplasm resources to mitigate these kinds of losses will be highlighted.