Primary Studies On Hybrid Origin And NrDNA Incomplete Concerted Evolution Of Aster Ageratoides Var.laticorymbus

Aster ageratoides Turcz.(Asteraceae),a taxonomically difficult taxon,includes a variety var.laticorymbus Hand.-Mazz..The variety is a polyploidy complex endemic to China.After the field sample collection and field observations of the variety,we investigated its speciation,formation of the geographical distribution pattern and incomplete concerted evolution of nrDNA,based on data of its cytology,nrDNA and cpDNA.Then,the taxonomical treatment of the variety was given and a new species was published.The main results are summarized as follows.1.Morphology and cytology studies of taxa related to A.ageratoides var.laticorymbusMorpgological comparisons were made between var.laticorymbus and its related taxa to determine their morphological differences and diagnostic characters,showing that var.laticoiymbus is distinctly different in morphology from its related taxa.Chromosome numbers of some related taxa that A.handelii,A.sp.Dehang and A.sp.Xiangcheng were first time reported.Further confirmation that some morpgological gaps in Aster ageratoides var.laticorymbus and related taxa.Its main features:stem leaves oblong,oval;green involucre trip,white ray florets,rare shallow-amaranth.2.Cytogeography studies of Aster ageratoides var.laticorymbus Aster ageratoides var.laticorymbus,endemic to the middle and lower latitude regions(22°-34°N)of China,was observed and collected for the samples in its 41 populations and its two cytotypes,tetraploid and hexaploid,wpopulationsere found.The tetraploid is similar to the hexaploid in distribution areas(103°-110°E)of western China although there is only the tetraploid at the range edge and in high altitude mountains.On the other hand,in the eastern area of the range area(110°-119°E),there are only hexapoid except for two tetrapoid populations of the north-eastern and southern edge of the range area.Therefore,the polyploid complex is developing towards its mature stage.3.Hybrid origin of A.ageratoides var.laticoiymbus and its putative parentsITS and ETS sequences from forty one populations of var.laticorymbus were obtained by PCR-direct sequencing and displayed dimorphism within these populations and individuals.Twenty-two and fourteen dimorphic sites were always found in ITS and ETS sequences,respectively,of almost all populations and individuals,which were similar in ITS and ETS cloned sequences of 32 plants from twenty populations.In polymorphic ITS cloned sequences,there were two main types,PIa and PIb while the remainder was intermediate types between PIa and PIb.The case was same for ETS cloned sequences with two main types PEa and PEb.And most of the plants had a predominant combination of PIa and PEa,while less plants belonged to combination of PIb and PEb.Therefore,it is hypothesized that var.laticorymbus has originated by hybridization from two parents with nrDNA of the type PIa-PEa(Putative Parent A)and the type PIb-PEb(Putative Parent B),respectively,and its nrDNA concerted evolution declined towards Putative Parent A.Because there are no differences in nrDNA dimorphic sequences and morphology between hexaploid and tetraploid populations of the variety,it was suggested that fusion between reduced gametes(2x)and unreduced gametes(4x)of the tetraploid produced the hexaploid.In the ITS and ETS phylogenetic trees,PIa and PEa were included within one clade(Clade A),respectively,with some cloned sequences of the variety,while PIb and PEb were nested in another clade(Clade B),respectively,with the other cloned sequences,indicating that the two parents of the variety belongs to one of the clade,respectively.According to the evidences from DNA,cytology and morphologies,we suggested that female parent of the variety might be A.ageratoides var.lasiocladus and the male parent might be related closely to A.homochlamydeus4.Incomplete concerted evolution studies of Aster ageratoides var.laticorymbusThe nrDNA ITS and ETS sequences determined by PCR-direct and cloned sequencing showed that there were incomplete concerted evolution within all the populations of var.laticorymbus and recovered some new facts.First,there existed bidirectional inter-population concerted evolution.Most of the populations biased the Putative Parent A(PIa-PEa)in ITS and ETS while less populations were inclined to the Putative Parent B with gene type PIb-PEb.Secondly,bidirectional concerted evolution between ITS and ETS were found in the population of Tianmushan,of some cloned copies had a combination of the type PIa of ITS and the type PEb of ETS.Thirdly,there were bidirectional concerted evolution between ITS1 and ITS2 in some copies.In one hand,in some cloned copies ITS1 was the type PIa of the Putative Parent A whereas ITS2 belonged to the type PIb of the Putative Parent B.On the other hand,other cloned copies had a combination of ITS1 of the type PIb and ITS2 of the type PEa.These different bidirectional concerted evolution revealed the complexity of concerted evolution and might provide a new explanation for mechanism of incomplete concerted evolution.5.trnL-F and trnK sequences of cpDNA analysisBased on cpDNA trnK and trnL-F of two hundred and ninety plants from twenty-two populations of the variety and twelve plants of var.lasiocladus,a phylogeographic analysis was carried out.The results showed that the variety had eleven haplotypes,of which three main haplotypes,including the most primitive,were also found in var.lasiocladus.Therefore,var.lasiocladus is the maternal parent of the tetraploid in its origin.The same conclusion was arrived at after a comparison of trnL-F sequences of some taxa including the paternal parent.Combined with the studies of the cytogeography and incomplete concerted evolution,the phylogeographic analysis can arrived at conclusions as follows,(1)the hexaploid originated from the tetraploid;(2)the two cytotypes originated in the eastern Yunnan-Guizhou Plateau and the adjacent areas;(3)the modern geographic distribution pattern of the two cytotypes might be attributed to stronger competitive power of the hexapoid than the tetraploid.In the vast eastern region,only two isolated tetraploid populations(SZ and LYG)at the edge of the distribution area remain in the diffusion process from west to east.6.Taxonomical treatment and studies of a new related speciesThe variety is suggested to restore its specific status with the name Aster laticorymbus Vaniot.because of its interspecific hybrid origin.During the field investigations to search for the parent species of A.laticorymbus(A.ageratoides var.laticorymbus),a new species was found and nemed as Aster oliganthus W.P.Li&Z.Li.