Phylogeny of two tetraploid Hordeum species, H. secalinum and H. capense inferred from physical mapping of 5S and 18S-25S rDNA

We previously showed that H. secalinum and H. capense are allotetraploids carrying the Xa genome of H. marinum and the I genome of an unidentified diploid species. In this study, intraspecific variation in each tetraploid species was investigated with regard to intergenomic translocations and chromosomal distribution of rDNA sites. Genomic in situ hybridization revealed that three H. secalinum accessions examined did not carry intergenomic translocations, but that two of three H. capsense accessions analyzed carried a pair of intergenomic translocations. Multicolour fluorescence in situ hybridization showed that H. secalinum? included two types of rDNA pattern differing in the presence or absence of an extra 5S rDNA site in a submetacentric chromosome pair of the Xa-genome origin. The extra 5S rDNA site was found in all H. capense accessions examined. This 5S rDNA site is characteristic of H. marinum ssp. gussoneanum, but is absent in ssp. marinum. Polymorphisms in the 5S rDNA site infer that H. secalinum included two types, one having ssp. gussoneanum 2x and the other having ssp. marinum, as the Xa-genome donor. We conclude that H. capense originated from a limited number of H. secalinum accessions introduced probably through migrating birds to South Africa.