2002 The American Genetic Association 93:185–192 Occurrence and Cytological Mechanism of 2n Pollen Formation in a Tetraploid Accession of

We evaluated a 4x accession of Ipomoea batatas (L.) Lam. and four 2x accessions of Ipomoea triloba for 2n pollen production. Approximately 90 % of the genotypes of accession 81.2 (I. batatas, 4x) produced 2n pollen with different frequencies. In contrast, none of the genotypes of I. triloba produced 2n pollen. The diameter of the 2n pollen was approximately 30 % ( ) larger than that of the n pollen, making32 it easy to identify, measure, and quantify. The correlation (r 0.93**) between the frequency of giant pollen and the frequency of dyads and triads was highly signif-icant, strongly suggesting that the giant pollen grains were 2n pollen. The 2n pollen producers presented either a parallel or tripolar spindle arrangement (Y shaped) at anaphase II instead of the normal 60 crossed spindle orientation. These two ab-normal spindle configurations produced dyads and triads, with different frequen-cies (13–67%), instead of tetrads. Occasionally a metaphase II spindle variation was found with a single fused spindle, which also forms a dyad. The correlation (r 0.89**) between the frequency of 2n pollen and the frequency of parallel, fused, and tripolar spindle arrangements was also highly significant, suggesting that these abnormal spindle configurations are involved in the production of 2n pollen in I. batatas. When we evaluated the efficiency of 2n pollen in polyploidization using 4x 4x (2n) crosses, all progenies were 4x, suggesting the existence of barriers to crossability between 4x genotypes and their 2n pollen-producer counterparts. The sweet potato, Ipomoea batatas (L.) Lam., is the ninth most important crop in the world in terms of production. In 2000 it yielded 141 106 metric ton