ISOZYMIC EVIDENCE BEARING ON THE ORIGIN OF NICOTIANA TABACUM L.

Isozyme patterns of plant organs and tissues are a direct reflection of particular genes that are operating at a given stage of growth and differentiation. Gene-enzyme systems affect survival and fitness in the course of evolution and consequently may provide useful information for studies of phylogenetic relationship. The application of isozyme patterns for biosystematics in plants has been reported in the recent literature (West and Garber, 1967; Sheen, 1970). The genus Nicotiana has been a favored material for studies on inheritance and evolution in higher plants. It comprises 64 presently recognized species (Smith, 1968), among which N. tabacum, an allopolyploid, is the only species cultivated around the world. The origin and evolution of N. tabacum has been deduced from morphological, ecological, genetic, and cytogenetic investigations. It is generally believed that interspecific hybridization, with subsequent amphiploidy as well as gene recombination, has played an important role in its evolution. However, N. tabacum is not found in nature (Goodspeed, 1954), particularly in the geographic area where the putative progenitor species overlap in their distribution. The origin of N. tabacum has, therefore, been a debatable subject for many years. In 1928 Clausen hypothesized that N. tabacum (N = 24) had originated from an interspecific hybrid between N. sylvestris (N = 12) and N. tomentosa (N = 12) after chromosome doubling. This hypothesis was later changed with regard to the progenitor species from the Tomentosae section. There are two currently recognized hypotheses regarding the origin of tobacco. Hypothesis I was proposed by Clausen (1932) and Kostoff (1938) who believed that N. tomentosiformis (N = 12) was one of the progenitors. In contrast, Goodspeed (1954) favored N. otophora (N = 12) (hypothesis II) as the closer relative of modern tobacco. Both hypotheses agreed that an ancestor of N. sylvestris was the other progenitor. Recently, Cameron (1965) added evidence that N. sylvestris contributed cytoplasm to the hybrid from which N. tabacum was evolved. The evolutionary process of N. tabacum may be clarified if synthesis of this species can be achieved through experimental hybridization and amphiploidy. Kostoff (1938) has conducted such experiments by backcrossing the F1 hybrid of N. sylvestris X N. tomentosiformis (2N = 24) first to N. sylvestris and then to N. tomentosiformis to reconstitute the respective genomes. He obtained two amphidiploids (2N = 48) which were taxonomically indistinguishable from N. tabacum and readily crossable with tobacco cultivars. However, some morphological features of the amphidiploids resembled the hybrids of a N. tabacum x (N. sylvestris x N. tomentosiformis) cross. Greenleaf (1942) produced similar amphidiploid from callus of the F1 hybrid. In contrast to Kostoff's amphidiploid which was self-fertile, the callus-induced one was completely female sterile but highly male fertile. Amphidiploids produced by colchicine treatments showed the same character of female sterility (D. U. Gerstel, pers. comm.). The present investigation was designed 1 Contribution from the Department of Agronomy, University of Kentucky, Lexington, Kentucky. Published with the approval of the Director as Paper No. (70-3-161) Journal series, Kentucky Agricultural Experiment Station.