Studies On The Origin And Evolution Of Polyploids In Gynostemma Pentaphyllum (Cucurbitaceae)

Polyploidy has occurred extensively during the evolution of plants,and it now appears that flowering plants and perhaps all eukaryotes possess genomes with considerable gene redundancy,much of which probably results from polyploidy or whole genome duplication.It is recognized that recurrent polyploidy is the rule,not the exception.According to the origin of different polyploids,it can be devided into two types,alloploids from between different species,whereas autoploids form within species,typically involving crossing between individuals.The genus Gynostemma(Cucurbitaceae) constitutes a polyploid group of perennial creeping herbs,which reproduce sexually and asexually,and its modern distribution center ranges from drainage areas of the Yangtze River to Southwest China's Yunnan Province.So far,16 species and 2 varieties have been identified,with 9 species and 2 varieties endemic to China.Based on the morphology of fruits,this genus can be divided into two subgenera, namely subgen.Gynostemma and subgen.Triostellum.Polyploids occur frequently in several species,and polyploidy is usually closely related to geographic distribution.Species with only diploids are usually distributed in pretty small regions,while those with polyploids frequently cover larger geographic ranges.With the largest variety of cytotypes(2n=22,44,66 and 88) in Gynostemma,G.pentaphyllum is also the most widespread species in this genus.In the present study,we inferred the origins of polyploids in G.pentaphyllum using sequences of the plastid intergenic spacers(trnL-trnF,psbB-psbF and rpl20-rps12) and cloned DNA sequences from two nuclear regions(RPB2 and nrDNA ITS).The objectives of this study were to:(1) infer the auto-vs.allopolyploid origins of G.pentaphyllum;(2) determine the number of polyploidization events responsible for the formation of tetra-,hexa-,and octoploids;and(3) investigate the relatedness between diploid and polyploid populations.Phylogenetic analyses of the separate and the combined nuclear gene datasets all supported autoploid origins of polyploids in G.pentaphyllum.Three polyploid populations were more closely related,indicating that significant genetic differentiation may have occurred between diploids and polyploids.We concluded that polyploidization might be an important evolutionary mechanism in the diversification of G.pentaphyllum.On the other hand,no chloroplast DNA(cpDNA) variation was detected in ingroups except the octoploid DL 8x, which possessed a different cpDNA haplotype from the other populations of G.pentaphyllum. The ITS phylogeny of G.pentaphyllum suggests no congruence in geographic distribution between polyploids and their diploid progenitors,and the present geographic distances between diploids and polyploids may not reflect patterns of relatedness.This can be explained by possible extinction of its diploid progenitors and/or the possibility that polyploidization occurred in G.pentaphyllum a sufficiently long time ago to allow substantial geographic movement of diploids and polyploids.However,the distribution of cytotypes in G. pentaphyllum was not as typical as many other autopolyploid complexes.Polyploidization failed to contribute significantly to the expansion of its geographic range.The geographic distribution of diploids and polyploids in G.pentaphyllum may be associated with the past ecological environments of different areas,especially during the glacial period.