A Cytogenetic Study of Heritable Tumors in Nicotiana Species Hybrids.

ENETIC tumors in Nicotiana species hybrids have been reported by several Ginvestigators. These tumors are formed in certain specific hybrid combinations on stems, roots, and occasionally on leaves of all or part of the F, progeny. Since the discovery by KOSTOFF (1930) of nine interspecific tumorous hybrids, almost 20 additional hybrid combinations have been reported in which the entire progeny is tumorous. In 23 additional species combinations, hereditary tumors are produced irregularly in some of the progeny or restricted to only certain parts of the plants (KEHR and SMITH 1954; SMITH 1958). Furthermore, it was noted that Nicotiana langsdorflii, N . glauca, N . rustica and N . alata enter into more tumorous hybrids than other Nicotiana species. NAF (1958) went a step further and tentatively and arbitrarily divided certain species of Nicotiana into two groups, namely plus group and minus group; tumors are formed when the species involved are from opposite groups. Thus, according to him, the plus group consists of N . forgetiana, N . langsdorflii, N . sanderae, N . alata, N . longiflora, N . noctiflora, N . bonariensis, and N . plumbaginifolia; the minus group species are N . tabacum, N . bigelouii, N . suaueolens, N . miersii, N . paniculata, N . debneyi, N . glauca, and N . rustica. It is of interest to note that seven of the eight plus group species (all but N . noctiflora) belong to the section Alatae. Morphologically, several types of tumors can be recognized. BURK and Tso (1960), working on the cytology of tumors, have described, on the basis of their appearance, two specific types of tumors, that is, LLPseudo” and “Cauliflower,” in F, N . glauca X N . langsdorflii (2n = 21). The “Pseudo” form had a characteristic chromosome number of 19 and produced leaf-like structures, while “Cauliflower” types produced only small shoot-like protuberances a few millimeters in length and characteristically had a chromosome number of 18. According to these authors, chromosome loss may be the cause rather than the result of genetic plant tumors. Cytogenetic studies of KEHR and SMITH (1954) have shown that tumors are produced on any diploid, triploid, or tetraploid combination of N . glauca (n = 12) and N . langsdorflii ( n = 9) possessing at least one complete genome of each of the parent species. In an effort to establish an association between tumor formation and particular chromosomes, KEHR and SMITH recurrently backcrossed