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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Tetraploid F1 hybrids between Ipomoea batatas, sweet potato (2n = 6x = ca. 90), and diploid (2n = 2x = 30) I. trifida (H. B. K.) Don. showed various degrees of fertility reduction. The present study aimed to clarify its causes by cytological analysis of meiotic chromosome behavior in the diploid and sweet potato parents and their tetraploid hybrids. The diploid parents showed exclusively 15 bivalents, and the sweet potato parents exhibited almost perfect chromosome pairing along with predominant multivalent formation. Their hybrids (2n = 4x= 57-63) formed 2.6-5.0 quadrivalents per cell, supporting the autotetraploid nature. The meiotic aberratios of the hybrids were characterized by the formation of univalents, micronuclei, and abnormal sporads (monad, dyad, triad, and polyad). The causes underlying these aberrations were attributed in part to the multivalent formation, and in part to a disturbance in the spindle function. Three hybrids showing serious meiotic aberrations were very low in fertility. The utilization of the sweet potato-diploid I. trifida hybrids for sweet potato improvement is described and, further, the role of interploidy hybridization in the study of the sweet potato evolution is discussed.