Ribosomal DNA spacer-length polymorphisms in barley: mendelian inheritance, chromosomal location, and population dynamics.

Abstract

Spacer-length (sI) variation in rRNA gene clusters (rDNA) was surveyed in 502 individual barley plants, including samples from 50 accessions of cultivated barley, 25 accessions of its wild ancestor, and 5 generations of composite cross II (CCII), an experimental population of barley. In total, 17 rDNA sI phenotypes, made up of 15 different rDNA sI variants, were observed. The 15 rDNA sI variants comprise a complete ladder in which each variant differs in length from adjacent variants by .apprxeq. 115 nucleotide pairs. Studies of 4 rDNA sI variants in an F2 population showed that these variants are located at 2 unlinked loci, Rrn1 and Rrn2, each with 2 codominant alleles. Wheat-barley addition lines were used to determine that Rrn1 and Rrn2 are located on chromosomes 6 and 7, respectively. The nonrandom distribution of sI variants between loci suggests that genetic exchange occurs much less frequently between than within the 2 loci, which demonstrates that Rrn1 and Rrn2 are useful as new genetic markers. Frequencies of rDNA sI phenotypes and variants were monitored over 54 generations in CCII. A phenotype that was originally infrequent in CCII ultimately became predominant, whereas the originally most frequent phenotype decreased drastically in frequency, and all other phenotypes originally present disappeared from the population. The sI variants and/or associated loci are under selection in CCII.