THE POPULATION-GENETICS OF THE SELF-INCOMPATIBILITY POLYMORPHISM IN PAPAVER RHOEAS .9. EVIDENCE OF AN EXTRA EFFECT OF SELECTION ACTING ON THE S-LOCUS

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. Thi s indicates that the S-locus is subject to an extra effect of selectio n over and above that due to incompatibility in these families, and su ggests that this is probably the chief cause of the unequal S-allele f requencies observed in the R102 and the other natural populations we h ave examined. Since, however, the selective advantage of an allele dep ends on the allele with which it is segregating in all of the families examined, the relationship between these results and the unequal S-al lele frequencies in populations appears to be complex. Furthermore, wh ile in one family the extra effect of selection appears to be of the z ygotic 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, rat her than to pleiotropy. Though it is suggested that among candidates f or linkage are genes controlling seed dormancy and albinism, the detec tion 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 un known effect are more likely to be involved.