Introduction

Based on a number of biochemical and functional considerations chloroplast DNA（ct-DNA）of higher plants has long been thought to be highly conserved（Kirk，1971）.This view should be re-examined in light of current evidence.The results obtained from restriction enzyme analysis reveal a wide range of variability（Atchison et al.，1976；Vedel et al.，1976）.For instance，in the genus Nicotiana ct-DNA exhibits a high degree of diversity within the realm of its common features（Rhodes et al.，1981）.The species specific variation in this genus offers an excellent opportunity to study the evolution of Nicotiana ct-DNA.One approach is to analyze all ct-DNA in this genus and trace its origin.This will undoubtedly involve a great deal of effort because there are over 60 species.Alternatively，investigation of ct-DNA from various cultivars of the commercial species，N.tabacum，may yield more revealing information because the ancestral parents of this species are known（Gray et al.，1974），as well as，the parentage of each of the cultivars.

S.D.Kung.Y.S.Zhu，K.Chen，G.F.Shen，and V.A.Sisson，Department of Biological Sciences.University of Maryland，Baltimore County.Catonsville.Maryland 21228.USA

此文发表于1981年“Mol.Gen.Genet”183：20-24.

Offprinr requests to：S.D.Kung(https://www.daowen.com)

It has been established that the major force of speciation in Nicotiana is interspecific hybridization（Goodspeed，1954）in which the ct-DNA is always transmitted maternally（Wildman et al.，1973；Frankel et al.，1979）.The cytoplasmic male sterilc（CMS）cultivars of N.tabacum were also derived from interspecific hybridizations between various species as the female and N.tabacum as the male followed by several generations of backcrossing.This scheme evolved because investigators were only interested in the manipulation of this cultivated species and the reciprocal cross was often difficult or impossibie to make.Therefore，there is a parallel between the origin of speciation of Nicotiana and a number of N.tabacum cultivars.The distinct advantage of using the cultivars is that the parents involved in each cross are known.Any alteration of ct-DNA in the cultivars can be detected and the mechanism involved can be elucidated.

Alteration of ct-DNA was recently reported in CMS lines of N.tabacum（Frankel et al.，1979）.This study is designed to investigate further ① whether alteration on ct-DNA exists only in the CMS cytoplasm or in the normal cultivars as well.② the common mechanism of such alteration.③the relative position of this altered region on the circular ct-DNA.The cultivars of N.tabacum employed in this study covered a wide range of genetic backgrounds including CMS lines of unusual origins.One was obtained from a rare cross（Pittarelli and Stavely，1975），the only reported case of a direct interspecific cross between these two species in the literature，and the other originated as the result of a spontaneous，cytoplasmic mutation（Berbec，1974）.The analysis of restriction patterns of ct-DNA generated by several enzymes reveals that there is a high frequency of ct-DNA alteration in the male sterile but not in the fertile cytoplasms or N.tabacum.The altered restriction patterns and maps suggest that recombination of ct-DNA may have occurred.Other possible alternatives which can cause rearrangements in the chloroplast genome such as deletions and insertions or，point mutations which create or eliminate restriction site sequences cannot be ruled out.