| Cotton is the world’s most important natural fiber crop.It is also a model system for studying the evolution of polyploidy.Because of the large number(2n=52)and small morphology of chromosomes,there is no obvious phenotypic difference in tetraploid cotton.Therefore,it is difficult to carry out the identification of chromosome and the analysis of karyotype.In this study,we isolated a complete set of bacterial artificial chromosome clones anchored to each of the 26 chromosome arms of the tetraploid cotton-Gossypium hirsutum.Combining these with telomere and centromere markers BAC 97G20,we constructed a standard karyotype for the G.hirsutum inbred line TM-1,and do the analyze of length variation about chromosomes:the longest chromosome is AT10 and the length is 3.51 m(131 Mb)accounting for the ratio of 5.40%;the shortest chromosome is DT04 and the length is 1.59 m(59.4 Mb)accounting for the ratio of 2.45%.Except the AT04 is shorter than its homologous chromosomes,the other AT chromosomes are longer than their homologous chromosomes.At the same time,BACs was used as probe for the karyotype analysis of Gossypium in different species.The results showed that the markers were suitable for the karyotype analysis of Gossypium.These results have the detailed analysis on the level of chromosomes and cotton genome structure.It expands the understanding of cotton genome structure,and provides a powerful tool for the study of plant cytogenetics.Centromeric chromatin in most eukaryotes is composed of highly repetitive centromeric retrotransposons and satellite repeats that are highly variable even among closely related species.The evolutionary mechanisms that underlie the rapid evolution of centromeric repeats remain unknown.Based on this,we used ChIP to get the centromeric repeats of G.raimondii which is the ancestral species of the tetraploid upland cotton and do related research.We use ChIP to explore the centromeric repeats and do the further analysis of ten rich sequences.Sequence analysis showed that the G.raimondii centromeric repeats originated from retrotransposon-related sequences.Comparative analysis showed that nine of the ten analyzed centromeric repeats were exist in the G.raimondii and G.hirsutum but absent from the centromeres in the A-genome and related diploid species(B-,F-and G-genomes),indicating that these sequences originated from D-genome and retained after forming the tetraploid.Notably,six repeats were present in both the A-and D-subgenomes in tetraploid G.hirsutum,and increased in abundance in both subgenomes.This finding suggests that centromeric repeats may spread and proliferate between genomes subsequent to polyploidization.Two repeats,Gr334 and Gr359 occurred in both the centromeres and nucleolar organizer regions(NORs)in D-and AD-genome species,yet localized to just the NORs in A-,B-,F-,and G-genome species.It hints that some of the features in NOR regions may be similar to that of centromeres,attracting the insertion of centromere specific transposons.In addition,we also map the centromere of G.raimondii and the location of centromere in genome.These results provide important clues for the study of formation and evolution of centromeric sequences and greatly promotes the development of cotton genome research.At the same time,it also expands the understanding of the formation mechanism about centromere specific sequence in plants. |
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