The population genetics of the self-incompatibility polymorphism in Papaver rhoeas. IX. Evidence of an extra effect of selection acting on the S-locus

Abstract

Investigation of the segregation ratios of the self-incompatibility (S-) alleles in three sets of full-sib families of Papaver rhoeas shows that these ratios depart significantly from Mendelian expectation. This indicates that the S-locus is subject to an extra effect of selection over and above that due to incompatibility in these families, and suggests that this is probably the chief cause of the unequal S-allele frequencies observed in the R102 and the other natural populations we have examined. Since, however, the selective advantage of an allele depends on the allele with which it is segregating in all of the families examined, the relationship between these results and the unequal 5-allele frequencies in populations appears to be complex. Furthermore, while in one family the extra effect of selection appears to be of the zygotic type, in the others it is of the gametic type - which, in most, involves the female gametophyte rather than the pollen. It is argued that the extra effect of selection is due to the linkage of the S-gene to one or more genes that are the chief target of this selection, rather than to pleiotropy. Though it is suggested that among candidates for linkage are genes controlling seed dormancy and albinism, the detection of an extra effect of selection acting on the female gametophyte and, in one case, on the pollen, implies that other genes of as yet unknown effect are more likely to be involved.