Ancient differentiation of the H and I haplomes in diploid Hordeum species based on 5S rDNA

Abstract

5S rDNA clones from 12 South American diploid Hordeum species containing the HH genome and 3 Eurasian diploid Hordeum species containing the II genome, including the cultivated barley Hordeum vulgare, were sequenced and their sequence diversity was analyzed. The 374 sequenced clones were assigned to "unit classes", which were further assigned to haplomes. Each haplome contained 2 unit classes. The naming of the unit classes reflected the haplomes, viz. both the long H1 and short I1 unit classes were identified with II genome diploids, and both the long H2 and long Y2 unit classes were recognized in South American HH genome diploids. Based upon an alignment of all sequences or alignments of representative sequences, we tested several evolutionary models, and then subjected the parameters of the models to a series of maximum likelihood (ML) analyses and various tests, including the molecular clock, and to a Bayesian evolutionary inference analysis using Markov chain Monte Carlo (MCMC). The best fitting model of nucleotide substitution was the HKY+G (Hasegawa, Kishino, Yano 1985 model with the Gamma distribution rates of nucleotide substitutions). Results from both ML and MCMC imply that the long H1 and short I unit classes found in the II genome diploids diverged from each other at the same rate as the long H2 and long Y2 unit classes found in the HH genome diploids. The divergence among the unit classes, estimated to be circa 7 million years, suggests that the genus Hordeum may be a paleopolyploid.Key words: 5S DNA gene, molecular clock, paleopolyploid.