Evolution and domestication of the sweet potato. Nishiyama, I. Bot. Maj. Tokyo, 84(1967):377–387, 1971.
Evolution and domestication of the sweet potato [link]Paper  doi  abstract   bibtex   
The discovery of a direct ancestor, Ipomoea trifida (6x), of the sweet potato (6x) in 1961-1962 has been followed by a further finding of its diploid (I, leucantha) and tetraploid (I. littoralis) predecessors. On the basis of cytogenetical study it was reasonably assumed that I. littoralis (4x), I. trifida-(3x)-6x, I. trifida (6x) and I. batatas (6x), sweet potato, are autoploids derived rather from the doubling of a set of 15 chromosome pairs (genome B) of I. leucantha (2x) than from segmental alloploidy. On the other hand an artificial hexaploid, I. littocantha, synthesized from I. leucantha (2x) and I. littoralis (4x) was proved to have the same genome constitution as I. trifida-(3x)-6x and sweet potato, their Fl hybrids having 45 bivalents or modified configurations. The hybrids were fertile. Another artificial hexaploid, I, lacuno-cilis, derived from I. lacunosa (2x) and I. gracilis (4x) was self-fertile, but usually did not hybridize either with I. trifida or sweet potato. The phylogenetic relationship was fairly well supported by the pattern of the mat-ing system in interspecific crosses. A hexaploid wild form of Ipomoea was collected by the author in Mexico in 1955. It was found to represent I. trifida (H.B.K.) Don. Based on the morphological, cytological and genetical evidences, this wild plant was considered to be the direct progenitor of the sweet potato, I. batatas (L.) Lam. (Nishiyama, 1961; Nishiyama et al., 1961a, b; Nishiyama and Teramura, 1962). It goes without saying that it corres-ponded very well to our long known conception of an ancestral type of sweet potato. The close relationship was confirmed by Jones (1967) on the evidence of crossing experiments and cytological observations of F1 hybrids between sweet potato and the wild form. On the other hand, as already reported by some workers, Jones also found that some sweet-potato seedlings exhibited occasionally such wild characteristics as nonedible roots, winding or climbing habit stems and pubescence on stems, petioles, peduncles and leaves. These findings have led him to a different conclusion from ours, namely that I. trifida (K123) is not an ancestor but a segregate of the sweet potato. In recent years a research project of improving the sweet potato by using the germ

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