Differential siring success of Pgi genotypes in Clarkia Unguiculata (Onagraceae).

Competition among pollen grains for the fertilization of ovules can play an important role in determining the male and female reproductive success of flowering plants. To examine the influence of pollen-donor genotype on male reproductive success, hand-pollinations were conducted on Clarkia unguiculata and the siring success of pollen-donor plants was compared between donors homozygous for different allelomorphs of the allozyme PGI (phosphoglucoisomerase). Donors homozygous for the B allele sired more seeds than C-allele donors. Single-donor crosses indicated that C-donor-sired seeds are aborted more often than are B-donor-sired seeds, suggesting that the B-allele donor's advantage in mixed pollinations was a result of differential abortion. A negative relationship between pollen load and the siring success of B-allele donors implies that pollen from B-allele donors has reduced performance relative to C-allele donors when pollen loads are high. These data demonstrate consistent differences in siring success between individuals homozygous for different alleles at a single locus and suggest that variation at the Pgi locus may be maintained by a post-pollination trade-off.