Genetic Diversity And Molecular Phylogeograpliic Study Of Rhododendron Aureum Georgi (Ericaceae) Endemic To Changbai Mountain

Rhododendron aureum Georgi (syn. R. chrysanthum Pall.), a family ofEricaceae, is a perennial evergreen dwarf shrub inhabiting the alpine tundra and theBetula ermanii population ecotone of Changbai Mountain. It has become thedominant species of the tundra ecosystem in this area. Based on the rich geographicvariation from the low to high elevations on Changbai mountain, we discussed theinfluence of the altitudinal gradient (environmental stress) on the spatial distributionof genetic variations and the genetic structure of this species using ISSR and RAPDmarkers. Based on the special history of volcanic eruption, we discussed theinfluence of volcanic events on the evolutionary processes and geographicaldistribution of R. aureum located on the north slope using the genealogicalinformation obtained from cpDNA trnL-trnF sequences.The main results are listed as following:1. We used ISSR and RAPD markers to describe the diversity and geneticstructure within and among four natural populations located at different altitudes onnorth slope of Changbai Mountain. DNA from66individuals was amplified with tenISSR markers and seven RAPD markers. Ten ISSR primers generated183bandswith87.43%(160) polymorphic fragments. Seven RAPD primers produced124bands. Among the total number of bands,95.16%(118) were polymorphicfragments. Shannon’s information index (I) was0.4593in ISSR and0.4794inRAPD. High genetic diversity was observed by these two techniques at the specieslevel. The genetic diversity of populations increased with altitudinal gradients fromlow to high. The coefficient of gene differentiation (GST,0.3652in ISSR and0.2511 in RAPD) and AMOVA analysis revealed that most genetic diversity wasdistributed within populations (61.96%in ISSR and70.23%in RAPD). The estimateof gene flow based on GSTwas0.8690in ISSR and1.4910in RAPD. The UPGMAclustering results using ISSR and RAPD showed that all individuals from the samealtitude were gathered together, and the two populations (TYD2a and YHLa) frommiddle altitudes always clustered together. Compared with populations fromdifferent altitudes, similar genetic diversity and low genetic differentiation wereobtained from populations at the same altitudes, as revealed by ISSR markers. Forinstance, two indexes of I and PPB indicated that the two populations at the2,300maltitude had similar genetic variation (0.3619and68.31%in TYD2a;0.3642and69.40%in TYD2b). The two populations (TYD2a and TYD2b) at the same altitudeexhibited a higher Nm (3.4595) and a lower differentiation (GST,0.1263). Thegenetic differentiation of populations increased with altitudinal gradients from lowto high (GST,0.0814,0.1263and0.1771). In addition to the reproductive strategy ofR. aureum, these data highlight that local environmental conditions may play animportant role in shaping the diversity and genetic structure of this species. Thegeneral increase in genetic diversity coincided with environmental stress. Themaintenance of genetic variation subject to environmental stress is of primeimportance in maintaining genetic diversity in natural populations because differentgenotypes display varying fitness under different environmental stresses. Highgenetic diversity can serve as the internal driving force allowing R. aureum toimprove its ecological tolerance and facilitate its adaptation along altitudinalgradients.2. We examined135individuals from10populations of R. aureum via thesequencing of the trnL-trnF region of chloroplast DNA (cpDNA). A total of909base pairs (bp), including9polymorphic sites, were sequenced, and5haplotypeswere identified. The ancestral haplotype A was the most common haplotype and wasshared in all populations (96%of R. aureum share the same haplotype A). Privatehaplotypes B and C were fixed in populations from high altitudes. Haplotype D were shared in populations from low and middle altitudes. Haplotype E was occurred onlyin population from low altitude. The nucleotide diversity and the haplotype diversityof cpDNA were generally low at the species level. The population differentiationwas relatively low in this species (FST=0.05206, P <0.001), as revealed by ananalysis of molecular variance (AMOVA). The results of the construction of aneighbor-joining tree showed that5haplotypes did not form lineages that weredistinct from each other due to the short evolutionary history related to the eruptions.The haplotype diversity was slightly higher in R. aureum occurring at low altitudesthan at the middle and high altitudes. A low-altitude area could represent the originalcolonization site of R. aureum following a volcanic eruption on the north slope. Oneprobable demographic historical scenario is that R. aureum might undergone recentpopulation expansion from low altitudes to high altitudes following bottleneckevents influenced by volcanic activity of Changbai Mountain. This conclusion wassupported by the combined findings of low haplotype diversity, low nucleotidediversity, significant Tajima’s D and Fu and Li’s D*values, and a star-likephylogeny of haplotypes.