Abstract:

Fluorescent in situ hybridization (FISH) was used to investigate the chromosomal location of 18S-5.8S-26S rDNA loci in Thinopyrum intermedium (Host) Barkworth et Dewey (2n=6x=42). In all accessions and individuals studied, 3 or 4 pairs of major loci were detected. Subsequent genomic in situ hybridization (GISH) analyses revealed that one pair was located on the ends of the short arms of one pair of homologous chromosomes of the St genome, while the other 2 or 3 pairs of major loci were located in the E genomes (including the E^e and E^b). It is suggested that 2 to 3 pairs of major loci were probably lost during the evolution of this hexaploid species. The variation in rDNA positions and copy numbers between the diploid donors and Th. intermedium, as well as the diversity among the accessions of Th. intermedium confirmed that the rDNA gene family conveyed the characters of DNA mobile elements. The internal transcribed spacer (ITS) regions of the rDNA in Th. intermedium were also investigated. Sequence data of seven positive clones from one individual suggested high degree of individual heterogeneity exists among ITS repeats. Phylogenetic analyses showed that there were two distinct types of ITS sequences in Th. intermedium, one with homology to that of Pseudoroegneria species (St genome) and the other to that of the E genome diploid species. This showed that the ITS paralogues in Th. intermedium have not been uniformly homogenized by concerted evolution. The limitation of using the chromosomal location of rDNA loci for phylogenetic analysis is discussed.

Key words: Thinopyrum intermedium , 18S-5.8S-26S rDNA, internal transcribed spacer (ITS), concerted evolution, fluorescent in situ hybridization (FISH), genomic in situ hybridization (GISH)