CHANGE IN RIBOSOMAL DNA SPACER-LENGTH COMPOSITION IN MAIZE RECURRENT SELECTION POPULATIONS .2. ANALYSIS OF BS10, BS11, RBS10, AND RSSSC

Four maize (Zea mays L.) populations selected for grain yield (BS10, I owa Two-ear Synthetic; BS11, formerly Pioneer Two-ear Composite; RBS10 , Illinois strain of BS10; and RSSSC, Illinois strain of Iowa Stiff St alk Synthetic) were assayed for molecular variation in the ribosomal D NA (rDNA) intergenic spacer (IGS) at initial and advanced cycles of se lection. RSSSC and RBS10 underwent reciprocal recurrent selection with an inbred tester in a high-yield environment, whereas BS10 and BS11 w ere subjected to full-sib reciprocal recurrent selection. Maize rDNA, which encodes the ribosomal RNA genes, is highly repetitive and shows IGS length variation within and among individuals. Five different ribo somal spacer-length variants (rslvs) and a polymorphic SstI restrictio n site in the IGS were detected in the four populations. The five rslv s and the polymorphic restriction fragment were observed in 20 differe nt combinations or hybridization fragment patterns (HP). RSSSC, RBS10, and BS11 showed significant changes in the overall rslv and HP freque ncies between cycle 0 and the advanced cycle of selection, whereas BS1 0 did not. In general, two specific HPs were more frequent in the majo rity of the advanced cycles of the four populations. The frequency cha nges between initial and advanced cycles were more dramatic for HPs th an rslvs. These results are consistent with earlier findings and furth er support the hypothesis that certain rDNA HPs and/or linked loci may be responding to selection for grain yield and may be associated with a selective advantage in US Corn Belt environments.