Genic diversity, genetic structure, and biogeography of Pinus sabiniana Dougl.

Pinus sabiniana Dougl. (grey pine) forms savanna forests in the foothills surrounding California's Great Central Valley. However, its fossil record, which dates from the late Miocene through the Pliocene and Pleistocene, is found exclusively in southern California, south of the species’ present range. A total of twenty-nine isozyme loci, representing eighteen enzyme systems, was assayed to analyse the genetic structure in eight populations of grey pine and attempt to track its migration history from southern to northern California. Expected heterozygosity in the two southernmost samples was 0.128 and 0.150, and heterozygosity tended to decrease with increasing latitude, suggesting the loss of diversity as grey pine dispersed northward. However, genetic distances between populations were very small, even on opposite sides of the treeless Great Central Valley; and estimated time since divergence was 900 to 9000 years at a maximum. Wright's F_ST, the proportion of total genetic diversity among populations, was only 0.057, which is similar to values found in many conifers with continuous distributions. Nm, the number of migrants among populations per generation, was 4.1 to 6.7, depending on estimator, and indicates that gene flow is extensive, or was so in the recent past. In every population, observed heterozygosity was less than expected heterozygosity, and the fixation index, F_IS, for the progeny was 0.128, which indicates a fairly high rate of inbreeding. The genetic similarity of disjunct populations, in combination with paleogeographic and paleoclimatic evidence, suggests that grey pine formed a continuous population throughout the Great Central Valley, perhaps between 12,000 and 8000 yrs BP . Its range became fragmented during the Xerothermic, when it ascended into the foothills. Gaps in its range correlate with late Pleistocene–early Holocene lakes in adjacent basins and with the Sacramento–San Joaquin Delta.