Studies On Karyotypes And Genome Sizes Of Saxifraga In The Alpine Belt From Hengduan Mountains

Saxifraga Tourn.ex L.is the largest genus in the Saxifragaceae.Saxifraga has about 450 species and mainly distributed in alpine areas of Asia,Europe,North America,and South America(Andes).There are about 216 species of China.The Hengduan Mountains are the center of modern distribution and differentiation of Saxifraga,and there are 72 species in the alpine belt regions(Over 2700 meters above altitude)of the Hengduan Mountains.The number of chromosomes in plants varies regularly among genera,species,and even within species,so it is often used as an important basis for plant taxonomy.Therefore,the study of karyotype and chromosomal are most significant to the plant taxonomy,and it is also one of the important bases for discussing the evolutionary relationship between species and genera and the pattern of intraspecific variation.The cytology of Saxifraga has a long history abroad,and the species that have been studied for chromosome numbers account for 64.66% of global,while only 24.07% of Saxifraga has been reported in China.The research in the alpine belt region accounts for only 11.11% of the Hengduan Mountains,and their research needs to be further strengthened.Flow cytometry(FCM)is a fast and efficient high-throughput method for measuring nuclear DNA content and rapidly estimating DNA ploidy.FCM is of great significance to the study of cytological evolution and genetic relationships between species and genus.However,the use of FCM to study Saxifraga has only performed flow analysis on seven species in abroad,not even in China.In this paper summarizes the global cytology of Saxifraga and analyzes the evolutionary characteristics of karyotypes.The karyotypes of 7 species and 8 populations of Saxifraga were studied in the alpine region of Hengduan Mountains.There are 61 species studied by flow cytometric method in this paper.The results are as follows:1.The chromosome numbers and karyotypes of 6 species of Saxifraga were reported for the first time(five endemics to China).The results showed that the different populations of the same taxon had different chromosome base numbers.The karyotype formula from Balangshan population of S.nigroglandulifera is 2n = 2x = 16 = 12 m + 4sm;and the karyotype formula of Bowashan is 2n = 3x = 18 = 13 m + 5sm.The karyotype formula of S.stellariifolia is 2n = 2x = 14 = 12 m + 2sm.The karyotype formula of S.egregia var.eciliata is 2n = 2x = 16 = 12 m + 4sm.The karyotype formula of S.deqenensis is 2n = 2x = 16 = 16 m.The karyotype formula of S.nigroglandulosa is 2n = 6x = 42 = 39 m + 3sm.The karyotype formula of S.signata is 2n = 2x = 16 = 12m(2sat)+ 4sm.Our results have provided cytology datas for Saxifraga,and provided basic data for the systematic classification and ecological adaptation of Saxifraga.2.We collected the global chromosome numbers and chromosome base numbers and ploidy levels of existing Saxifraga and its related genera.The result showed that the chromosome numbers and chromosome base numbers and ploidy levels have rich diversity characteristics.The main display as the number of chromosomes for 2n = 10 to 208;chromosome base x = 5 to 52;At the proportion of chromosomal ploidy level,42.82% are diploidy,57.18% are polyploidy.It is presumed that the evolution of Saxifraga at the cell level is mainly reflected in the diversity of chromosome number,base number,aneuploidy,and ploidy level,which may be an important mechanism for the rapid differentiation and adaptation to extreme alpine environments of Saxifraga.3.The genome sizes and ploidy levels of 61 taxa of Saxifaga were reported here for the first time by FCM.The results showed that the genome sizes of the 61 species range from 232.32 Mbp to 20010.82 Mbp;ploidy included diploid,tetraploid,and triploid.Most of them are diploid as accounting for 90.16%.With the existing phylogenetic tree frameworks,it was found that the genome sizes of S.filicaulis and S.lychnitis were significantly different,indicating that the species were different each other;S.nigroglandulifera was closer to S.montana,farther away from S.pseudohirculus and S.lychnitis;S.umbellulata,S.wallichiana,S.melanocentra,S.stolonifera and S.finitima had similar genome sizes;indicating a close genetic relationship,and they were not closely related to S.hispidula and S.tangutica.As a whole,it was speculated that the genome sizes of Saxifraga might have evolved in an increasing direction.It was further speculated that genome size,ploidy,and altitude may be related.4.Based on the analysis of all studies,it was found that the polyploidy of Saxifraga was not evident in the chromosome evolution,which did not support the hypothesis of the "polyploid preadaptation mechanism",that is polyploids are more successful than diploids in alpine or extreme environments.At the same time,the theory of "accelerating polyploids during the glacial period" has not been supported by this study,which also provides cytological evidence speculation that there is no large ice sheet or less affected by the glacial period in the geological history in the alpine belt of Hengduan Mountains.