Geographic divergence in a species-rich symbiosis: interactions between monterey pines and ectomycorrhizal fungi.

A key problem in evolutionary biology is to understand how multispecific networks are reshaped by evolutionary and coevolutionary processes as they spread across contrasting environments. To address this problem, we need studies that explicitly evaluate the multispecific guild structure of coevolutionary processes and some of their key outcomes such as local adaptation. We evaluated geographic variation in interactions between most extant native populations of Monterey pine (Pinus radiata) and the associated resistant-propagule community (RPC) of ectomycorrhizal (EM) fungi, using a reciprocal cross-inoculation experiment with all factorial combinations of plant genotypes and soils with fungal guilds from each population. Our results suggest that the pine populations have diverged in community composition of their RPC fungi, and have also diverged genetically in several traits related to interactions of seedlings with particular EM fungi, growth, and biomass allocation. Patterns of genetic variation among pine populations for compatibility with EM fungi differed for the three dominant species of EM fungi, suggesting that Monterey pines can evolve differently in their compatibility with different symbiont species.

FINE-SCALE GENETIC STRUCTURE OF WHITEBARK PINE (PINUS ALBICAULIS): ASSOCIATIONS WITH WATERSHED AND GROWTH FORM.

The fine-scale genetic structure of a subalpine conifer, whitebark pine (Pinus albicaulis Engelm.), was studied at nested geographic levels from watershed to adjacent stems in the eastern Sierra Nevada Range of California. A combination of several characteristics contributed to unpredicted genetic structure in this species. This includes being one of only 20 pine species worldwide with wingless, bird-dispersed seeds; having the reputed capacity to reproduce vegetatively; and forming distinct growth morphologies at different elevations in this part of its natural range. Genetic differentiation, as measured with 21 allozyme loci, among the three studied watersheds is virtually negligible (FST = 0.004). This is a surprising result because the upper-elevation sites vary somewhat in slope aspect; thus, aspect was confounded with watershed effect. Differentiation between the upper-elevation prostrate krummholz thickets and lower-elevation upright tree clump growth forms is modest (FST = 0.051). Much stronger differentiation was measured among the individual thickets and clumps within their sample sites (FST = 0.334). Within krummholz thickets, multiple individuals are present and genetic relationships often resemble half- to full-sibling family structure (mean r = 0.320). Canonical trend surface analysis in two intensively sampled thickets indicates greatest genotypic variation in the direction of the prevailing wind. At lower elevations, most (72%) of the tree clumps contained more than one genotype; the remaining clumps are probably multistemmed trees. Within tree clumps, family relationships are closer than those for krummholz thickets-commonly full-sibling to selfed structure (mean r = 0.597). Genetic structure is apparently profoundly influenced by the seed-caching behavior of Clark's nutcracker (Nucifraga columbiana Wilson). Western pine species typically show little among-population differentiation and high levels of within-population genetic variation. In whitebark pine in the eastern Sierra Nevada of California, genetic variation is highly structured, especially within the natural groupings-krummholz thickets and upright tree clumps.