INDIRECT SELECTION OF STIGMA POSITION IN IPOMOPSIS-AGGREGATA VIA A GENETICALLY CORRELATED TRAIT

Experimental manipulation of a trait can be used to distinguish direct selection from selection of correlated traits and to identify mechani sms of selection. Here we use experiments to investigate phenotypic se lection of stigma position in angiosperm flowers. In natural populatio ns of the subalpine herb Ipomopsis aggregata, plants with more strongl y exserted stigmas receive more pollen per flower, indicating selectio n favoring stigma exsertion during the pollination stage of the life c ycle. We pose four hypotheses for this association, two involving dire ct selection on stigma position and two involving indirect selection o f a correlated floral trait. The first three hypotheses were tested us ing hand pollinations that mimicked natural hummingbird visitation, an d by presenting captive hummingbirds with a series of flowers that dif fered in stigma and anther positions, sex ratio, and presence of anthe rs. In these experiments, pollen deposition either was independent of stigma exsertion or was highest on inserted stigmas, suggesting direct selection against exserted stigmas. In natural populations, however, stigma exsertion is highly correlated with time spent by the protandro us flowers in the pistillate phase. When we manipulated the latter tra it in the held, pollen deposition increased with duration of exposure to hummingbirds, indicating indirect selection for stigma exsertion. S tigma exsertion and time spent in the pistillate phase are genetically and phenotypically correlated, as shown by a quantitative genetic exp eriment conducted in the field with paternal half sibships. Our result s suggest that the evolution of stigma position can be driven by selec tion of a genetically correlated trait.