ECOTYPIC DIVERGENCE IN ALPINE <i>POLEMONIUM VISCOSUM</i> : GENETIC STRUCTURE, QUANTITATIVE VARIATION, AND LOCAL ADAPTATION

Abstract

Polemonium viscosum has a continuous distribution from 3,500 m in the krummholz to 4,025 m on the summit ridges of Pennsylvania Mountain, Colorado. Seeds produced by plants at opposite ends of this cline, 1.5 km apart, differed significantly at allozyme loci in two consecutive breeding seasons. Mean multilocus Fst values for both years (0.015 and 0.069) were significantly different from zero, indicating restricted gene exchange between subpopulations. Average allele frequencies at two individual loci also differed significantly between families comprising krummholz and summit subpopulations. Progeny of plants growing on the summit had higher leaf production rates, more densely packed leaflets, and lower resistance to aphids than progeny of plants growing in the krummholz site, when tested under greenhouse conditions. These differences probably reflect the restricted opportunities for growth and severe exposure at high elevations, and the increased risk from herbivores near timberline. The two subpopulations did not differ in leaf length (stature), leaf width, or pubescence. Reciprocal transplanting of seedlings between krummholz and summit sites confirmed that the differences were adaptive, since progeny from each subpopulation performed significantly better in their parent's habitat. Coordinated studies of genetic structure, quantitative variation, and local adaptation across the elevational range of P. viscosum provide a comprehensive view of ecotypic divergence in this widespread alpine plant.