BRIDGE SPECIES FOR INTROGRESSION OF WILD GERMPLASM IN SWEET POTATO BREEDING.

The species of section Batatas represent the primary potential source of wild germplasm for sweet potato breeding. Their wide distribution and adaptation make their use desirable, but their direct use is constrained by their ploidy level (2x, and 4x, in contrast with cultivated I . batatas which is 6x) and lack of storage root production. We investigated 4x clones of interspecific origin (6x I. batatas 2x I. trifida ) as bridges to introgress genes from several wild species of the section Batatas to I . batatas . Six 2x wild species from different origins were reciprocally crossed to the 4x clones. The crosses were successful only when the diploid wild species were used as females (2x × 4x) indicating unilateral interspecific incompatibility. The unsuccessful types of crosses (4x × 2x) were repeated using wild hybrid males containing I . trifida in their parentage. This time only those crosses involving a hybrid having I . trifida as its female parent produced seeds. Progenies from the successful 2x × 4x crosses were diploid instead of the expected triploid. Triploid progenies were obtained only in the 4x × (2x hybrid) crosses. Some triploid progenies produced 2n pollen in varying frequencies. These results show different levels of crossing barriers among Ipomoea species and give us alternative pathways for overcoming them. I . trifida was identified as a possible bridge species.