Self-incompatibility genotypes of 224 plants of Ipomoea trifida from six populations in Central America have been determined by genetic analysis of segregants in F1 families derived from crosspollinations with the most recessive homozygote. A total of 49 different S-alleles was identified in these populations. From analyses of S-allelic interactions in heterozygous plants which were generated from cross-pollinations between plants possessing different S-alleles, a linear dominance hierarchy with six levels has been established among 28 S-alleles in both pollen and stigma.Codominance of alleles occurred more frequently in the stigma (9.2 per cent) than in the pollen (4.9 per cent). Nonlinear dominance relationships were rarely observed. Unequal frequencies of Salleles have been found in all populations examined, the most common 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 study also suggests that the southern area of Mexico to Guatemala is a centre of genetic variation in diploid I. trijIda.

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