tion, as might be the case when pollinators avoid particular floral displays.] The degree of advantage of self-fertilization genes, if any, clearly hinges on several additional factors, including (i) the extent of pollen discounting; (ii) differential maternal fertility associated with autopollination; and (iii) reduced viability of progeny derived through self-fertilization because of inbreeding depression. A number of experiments have been conducted to investigate each of these additional factors.
Rausher et al. (1993) found no evidence for pollen discounting in experimental populations of morning glory and instead found a nonsignificant excess of white gene fertilizations of ovules of non-white parents. In a different set of experiments, Epperson and Clegg (unpublished data) found a nonsignificant deficit of white gene fertilizations of ovules of non-white parents. Much larger experiments with greater statistical power are needed to provide a definitive answer about the role of pollen discounting, but at present there is no clear evidence that the advantage of white genes is offset by pollen discounting. Epperson and Clegg (unpublished data) have also measured the fertility of white maternal parents subjected to autopollination and find a reduction in maternal fertility that could act to moderate or eliminate the transmission advantage of white genes. Experiments to measure inbreeding depression provide evidence for a depression in fitness associated with self-fertilization, but the magnitude is not sufficient to offset the selfing advantage of white genes (Chang and Rausher, 1999). Other studies have suggested heterogeneity among families in segregation bias favoring the transmission of either white or dark alleles in different heterozygous parents (Fry and Rausher, 1997). Finally, overdominance in seed size among the progeny of white maternal parents has also been documented, but this evidently does not translate into a fitness advantage (Mojonnier and Rausher, 1997).
In summary, there is clear and convincing evidence that white phenotypes suffer some disadvantage in natural populations based on spatial autocorrelation analyses, and there is clear and convincing evidence that white genes have a transmission advantage when white maternal plants are infrequent in populations. The transmission advantage is associated with pollinator preferences and a consequent increased rate of self-fertilization among white maternal parents, but this advantage is one-sided and should diminish to zero as the frequency of white maternal types approaches 50%. Many lines of evidence argue for a compensatory disadvantage of white types to account for the global frequency in the southeastern U.S. of approximately 10%, but the precise nature of the disadvantage is unresolved. It may be that a number of separate components such as reduced maternal fertility under autopollination, inbreeding depression, and segregation bias all sum to provide the needed balance. Because small effects demand very large experiments to provide adequate statisti-