Genome Size Evolution In Allium(Amaryllidaceae) From Qinghai-Tibetan Plateau

Genome size is an important trait of plants and is highly variable among interspecies.However,it variant patterns and driving factors are important problems in evolutionary biology.In plant,genome size is affected by ecological factor except its own.Furthermore,genome size closely correlates with phenotype,karyotype,and seed traits.Allium is a large genus of monocotyledons.There are about 971 species in the world,mainly distributed in the northern hemisphere.At present,the genus has been studied systematically.It is a monophyletic group with clear phylogenetic relationship and geographical distribution.There are at least 111 species of this genus in the Qinghai Tibetan Plateau,and the species diversity is rich and highly differentiated in this area Habitat diversity resulting from the uplift of the Qinghai-Tibetan plateau has promoted severely differentiation of Allium L.;moreover,karyotypic traits and phylogenetic relationship in Allium are clear.Therefore,Allium is an ideal model to study genome size evolution in Qinghai-Tibetan Plateau.In this study,we first reconstruct a phylogeny for Allium in the Qinghai-Tibetan Plateau.In the phylogenetic framework,we then analyze genome size and its relationships to karyotypic traits,seed traits(seed size and size mass),and ecological factors(altitude,mean annual temperature,mean annual precipitation,and phenology)to insight into model driving genome size evolution and relationship of environmental adaptation,as well as provide a value model of genome size evolution in Qinghai-Tibetan Plateau.The results are as follows:(1)The genome size of Allium in Qinghai-Tibetan Plateau was strongly variant,about 21.37-fold(from 2.61 pg(A.rude)to 55.78 pg(A.cyathophorum var.farreri)).The genome size of the genus also varied greatly in different ploidy levels,and the diploid genome size varied about 7.59-fold(from 7.35 pg(A.chrysocephalum;2n =16)to 55.78 pg(A.cyathophorum var.farreri;2n = 16)),the mean genome size was18.08 pg;the variation of triploid genome size was about 4.28-fold(from 5.69 pg(A.korolkowii;2n = 24)to 24.34 pg(A.ovalifolium;2n = 24)),and the average genome size was 13.21 pg;The tetraploid genome size variation was about 4.03-fold(from4.20 pg(A.ramosum;2n = 28)to 16.91 pg(A.bidentatum;2n = 32)),the mean genome size was 9.62 pg;the variation of hexaploid genome size was about 2.54-fold(from 2.97 pg(A.fasciculatum;2n = 42)to 7.55 pg(A.polyrrhizum;2n = 54)),and the average genome size was 5.74 pg;octoploid has only one species(A.rude),with the smallest genome size of 2.61 pg.In addition,the genome size of Allium decreased with the increase of ploidy level.There was a significant positive correlation between genome size and chromosome length of Allium in Qinghai-Tibetan Plateau.Repeated sequences,heterochromatin and chromosome doubling fusion may be responsible for the variation of genome size in Allium from Qinghai-Tibetan Plateau.(2)Genomic size evolution model of Allium in the Qinghai-Tibetan Plateau was gradual evolution(? < 1),followed by adaptation radiation(? < 1).The results showed that the ancestral genome size of Allium was 18.781 pg.The phylogenetic tree based on ITS can be divided into three evolutionary branches: the genome size of clade I was mostly close to that of the ancestral genome size;clade II tended to expand,it may occur adaptation radiation;clade III tended to contraction,this branch including diploid,triploid,tetraploid,hexaploid,and octoploid,the loss of DNA during polyploidization may be the main reason of genome size contraction.(3)There was a significant positive correlation between the genome size and seed mass of Allium in the Qinghai-Tibetan Plateau,which might be regulated by transcription factors.The genus was positively correlated with altitude and had no correlation with 19 bioclimatic factors.The results of parameter ?(? = 0.0003)showed that genomic size of Allium in Qinghai-Tibetan Plateau was very weak affected by phylogenetic signals.Therefore,genomic size of Allium in the Qinghai-Tibetan Plateau may be very weak affected by ecological factors.We speculated that genome size variation Allium might be influenced by the selection pressure of the special habitat(low temperature,high UV,and rich phosphate content in soil)in the Qinghai-Tibetan Plateau.