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Keywords:

  • Asymmetric mating;
  • distyly;
  • floral display size;
  • legitimate pollination;
  • pollen quality;
  • pollen quantity;
  • seed set;
  • sexual polymorphism;
  • spatial isolation

Abstract

In animal-pollinated plants, both the spatial distribution of flowering individuals and the number of flowers that an individual displays affect pollen deposition rates and female reproductive success. Heterostylous species are likely to be particularly sensitive to the contingencies of spatial distribution, as they are reproductively subdivided into distinct mating groups, which usually exhibit self- and intra-morph incompatibility and differ in floral morphology. In this paper, we explore the joint effects of both spatial distribution of potential mates and floral display size on morph-specific pollen deposition rates and seed set patterns in two natural populations of Pulmonaria officinalis, a distylous species with a weak self-incompatibility system. Both total stigmatic pollen load and the proportion of legitimate pollen decreased with increasing spatial isolation. Legitimate (intermorph) pollen transfer was, however, asymmetric and decreased more rapidly with decreasing proximity to a compatible legitimate mating partner in the S-morph than in the L-morph. Total stigmatic pollen loads per flower increased with increasing floral display size, indicating that large plants are disproportionately more visited than smaller individuals. However, because legitimate pollen deposition decreased with increasing floral display size, these results also suggest that larger numbers of flowers increase the degree of geitonogamous pollination. In both the L- and S-morph, seed set significantly decreased with increasing isolation from a legitimate mating partner, but in the L-morph seed set was less dependent on the spatial distribution of the S-morph. In addition, seed set significantly increased with floral display size in the L-morph, but not in the S-morph. These findings indicate that the spatial distribution of potential mates and variation in floral display size may cause morph-specific differences in pollen deposition rates and female reproductive success.