Alfred Jones

The species ofIpomoea closely related to the sweet potato

The problem of the origin of the sweet potato, Ipomoea batatas (L.) Lam., has both geographic and biosystematic aspects. The geographical locality where the sweet potato first occurred, or was first domesticated, is generally accepted to be the New World. Hypotheses of an Oceanian or African origin have been discounted by Conklin (2). Nevertheless, the sweet potato was evidently well distributed throughout the Pacific islands and reached even New Zealand in pre-Columbian times (16). Spanish and Portuguese travelers are believed to have disseminated the sweet potato through the tropics so rapidly that the impression was created that the sweet potato was pantropical in distribution prior to discovery of the New World. Although the actual geographical origin and the subsequent spread of the sweet potato throughout the world have ethnological implications, the geneticist or plant breeder is much more concerned with the origin of the sweet potato from related species. This interest is not a matter of idle curiosity. Reconstruction of sweet potato-like plants from related species should provide a new method of introducing useful variation, including disease resistance, to the sweet potato gene pool. In recognition of this fact, considerable emphasis has been placed on the species of Ipomoea

Discovery of wild tetraploid sweetpotatoes

Specimens in the germplasm collection at the U.S. Vegetable Laboratory, United States Department of Agriculture (USDA) in Charleston, SC, were studied to examine phylogenetic relations of the tetraploid accessions inIpomoea sectionBatatas. This collection contains tetraploidsfrom a wide geographic range and most were tentatively identified by the collector asI. trifida. This study shows that corolla and sepal traits may be used to distinguish the tetraploidsfrom known specimens ofI. trifida (diploid) andI. batatas (hexaploid). All but one tetraploid accession examined (CH67.50) had corolla tubes and sepals shaped likeI. batatas and more closely resembled that species thanI. trifida. Use of corolla tube diameter allowed the hexaploidI. batatas and tetraploid accessions to be distinguished fromI. trifida because the corolla tubes were wider immediately above the calyx. Differences in sepal shape were quantified using the angle at the sepal apex. This angle was consistently obtuse in theI. batatas hexaploids and the tetraploids, but was acute in theI. trifida accessions. Due to similarities in sepal and corolla traits, these tetraploids should be reidentified as tetraploidI. batatas, a cytological race of the hexaploid I. batatas (the sweetpotato).ZusammenfassungEl banco de germoplasma del Laboratorio Vegetal U.S. del Departamento de Agricultura de los Estados Unidos en Charleston, S.C., en su colección posee especímenes de entradas tetraploides deIpomoea batatas (identificadas comoIpomoea trifida,) con amplia distributión geográfica. El objetivo príncipal del presente estudiofue verificar la relatión filogenética de las entradas tetraploides con especímenes deI. batatas (hexaploide) eIpomoea trifida (diploide). Las características relacionadas a la forma y tamaño del corola y los sepalos fueron considerados para diferenciar tetraploides, diploides, y hexaploides. La forma del sepalo y tubo de la corola de la mayoría de tetraploides, excepto la entrada CH67.50, poseen estrecha similitud con la batata cultivada diferenciandosé deIpomoea trifida. Para cuantificar las diferencias en el tamaño de corola fue considerado el diámetro del tubo. Así, la batata cultivada y las entradas tetraploides se diferencian deI. trifida debido al ensanchamiento del tubo de la corola sobre el cáliz. Asímismo, para cuantificar las diferencias en la forma de sepalo fue considerado la forma del ápice. La batata cultivada y las entradas tetraploides poseen forma considerablemente obtusa mientras queI. trifida posee forma aguda. Debido a las similitudes entre tetraploides y hexaploides en la forma y tamaño de sepalo y corola, los tetraploides serían re-identificados como I. batatas tetraploide, considerandosé como una raza citológica de la batata cultivada.

A WildIpomoea Species Closely Related to the Sweet Potato

In a brilliant study of certain species of Ipomoea closely related to the sweet potato Ipomoea batatas (L.) Lamn., Nishiyama (1971) claims not only to have found the primitive wild type of 90 chromosomes from which the sweet potato originated, but also to have resynthesized the wild type from a diploid and a tetraploid species. In contrast, in a very careful study of many of the diploid and tetraploid relatives of the sweet potato, Martin and Jones (1972) point out that none of the species so far found resembles the sweet potato enough to be its ancestor. They hypothesize that the described diploid species are closely related to each other, but that the tetraploid species show no close resemblances to each other, to the diploids, or to the sweet potato. Nishiyamas (1971) hypothesis is that the tetraploid progenitor of sweet potato is I. littoralis Blume, a species that when grown in Mayaguez, Puerto Rico, proved to be equal to what Martin and Jones (1972) have called I. gracilis R. Br. Matuda (1963) and van Oostroom (1953) consider I. littoralis to be a synonym for L gracilis. As they point out, the species has forms with cordate leaves as well as with lobed leaves. Nevertheless, Nishiyama apparently recognizes other differences, because he treats L littoralis and L gracilis as separate species (1971).

Should nishiyama’s K123 (Ipomoea trifida) be designatedI. batatas?)

An interesting Ipomoea was collected and designated K123 by Nishiyama (9) while he was on tour in the United States and Mexico in 1955. It has since been extensively discussed (10, 11, 12, 13). Following preliminary observations, K123 was tentatively considered as Ipomoea commutata Roem., and, because of its 90 somatic chromosomes, possibly an ancestral species to the sweet potato, I. batatas (L.) Lam. (9). In this first report of K123, Nishiyama recognized that it may actually be a direct prototype of the modern cultivated sweet potato. In subsequent reports, it has been considered to represent I. trifida (H.B.K.) G. Don and has received considerable attention in phylogenetic discussions of the sweet potato (4, 10, 11, 12, 13). Observations of K123 (very graciously supplied to me by Dr. Nishiyama) lead me to the inescapable conclusion that it should be considered an extreme segregate of the sweet potato rather than a different species as previously postulated. This paper presents information relative to that conclusion.