Progress on Mu tagging of ij --Chang-deok Han and Edward H. Coe, Jr. In the last MNL (63:65), we reported one ij mutant from Robertson's Mu background, which met the following genetic criteria: first, all the selfed progeny of the backcrosses of the ij-Mu plant to inbred Ky21 segregated for ij phenotype; secondly, the backcross progeny showed 1:1 segregation of ij-linked RFLP markers from the Mu and ij-ref parents; lastly, allelism tests confirmed that the new ij gene from Mu background is allelic to ij-ref.

To find a Mu element that cosegregates with the ij-Mu plants, we performed Southern blot hybridizations with an internal sequence (pA/B5; from Dr. Loverine Taylor) of Mu1. 47 ij-Mu and 19 normal plants from the selfed progeny after 2 consecutive backcrosses to inbred lines Ky21 and Mo17 were examined. In a preliminary experiment, DNA from each of 14 ij and 10 normal seedlings was digested with each of 5 different restriction enzymes that do not cut Mu1, EcoRI, HindIII, XbaI, SacI, and SstI (an isoschizomer of SacI). EcoRI and HindIII generate 2 bands unique to ij seedlings. One Mu-hybridizing band cosegregated with the ij seedlings on Southern blots of SstI, SacI, or XbaI-digested genomic DNA. Using SstI or SacI, an additional 33 ij and 9 normal seedlings were examined. All 47 ij-Mu plants examined with the SstI DNA blots contained the unique Mu fragment that was absent in all 19 normal plants (see Figure). Our data suggest that the DNA containing a Mu transposon might be located at or near the ij locus. The verification whether the Mu resides in the ij locus needs further characterization of the following materials: 1) genetic and molecular studies on another ij-Mu plant from the second screening, 2) molecular studies on germinal revertants of the ij-ref allele (Coe et al., 1988; see Coe and Han, MNL, this issue).

Figure. Southern analysis of the ij-Mu*1 plants with pA/B5 (internal sequence of Mu1). Around 6 ug of total cellular DNA from individual seedlings were digested with SstI enzyme that does not recognize the DNA sequence of the Mu1 transposon. The blot was probed with an internal DNA sequence of Mu1. A unique Mu-hybridizing band that is present only in ij-Mu*1 plants is marked with an arrow.
 

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