Diversity and Evolution

Based on a number of biochemical and functional considerations ct-DNA of higher plants has long been thought to be highly conserved.This concept originated from observations that ct-DNA exists in multiple copies per chloroplast，contains similar coding information and exhibits uniform physico-chemical properties.This view should be re-examined in light of current evidence.The results obtained from restriction enzyme analysis revealed a wide range of variability of ct-DNA［10，11］.Nicotiana ct-DNA exhibits a high degree of diversity but an overall similarity.For instance the individual restriction pattern is species specific for any given species，while the general configuraton is characteristic of the genus Nicotiana［12］.

Among the many restriction enzymes used，EcoRI produces the largest number of fragments and therefore has the highest resolving power to uncover differences，Of the 40 fragments generated by EcoRI enzyme the first 10 are worth noting［6］.Fragments 3 are stable and present in all 40 Nicotiona species examined so far.In contrast，fragments 4 and 5 are extremely variable one or both can be altered；one or both may be absent.It is such a combination of diversity and similarity in fragment pattern that forms the basis of species specificity and overall identity of Nicotiana ct-DNA.

The fragment pattern generated by BamHI revealed a similar degree of diversrty in Nicotiana ct-DNA.For example，N.gossei and N.otophora ct-DNAs differ in 13 of the 27 BanHI bands.This matches the extent of variation in their EcoRI fragments in which 18 of the 40 bands are different.Therefore，both EcoRI ard BamHI can be used to measure accurately the degree of diversity in Nicotiana ct-DNA.(https://www.daowen.com)

Alteration of DNA is the main force of evolution.Judging from the wide diversity of Nicotiana ct-DNA it is evident that there have been considerable changes through the course of evolution.It has been demonstrated in Nicotiana that the mechanism of ct-DNA alteration involves point mutation，inversion，deletion，duplication and rarely recombination［13］.Point mutations are primarily responsible for the observed gain and elimination of many restriction sites.They occur rather frequently in relation to other mechanisms and are clustered in one region which can be described as a hot spot［6］.A large（11 kb）and a small（0.5 kb）deletion have also been discovered［14］.Whether these segments contain any structural genes is not known.The example of ct-DNA duplication is the existence of the large inverted and the small scattered repeats.

The limited differences in the restriction patterns generated by Sma Ⅰ provide excellent markers for phylogenetic studies.It is a better system than the isoelectric focusing patterns of fraction I protein with relation to identifying evolutionary steps in ct-DNA.By using a single restriction enzyme，Sma I，eliminations and sequential gains of its recognition sites during the course of ct-DNA evolution are clearly demonstrated6］.