POPULATION-STRUCTURE ALONG A STEEP ENVIRONMENTAL GRADIENT - CONSEQUENCES OF FLOWERING TIME AND HABITAT VARIATION IN THE SNOW BUTTERCUP, RANUNCULUS-ADONEUS

Few studies have determined how gene flow and selection interact to ge nerate population genetic structure in heterogeneous environments. One way to identify the potential role played by natural selection is to compare patterns of spatial genetic structure between different life c ycle stages and among microenvironments. We examined patterns of spati al structure in a population of the snow buttercup (Ranunculus adoneus ), using both adult plants and newly emerged seedlings. The study popu lation spans a steep environmental gradient caused by gradual melting of snow within a permanent snowbed. Early-melting sites are characteri zed by denser vegetation, more fertile soils, and a longer growing sea son than late-melting sites tens of meters away. The flowering time of R. adoneus is controlled entirely by time of snowmelt, so the contigu ous population is phenologically substructured into a series of succes sively flowering cohorts, reducing the opportunity for direct pollen t ransfer between early- and late-melting sites. For four highly polymor phic enzyme loci in this tetraploid species, there was subtle, but sta tistically significant, genetic differentiation between early, middle, and late-melting cohorts; adults usually showed greater differentiati on among snowmelt zones than did seedlings. At two loci in adults and one locus in seedlings, homozygotes were more common than predicted at Hardy-Weinberg equilibrium, even when assuming maximum levels of doub le reduction during meiosis. This pattern suggests the occurrence of s elf-fertilization and/or population substructure. To determine how spa tial isolation and phenological separation each contribute to genetic substructure, we used bivariate regression models to predict the numbe rs of allele differences between randomly paired individuals as a func tion of meters separation in space and days separation in flowering ti me. For newly emerged seedlings, we found that spatial separation was positively associated with genetic difference, but that the additional contribution of phenological separation to genetic difference was not significant. This implies that seeds and/or pollen move effectively a cross the snowmelt gradient, despite differences in flowering time. As was true for seedlings, spatial separation between paired adults cont ributed to greater genetic difference, but for a given spatial separat ion, the genetic difference between adult plants was reduced by phenol ogical separation. This result implies that postemergence selection is favoring at least some seeds that migrate across the snowmelt gradien t. Directional gene flow across the snowmelt gradient probably results from a genetic source-sink interaction, that is, the colonization of ecologically marginal late-melting sites by high quality seeds produce d by the larger subpopulation in early-melting sites. Effective gene f low from high to low quality microenvironments is likely to impede ada ptation to late-melting locations.