Composition of the Genome Repeat Sequences of Psathyrostachys juncea and Its Distribution on the Chromosomes in Leymus Species

Psathyrostachys juncea was considered an important ancestral species of Leymus. The genomic Cot-1DNA library with highly repeat sequences of Psa. juncea was constructed, and sequenced and characterized, and the results showed that the library could be classified into six types: retrotransposons, transposons, satellite DNA, LZ-NBS-LRR, uncharacterized sequences, and retrotransposon LTR mixed LTR/Copia, and the proportions were 49.5%, 1.0%, 28.7%, 5.9%, 13.9% and 1.0% of the Cot-1 DNA, respectively. Furthermore, two satellite DNAs and five retrotransposon sequences were used as probes, and chromosomes in Psa. juncea, Leymus secalinus and L. racemosus were detected by florescence in situ hybridization（FISH）. The results showed that the hybridization signals of satellite DNA sequences TaiI-family and pSc250-family were mainly distributed at the ends of chromosomes, and the number of TaiI-family hybridization signals were 20, 16, 7, and 18 in Psa. juncea, L. secalinus, L. racemosus（PI 531811） and L. racemosus（PI 531812）, respectively, and the number of pSc250-family hybridization signals were 17 and 24 in Psa. juncea and L. secalinus, respectively, but no signal was detected in two L. racemosus samples. The distribution of retrotransposon sequences on the chromosomes of three detected species was basically dispersed, and the distribution of chromosome sequences in Leymus species showed a tendency of homogenity. Especially, clone sequences pPj-44 and pPj-28 showed significant differences in signal distribution between Psa. juncea and Leymus species, suggesting sequence expansion and contraction, respectively, during allopolyploidization, and clone sequence pPj-77 only differentiated the hybridization intensity of 10 chromosomes from the rest of the chromosomes in one L. racemosus（PI 531812）. The results suggested the repeats of Psa. juncea evolved rapidly and had homogenous diffusion during the polypoid formation of Leymus species and the repeat composition might be significant different among different Leymus species.