Production of 2n pollen in diploid Ipomoea trifida, a putative wild ancestor of sweet potato

We screened 429 individuals from 31 diploid Ipomoea trifida accessions from different countries of Central and South America for 2n pollen production. Almost 20% of this population was found to produce 2n pollen in different frequencies. The production of 2n pollen is not an isolated incident but a common phenomenon in this taxon. Pollen diameter was measured in normal and 2n pollen-producing clones. The diameter of 2n pollen is 30% larger than that of normal pollen, making it very easy to detect 2n pollen producers and measure their frequency. Production of 2n pollen was confirmed by the finding of dyads and triads in anther samples at the tetrad stage of meiosis. Furthermore, Ax individuals were found among progenies of 4x × 2x crosses, providing direct evidence of 2n pollen production in the diploid male parent. The occurrence of 2n pollen in 2× I. trifida provides a clue to understanding how polyploidization and gene flow between different ploidy levels can occur in nature. These findings are discussed in relation to the origin of the polyploid series in this species and of the cultivated sweet potato, I. batatas.