ISOLATION AND CHARACTERIZATION OF GENOME-SPECIFIC DNA-SEQUENCES IN TRITICEAE SPECIES

Two contrasting genome-specific DNA sequences were isolated from Aegil ops speltoides (wild goat grass) and Hordeum chilense (wild barley), e ach representing more than 1% of the genomes. These repetitive DNA fra gments were identified as being genome-specific before cloning by geno mic Southern hybridization (using total genomic DNA as a probe), and h ence extensive screening of clones was not required. For each fragment , up to six recombinant plasmid clones were screened and about half we re genome-specific. Clone pAesKB52 from Ae. speltoides was a 763 bp Ec oRI fragment, physically organized in simple tandem repeats and shown to localize to sub-telomeric chromosome regions of species with the Tr iticeae S-genome by in situ hybridization to chromosomes. The sequence data showed an internal duplication of some 280 bp, which presumably occurred before sequence amplification and dispersion, perhaps by uneq ual crossing-over or reciprocal translocation. In situ hybridization s howed that the sequence distribution varied between closely related (S -genome) species. Clone pHcKB6 was a 339 bp DraI fragment from H. chil ense, also tandemly repeated but more variable; loss of the DraI site resulting in a ladder pattern in Southern blots which had little backg round smear. In situ hybridization showed that the tandem repeats were present as small clusters dispersed along all chromosome arms except at a few discrete regions including the centromeres and telomeres. The clone hybridized essentially specifically to the H-genome of H. chile nse and hence was able to identify the origin of chromosomes in a H. c hilense x Secale africanum hybrid by in situ hybridization. It has a h igh A + T content (66%), small internal duplications, and a 50 bp dege nerate inverted repeat. We speculate that it has dispersed by retrotra nsposition in association with other sequences carrying coding domains . The organization and evolution of such sequences are important in un derstanding long-range genome organization and the types of change tha t can occur on evolutionary and plant breeding timescales. Genome-spec ific sequences are also useful as markers for alien chromatin in plant breeding.