The presence of sequences homologous to the plasmid-like mitochondrial DNAs (mtDNAs) in the nuclear genome of maize (R.J. Kemble et al., Nature 304:744-747, 1983) has been examined further. In various cytoplasm and restorer combinations within hybrid background M825/OhO7, seven BamHI nuclear DNA (nDNA) restriction fragments (designated A-G) consistently exhibit homology at stringent hybridization conditions to pZmS21 (R.J. Kemble et al., 1983; and unpublished observations). PZmS21 is a clone containing a 4.0 kb S1 insert (R.D. Thompson et al., Nucl. Acids Res. 8:1999-2008, 1980), including part of the region of homology with S2 and psbA chloroplast gene homology (P. Bedinger et al., personal communication).
While all the S1 homologous BamHI nDNA fragments hybridize with pZmS21, pZmS42 hybridizes to all fragments except fragment D. PZmS42 is a clone containing a 1.5 kb insert of "unique" S1 DNA, i.e. neither containing homology with S2 nor with the psbA gene. BamHI restriction digests of chloroplast DNA (ctDNA) exhibit homology to clone pZmS21 in only one fragment, which is identical in size (4.7 kb) to the M825/OhO7 BamHI fragment D. Thus fragment D most likely represents contaminating ctDNA (i.e. the psbA gene; observation initially presented by P. Bedinger, 1984 Maize Genetics Conference). Alternatively, fragment D in BamHI nDNA digests may be composed both of psbA gene sequences residing in the nuclear genome and ctDNA contamination. Clones of fragment D are presently being screened. Examination of a number of additional maize lines indicates that there is variability in distribution of S1 homologous nDNA sequences between lines. Some lines that contain all of the fragments A-G identified in M825/OhO7 differ in their relative amounts of the bands. For example, compared to M825/OhO7, inbred WF9 contains lower levels of band C while band B is diminished in B37. Other lines contain S1 homologous fragments which have slightly different mobilities than fragments A, B and C. This variability could have resulted from differential sequence rearrangements among lines descended from an original S1 insertion event into nDNA. Furthermore, other lines, such as Tr, do not possess A, B or C fragments.
When two such lines (e.g. WF9 and M825) that differed in S1 nDNA sequence arrangement were crossed, the resulting hybrids contained S1 nDNA fragments from both parents. This demonstrates a stability in inheritance of the S1 nuclear sequences. Examination of S1 nDNAs following several generations of reciprocal crosses will be necessary to fully ascertain stability. In collaboration with D.B. Walden and C.A. Rees at the University of Western Ontario in London, we are using r-x1 generated monosomics for each of the ten maize chromosomes to physically map the distribution of the S1 homologous sequences within the nuclear genome of selected lines.
C.L. Baszczynski, J.E. Carlson and R.J. Kemble
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