Study On Introgression Of Natural Hybridization Among Main Wild Species Of Rhododendron In Baili Rhododendron Nature Reserve Of Guizhou Province

Rhododendron is a large genus of Ericaceae,which contributes a plenty of beautiful ornamental resources for horticulture.Baili Rhododendron Nature Reserve of Guizhou is the largest concentrated distribution area of native rhododendron forest in China so far.It is the only nature reserve in China that takes Rhododendrons as the object of protection,and also one of the largest concentrated and sympatric distributed areas of natural Rhododendrons in the world at present.However,the population of Rhododendron in Baili Rhododendron Nature Reserve of Guizhou tends to deteriorate rapidly,and the genetic background of Rhododendron resources is not clear,which seriously hinders the protection and sustainable self-use of Rhododendron resources.In order to solve the above problems,four major species（R.delavayi,R irroratum,R.decorum and R.denudatum）and its suspected natural hybrid offsprings related species were used as the research object.Morphological methods combined with nuclear genome SSR markers and chloroplast genome SNP markers were applied to study its genetic background and natural hybridization introgression.The main findings were as follows:（1）Four major species of Baili Rhododendron Nature Reserve（R.delavayi,R irroratum,R.decorum and R.denudatum）and their suspected natural hybrid offsprings were collected according to morphological characteristics.A total of 13 characteristics were measured in each representative.Morphological characteristics results suggest that R.agastum is more like a hybrid between R.delavayi and R.irroratum rather than between R.delavayi and R.decorum.（2）In this study,20 pairs of SSR primers were used to analyze the genetic diversity of a total of 219 individuals from 4 main species of Rhododendron,R.delavayi,R.irroratum,R.decorum,R.denudatum and their suspected natural hybrid offsprings,in Baili Rhododendron Reserve,Guizhou province.The results showed that percentage of polymorphic loci in natural population of Baili Rhododendron Nature Reserve（P）was97.5%;the average number of alleles（N_A）was 4.875;the average number of effective alleles（N_E）was 2.700;the average number of Shannon’s information index（I）was 1.057;the average number of observed heterozygosity（H_O）was 0.291;the average number of expected heterozygosity（H_E）is 0.524;the average number of the fixed index（F）was0.396.The genetic diversity of R.agastum and R.decorum was the highest.The genetic differentiation coefficient of Rhododendron populations in Baili Rhododendron Nature Reserve was 0.135,indicating that the genetic differentiation of Rhododendron populations in Baili Rhododendron Nature Reserve was small.The results of molecular analysis of variance（AMOVA）showed that Reserve mainly came from within population（78%）,and genetic variation among the populations was 22%,which was consistent with the results of genetic differentiation coefficient（3）The structure results of SSR markers and origin scenario estimation support that R.agastum is a natural hybrid offspring of R.delavayi and R.irroratum,which is consistent with the results of morphological analysis.Besides,hybridization analysis indicates that R.agastum is dominated by F₂ generation in the Reserve.（4）The analysis results based on SSR markers and chloroplast genome SNP markers showed that the natural hybridization between R.delavayi and R.irroratum was bidirectional but asymmetric;R.delavayi is the main maternal parent of R.agastum.（5）The structure results of SSR markers were not supported that there had been extensive natural introgression of hybridization between R.irroratum and R.denudatum;it was not supported that there had been extensive natural introgression of hybridization between R.denudatum and R.delavayi;it was not supported that there had been extensive natural introgression of hybridization between R.denudatum and R.agastum.