The evolutionary genetics and adaptive significance of intraspecific variation in pollen grain size in Brassica rapa L

Pollen size commonly varies within and among angiosperm species, but the biological significance and genetic basis of this variation remain largely unknown. A pollen-mixture experiment in Brassica rapa revealed that donors producing large pollen enjoyed a 20% siring advantage over donors producing small pollen during post-pollination competition for access to ovules. For selection to respond to such fitness differences, interindividual variation in pollen size must be genetically determined and heritable. Artificial selection for large and small pollen over three generations caused significant divergence in pollen diameter, with realized heritability ranging from 0.13 to 0.61. In addition, selection on pollen size led to correlated responses in pollen number (-), flower size (+), style length (+), and ovule number (+), suggesting that pollen size cannot evolve independently. The heritable genetic control of pollen size variation and consequent effects on siring success suggest that post-pollination processes provide the primary impetus for pollen-size evolution in angiosperms.