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Delayed stigma receptivity in Collinsia heterophylla (Plantaginaceae): genetic variation and adaptive significance in relation to pollen competition, delayed self-pollination, and mating-system evolution¹

Department of Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway; Department of Plant Ecology and Systematics, Lund University, Ecology Building, 223 62 Lund, Sweden; Institute of Arctic Biology and Department of Biology and Wildlife, University of Alaska, Fairbanks, Alaska 99775 USA; School of Biological Sciences, University of Portsmouth, King Henry Building, Portsmouth, PO1 2DY, UK

ABSTRACT

To increase our knowledge about mating-system evolution, we need to understand the relationship between specific floral traits and mating system. Species of Collinsia (Plantaginaceae) vary extensively in mating system; this variation is associated with variation in floral morphology and development and with the timing of self-pollination. Counterintuitively, large-flowered, more outcrossing species tend to have delayed stigma receptivity, reducing the amount of time that the stigma is receptive to cross-pollination before autonomous self-pollination. To understand how the timing of stigma receptivity is related to mating-system evolution, we studied in detail the timing of both stigma receptivity and self-pollination (anther–stigma contact) in two greenhouse-grown populations of large-flowered Collinsia heterophylla. Crosses on emasculated flowers at different stages of floral development always produced seeds, suggesting that cross-fertilization can be effected by pollen arriving prior to physiological receptivity. Phenotypic and genetic variation within populations in the timing of stigma receptivity and anther–stigma contact was substantial, although slightly less for the contact. Despite strong interspecific and interpopulation correlations, we did not find an among-genet phenotypic correlation between the traits. This indicates that each trait may respond independently to selection, and the trait association may be the result of correlational selection.

Key Words: Collinsia heterophylla • delayed selfing • delayed stigma receptivity • evolvability • flower development • heritability • mixed mating system • Plantaginaceae