The Bg rbg transposable element system was first detected when studying a case of somatic instability at the opaque2 (o2) locus. It was demonstrated that the instability encountered was due to the presence of a receptor element (rbg) at the o2 locus. The responsive recessive allele, called o2-m(r), could revert somatically to wild-type in the presence of a regulatory element termed Bg. One striking feature of the Bg rbg transposable element system, reported by genetic analyses, is its apparent widespread occurrence in natural maize populations. A search for the presence of Bg elements, by their ability to induce instability of the o2-m(r) allele, revealed active autonomous elements in maize populations from distinct geographical areas (Montanelli et al., Mol. Gen. Genet. 197:209, 1984).

We have assessed thirty-four accessions from Zea for the presence of Bg. Bg-like sequences, identified as hybridizing bands on Southern blots, were visualized in all Zea accessions and were present in approximately equal numbers in teosinte and maize. Analysis of 16 inbred lines with the EcoRI and HindIII restriction enzymes using three molecular probes, covering 75% of the Bg transposable element, revealed an average of 8.7 ± 2.1 Bg-like sequences per maize genome. Teosinte accessions and maize races, analyzed with EcoR1 and HindIII in combination with a centrally located Bg probe, disclosed an average of 12.5 ± 4.3 and 10.4 ± 3.3 homologous sequences, respectively. An average of 9.5 ± 3.1 Bg-like sequences were present among the 34 Zea accessions analyzed.

All Zea accessions displayed a restriction fragment characteristic for a full-length Bg element upon cleavage with BstEII. Southern analyses performed with methylation-sensitive enzymes demonstrated that these Bg-like sequences are methylated, although present in unmethylated DNA regions. Hence, it can be deduced that autonomous Bg elements are not uncommon among Zea accessions, while active elements are.

A comparison of the restriction patterns of related inbred lines revealed numerous common hybridizing fragments. An index of molecular similarity (MS), computed as the ratio of the number of bands common to two hybrids over the total band number for those two hybrids, was used to determine the degree of similarity between pairs of inbred lines.

MS values were computed for the LSC inbred lines H99, Lo924 (Mo17 x H992) and Mo17. Considering the pedigree of Lo924, a higher similarity of this line with H99 than with Mo17 was expected. However, a comparison of the hybridization patterns reveals a high MS value (95.9) for Lo924 and Mo17. On the contrary, Lo924 and H99 exhibit an MS value of 58.8, while H99 and Mo17 reveal an MS value of 54.0. Comparison of the restriction patterns obtained for inbred lines C103, Va17, and Va22 (Va17 x C1032), revealed an intermediate pattern for Va22. In fact, Va22 and Va17 exhibit an MS of 82.6, a value only slightly higher than that calculated for Va22 and C103 (73.3), while C103 and Va17 reveal an MS value of 58.2. These MS data are in good agreement with earlier findings, which indicate the prominence of Mo17 germplasm in Lo924 despite its pedigree and a low association between Va22 and C103 (Ajmone-Marsan et al., Euphytica 60:139, 1992). 

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