Variation in the ribosomal DNA intergenic spacer of a maize population mass-selected for high grain yield.

Variation in the intergenic spacer of ribosomal DNA (rDNA) was detected among individual plants of the open-pollinated maize variety 'Hays Golden' and populations derived from this variety. rDNA intergenic spacer-length variants were detected at approximately 200 bp intervals, consistent with the number of 200 bp subrepeats as the basis for this variation. Inheritance data revealed that more than one spacer-length class may be present on an individual chromosome. Fourteen different predominant rDNA intergenic spacer hybridization fragment patterns were detected. C-29, a population developed by 29 cycles of mass-selecting Hay Golden for high grain yield, exhibited a significant change in rDNA intergenic spacer hybridization fragment pattern composition in comparison to Hays Golden. This change included a reduction in frequency of the shortest predominant space-length variant (3.4 kb) and an increase in a 5.2 -kb hybridization fragment. I-31, a population developed through thermal neutron irradiation of Hays Golden and 31 generations of mass selection for high grain yield, did not exhibit a significant change in overall rDNA intergenic spacer composition. I-31 did exhibit an increase in frequency of the 5.2-kb hybridization fragment and a significant change in two specific hybridization fragment patterns that had also changed in C-29. These data, particularly for the C-29 population, suggest that rDNA intergenic spacer-length variants and/or associated loci were influenced by selection.