Introduction

Nicotiana chloroplast genomes possess many distinct properties and unique features.The fragment patterns generated by restriction enzymes exhibit a high degree of diversity as well as a general similarity（Rhodes et al.，1981；Kung et al.，1981）.For example，the individual restriction pattern is species specific for any given species while the overall configuration is characteristic of the genus Nicotiana（Kung et al.，1981）.In many species，even variation of a fragment contains sufficient information to indicate its taxonomic position or to reflect its evolutionary relationship.Therefore，restriction patterns of Nicotiana chloroplast DNA（ct-DNA）can be used in this context to identify species in a manner similar to the taxonomic，cytogenetic（Goodspeed，1954）and biochemical（Chen et al.，1976）classifications.This is due mainly to the existence of the unusual taxonomic and evolutionary relationships among Nicotiana species.Many of the 64 Nicotiana species have originated through interspecific hybridization followed by doubling of the chromosomes（Goodspeed，1954）.In each hybridization the ct-DNA is generally inherited maternally（Wildman et al.，1974）with only infrequent alteration（Kung et al.，1981）.Thus，the present-day species with 24 pairs of chromosomes are more advanced than those possessing 12 pairs.Consequently，if a 12-paired chromosome species shares an identical ct-DNA fragment pattern with that of a 24-paired chromosome species a close phylogenetic relationship between them is implied.It is likely that the former is an ancestor（female）of the latter.The best studied case is the origin of N.tabacum（N=24）in which N.sylvestris（N=12）is the female parent in the original cross（Gray et al.，1974）and both of these species possess identical ct-DNA（Rhodes et al.，1981）as directed by the mode of maternal inheritance.

此文发表于“Theor.Appl.Genet”61：73-79.(https://www.daowen.com)

This study is designed to investigate the restriction pattern of ct-DNA from many closely related species having different levels of ploidy.From these results the sequence of ct-DNA evolution in Nicotiana is formulated.This is accomplished by a simple procedure using a single restriction enzyme without radiolabelling the fragments.In this paper we present our findings and discuss their significance.