Muelements as molecular markers for genomic stability in culture --Martha James and Joan Stadler Somaclonal variation in plants regenerated from tissue culture is known to stem, in part, from chromosome instability. Reports have suggested that this instability, which can result in breakage and/or rearrangements, occurs in late-replicating DNA (heterochromatin) (Lee and Phillips, Annu. Rev. Plant Physiol. Plant Mol. Biol. 39:413-437, 1988). Little is known, however, about the effects of tissue culture on euchromatic, or active, regions of the genome.

Mu elements are present in approximately 10-70 copies in Mutator lines of maize. They have been shown to reside primarily in active regions of the genome and to exhibit a gene-specific insertion preference (Alleman and Freeling, Genetics 112:107-119, 1986; Bennetzen et al., UCLA Symp. Mol. Biol. 62:183-204, 1987). Thus, Mu elements may serve as useful molecular markers for those active portions of the genome in which they are found. We have used the internal MluI fragment of the Mu1 element as a hybridization probe in Southern analyses of DNA from Mutator embryogenic callus tissues. The six callus lines examined were derived either from the F1's of H99/Mu32 crosses or the self-pollinations of a bz-Mum8 plant. Each of these lines contained approximately 20-40 Mu elements that were modified, as determined by the failure of the restriction enzyme HinfI to cleave the DNA at sites located within Mu-element inverted repeats. The hybridization profiles of each line obtained by restriction digestion with various enzymes that cut outside Mu1 and hybridization with the Mu1 probe revealed restriction fragment uniformity and stability over approximately one year in culture (10-60 weeks). This stability suggests that gross rearrangements of regions of the genome occupied by Mu elements did not occur in culture. Because Mu elements are associated with active regions of DNA, stability of the active, or genic, portion of the genome during culture is implied.

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