NUMBER, FREQUENCY AND DOMINANCE RELATIONSHIPS OF S-ALLELES IN DIPLOIDIPOMOEA-TRIFIDA

Self-incompatibility genotypes of 224 plants of Ipomoea trifida from s ix populations in Central America have been determined by genetic anal ysis of segregants in F-1 families derived from cross-pollinations wit h the most recessive homozygote. A total of 49 different S-alleles was identified in these populations. From analyses of S-allelic interacti ons in heterozygous plants which were generated from cross-pollination s between plants possessing different S-alleles, a linear dominance hi erarchy with six levels has been established among 28 S-alleles in bot h pollen and stigma. Codominance of alleles occurred more frequently i n the stigma (9.2 per cent) than in the pollen (4.9 per cent). Nonline ar dominance relationships were rarely observed. Unequal frequencies o f S-alleles have been found in all populations examined, the most comm on S-allele being, as expected, the most recessive. This suggests that recessive S-alleles are widely distributed throughout Central America . The diversity of the multiple S-alleles observed in the present stud y also suggests that the southern area of Mexico to Guatemala is a cen tre of genetic variation in diploid I. trifida.