the ww phenotype. Limited sampling across the southeastern U.S. indicates very low frequencies of albino alleles (<1%) in most local populations (Epperson and Clegg, 1992). In addition, unstable alleles also appear to be rare in most local populations. An exceptional local population near Athens, GA is the only population sampled with moderate frequencies of both albino (a) and unstable (a*) alleles (Epperson and Clegg, 1992).
Because flowering plants often depend on insect pollinators for reproductive success, a natural question to investigate is whether pollinators discriminate among the various flower color phenotypes in morning glory populations. A number of experiments conducted in different years and by different investigators in both natural and experimental populations agree in revealing a bias by bumblebee pollinators against visiting white flowers when white is less than 25% of the population (Brown and Clegg, 1984; Epperson and Clegg, 1987a; Rausher et al., 1993). In contrast, there is no evidence of pollinator discrimination among P/p (blue/pink) or I/i (dark/intense) locus phenotypes (Brown and Clegg, 1984; Schoen and Clegg, 1985). The under visitation of white phenotypes is correlated with an increased frequency of self-fertilization by white maternal parents based on estimates using isozyme marker loci (Brown and Clegg, 1984; Epperson and Clegg, 1987a). However, Schoen and Clegg (1985) discovered that pollinator visitation rates and outcrossing estimates did not differ between white and pigmented phenotypes when the two forms were in equal frequency. Later, Epperson and Clegg (1987a) and Rausher et al. (1993) showed that the pollinator discrimination against whites, and the reduced outcrossing rate of white maternal plants, was frequency-dependent and disappeared when the frequency of the white phenotype approached 50%. These observations and experiments indicate that white loci act as mating system modifier loci and consequently bias their own transmission to subsequent generations.
There is substantial theoretical literature on the population genetics of modifier loci (Fisher, 1941; Holsinger, 1996; Karlin and McGregor, 1974). According to this literature, white genes that increase the frequency of self-fertilization are expected to increase in frequency to fixation within populations because the white gene should be transmitted differentially to progeny of white maternal plants via self-fertilization. This assumes that the selfing (white) gene is also transmitted to the outcross pool in proportion to its frequency in the population, where it can fertilize ovules of non-white maternal phenotypes (absence of pollen discounting). [“Pollen discounting” refers to the situation in which pollen transmission to other (nonmaternal) plants is reduced by a gene promoting self-fertiliza-