Abstract: The objective of this paper is to summarize the work in my group on FISH (fluorescent in situ hybridization) mapping of Ns-specific repetitive DNA sequences fromLeymus and discuss the results in the context of classification based on the genome system currently used among Triticeae researchers. The key question here is whether the genome composition of a tetraploid Leymus species should be NsXm or NsNs (Ns₁Ns₂). Different types of Leymus-specific dispersed retroelement-like repeats have been isolated and characterized. Because the sequences occur in significantly high copy number in Leymus, based on strong hybridization signal in Southern blots, they are considered essentially specific to Leymus. They are also abundant in Psathyrostachys, the progenitor of Ns genome in Leymus. These dispersed repeats are found to distribute over the whole of all Leymus chromosomes, without any differentiation between chromosomes that have been suggested to be of different genomic origins, meaning that all genomes in Leymus are the same. GISH (genomic in situ hybridization) experiments on Leymus chromosomes using Psathyrostachys genomic DNA as probes further support the NsNs (Ns₁Ns₂) genome constitution for Leymus. The Xm genome of an unknown origin might have been there in the beginning of the allopolyploidization process, but the Ns genome-specific elements must have spread predominantly and rapidly across genomes, thus homogenizing the nuclear genomes of Leymus. I present here for the first time evidence that Ns-specific dispersed repeats can spread in a very short time, from Leymus over to wheat in Triticum × Leymus hybrids growing in artificial conditions.

Key words: FISH, GISH, Leymus, Ns genome, Psathyrostachys.