Functional Centromere Sequence Composition And Evolutionary Analysis Of Brassica Genus

The centromere is a special component of eukaryotic chromosomes,which provides the assembly site for the kinetochore protein complex during cell division,and plays a crucial role in the maintenance and accurate pairing and separating of chromosomes.Comparing with the conservative function of centromere,its DNA sequence evolves rapidly in different species.Brassica genus contains three basic diploid species and their allotetraploid species,which is an ideal material for the study of the evolution of polyploids.Therefore,the study of Brassica genus centromeric sequence is helpful to deepen our understanding of the evolution of polyploid,and can provide valuable reference for the genome assembly of Brassica genus.This study uses the centromeric special histone(CENH3)antibodies in Brassica genus to recognize functional centromere regions,And then the Ch IP assays were carried followed by NGS technology(Ch IP-seq)to acquire functional centromeric DNA sequences of six species of Brassica genus,then by using fluorescence in situ hybridization(FISH)technique to identify the specific centromeric sequences of these six Brassica species.the following results are obtained:(1)The proportion of repeat sequences in the genome of six species of Brassica genus was evaluated,and the proportion of repeat sequencesin the genomes of Brassica genus was as high as over 89%.(2)centromere-specific repeats were obtained from six Brassica species.Among them,Chinese cabbage(B.rapa,AA,2n = 20)has 4centromere-specific repeats;black mustard(B.nigra,BB,2n = 16)has 13centromere-specific repeats;cabbage(B.oleracea,CC,2n = 18)has 8centromere-specific repeats;mustard rape(B.juncea,AABB,2n = 36)has13 centromere-specific repeats;Brassica napus(B.napus,AACC,2n = 38)has 8 centromere-specific repeats;and Ethiopian mustard(B.carinata,BBCC,2n = 34)has 14 centromere-specific repeats.(3)Analysis and comparison of six centromere-specific repeats revealed that most of the centromeric sequences of Chinese cabbage were similar to the centromeric sequences of cabbage,while black mustard had only 2 sequences similar to Chinese cabbage or cabbage.Among the three tetraploid centromeric sequences,only one centromeric repeat in Ethiopian mustard was obtained after tetraploid,and the rest other centromeric repeats derived from the corresponding diploid ancestor species.In addition,all the other five species had relatively conservative tandem repeats except the black mustard.Moreover,LTR/Copia has a great advantage in the retrotransposons of the centromeric region of Brassica genus.This study provides a comprehensive understanding of the sequence composition and evolution of Brassica functional centromere,and at thesame time,it provides an insight for the variation of centromeric sequence in the process of polyploidization.In addition,this study also provides a reference to fill the gap of centromeric region of Brassica genome.