Possible insertion of Mu DNA in the chloroplast of a variegated mutant

In the summer of 1982, a plant was found with green and yellow-green striping. This plant occurred in an outcross progeny in which the male parent was a Mu stock and the female parent was a Q60 standard line. The plant was reciprocally-crossed to a B70 standard. Fifty seeds of each of the outcrosses were planted in the field in 1983. By the time D.S.R. returned from the GSA meetings, there were in the female outcross progeny 17 dead or dying plants, one severely striped plant, 16 moderately striped plants, and 16 green plants with no stripes. The viable tissue that could be observed in the dying plants was yellow-green. It seems reasonable that the plants in the "dead or dying" class were wholly yellow-green or very heavily striped. The progeny from the reciprocal cross (i.e., using the striped plant as a male) consisted of only green plants. Thus, it seems that the yellow-green condition is due to the maternal inheritance of defective chloroplasts.

Chloroplast DNA was isolated from the variegated mutant (line 3366), as well as its maternal parent (standard Q60), Q67 and Q66 (the female and male inbred parents of the Q60 standard, respectively), and the B70 standard. The chloroplast DNA's were digested with several restriction endonucleases and blotted onto nitrocellulose using the Southern technique. The recombinant plasmid, pMJ9 (kindly provided by M. Freeling, Berkeley) containing the Mu sequence from the Adh1 locus of maize, was nick-translated and used in the hybridization reaction to probe for homologous Mu sequences in the chloroplast DNA's. Hybridization to line 3366 DNA was very intense, while there was slight hybridization to both Q60 and Q67 (the female parent of the Q60 standard), and no detectable hybridization to B70 or Q66. These data suggest that Mu DNA has inserted in the chloroplast genome in the mutant line 3366, resulting in the mutant phenotype. We are presently cloning chloroplast DNA from lines 3366 and Q67 for molecular characterization of the Mu-homologous sequence in the mutant line, and for comparison between the corresponding clones.

M. J. Skogen-Hagenson, Donald S. Robertson and David W. Morris

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