Summary Pollination failure has been proposed to be an important determinant of plant species’ range limits if pollinator activity declines along an environmental gradient, directly limiting plant populations, or if plant populations decline along an environmental gradient and subsequently fail to attract sufficient visitation. Both mechanisms predict reduced pollinator visitation, increased pollen limitation and decreased seed production towards range limits, and the first additionally predicts declining pollinator abundance independent of any particular plant species. However, many self‐compatible species have some capacity for autonomous self‐fertilization, which may buffer reproductive success from declining pollinator visitation if inbreeding depression is mild. Thus pollinator‐mediated limits may also predict selection for reduced reliance on pollinators towards range limits. We tested these predictions towards the high‐elevation limit of the self‐compatible, bumblebee (Bombus)‐pollinated Rhinanthus minor, along two elevation transects in the Rocky Mountains of Alberta, Canada. Bombus abundance was highest at mid‐ (range‐centre) and high‐elevation (range limit) sites, so declining pollinator abundance is unlikely to impose high‐elevation limits for bumblebee‐pollinated species in this area. Flowers per plant and per m2 declined at upper range limits, potentially rendering edge populations less attractive. However, visitation rate did not decline towards the range limit at either transect. Stigmatic pollen receipt declined with increasing elevation, but seed set did not, nor did outcross pollen supplementation increase seed set at any site. Investment in floral attractiveness (corolla area/ovary area) increased towards range limits, but capacity for high‐quality autonomous seed set and adult inbreeding coefficients inferred from genetic markers were uniformly high, suggesting frequent self‐fertilization and weak inbreeding depression throughout the range. Synthesis. We found no evidence for pollination failure towards the upper range limit of R. minor. Moreover, unlike some species with a capacity for autogamy, autonomous selfing makes a major contribution to R. minor's mating system and demography, and likely buffers reproductive success from stochasticity in pollination. Continued investment in floral attractiveness despite high autonomous selfing suggests some evolutionary benefit to pollinator‐mediated outcrossing, rather than ecological benefits via increasing seed quantity or quality. Given that >50% of angiosperms are self‐compatible, the reproductive assurance provided by selfing may reduce the importance of pollination in limiting plant distributions compared to other biotic interactions.

Heterotrimeric guanine nucleotide-binding proteins (G proteins) consisting of alpha, beta, and gamma subunits mediate signalling between cell surface receptors and intracellular effectors in eukaryotic cells. To define signalling functions of G gamma subunits (STE18 gene product) involved in pheromone response and mating in the yeast Saccharomyces cerevisiae, we isolated and characterized dominant-negative STE18 alleles. We obtained dominant-negative mutations that disrupt C-terminal sequences required for prenylation of G gamma precursors (CAAX box) and that affect residues in the N-terminal half of Ste18p. Overexpression of mutant G gamma subunits in wild-type cells blocked signal transduction; this effect was suppressed upon overexpression of G beta subunits. Mutant G gamma subunits may therefore sequester G beta subunits into nonproductive G beta gamma dimers. Because mutant G gamma subunits blocked the constitutive signal resulting from disruption of the G alpha subunit gene (GPA1), they are defective in functions required for downstream signalling. Ste18p bearing a C107Y substitution in the CAAX box displayed reduced electrophoretic mobility, consistent with a prenylation defect. G gamma subunits carrying N-terminal substitutions had normal electrophoretic mobilities, suggesting that these proteins were prenylated. G gamma subunits bearing substitutions in their N-terminal region or C-terminal CAAX box (C107Y) supported receptor-G protein coupling in vitro, whereas C-terminal truncations caused partial defects in receptor coupling.

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The STE4 gene of the yeast Saccharomyces cerevisiae encodes the beta subunit of a heterotrimeric G protein that mediates response to mating pheromones and influences recovery from pheromone-induced growth arrest. To explore how G beta subunits regulate response and recovery (adaptation), we isolated and characterized signaling-defective STE4 alleles (STE4sd). STE4sd mutations resulted in amino acid substitutions in the N-terminal region of Ste4p, proximal to the first of seven repeat units conserved in G protein beta subunits. Genetic tests indicated that STE4sd mutations disrupted functions of Ste4p required for inducing pheromone responses. Wild-type cells that overexpressed STE4sd alleles displayed apparently normal initial responses to pheromone as judged by quantitative mating, G1 arrest and transcriptional assays. However, after undergoing initial G1 arrest, wild-type cells overexpressing STE4sd alleles recovered more quickly from division arrest, suggestive of a hyperadaptive phenotype. Because hyperadaptation occurred when STE4sd alleles were overexpressed in cells lacking Sst1p (Bar1p), Sst2p or the C-terminal domain of the alpha-factor receptor, this phenotype did not involve three principal modes of adaptation in yeast. However, hyperadaptation was abolished when STE4sd mutations were combined in cis with a deletion that removes a segment of Ste4p (residues 310-346) previously implicated in adaptation to pheromone. These results indicate that G beta subunits possess two independent activities, one required for triggering pheromone response and another that promotes adaptation. Potential models for G beta subunit-mediated adaptation are discussed.

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