Preliminary data on Mu tagging of ij

--Chang-deok Han and Ed Coe

To explore the establishment of cell fate (position-dependent) and its maintenance (path-dependent) during initial leaf ontogenesis, we are studying striping mutants (ij, j1, j2 and sr2) and Isr. To be able to carry the studies to the molecular level, we have begun transposon tagging experiments on the striping genes with Mu for ij, j1, and j2 and with Ac for sr2. Based on genetic criteria, we have obtained tentative Mu-tagged ij and j2 mutants. Here we report preliminary genetic data on ij mutants derived from Robertson's Mu stocks.

We screened around 30,000 F1 seedlings from the cross of Mu stock (female) x ij-ref. Four independent striped seedlings, and one ear that was segregating for striped seedlings, were found. But 2 of the 4 seedlings, and the plants from the latter ear, had other than ij phenotype, displaying pale green stripes extending over whole leaves. Progeny tests from the ear revealed the following interesting genetic behavior: 1) there was no co-expression of pale stripes and ij among selfed progeny of the F1 pale striped plants; 2) when the F1 plants were crossed as female with normal plants (Ij Ij), pale striped seedlings were obtained, which indicates that the phenotype of pale green stripes may be maternally inherited; 3) when the F1 plants were used as male parent, all progeny showed normal, which eliminates the possibility of a dominant, male-transmissible factor for the pale striping phenotype. Further genetic tests are being undertaken to examine whether this mutant is a new allele of ij.

Two of the 4 plants selected as seedlings from the original screening showed ij-like striped patterns. Both plants grown in the greenhouse failed to set seeds, which frequently happens in the winter greenhouse. But we succeeded in propagating the plants as pollen parent by crosses to Ky21 and W23 inbreds. These backcross progeny were obtained in winter, 1986. Genotypes of the progeny will be ij-Mu/+ and ij-ref/+ if Ij is tagged by Mu.

Based on two genetic criteria, one of the two plants has been identified as a new ij mutant: 1) All the selfed progeny of the backcrosses, as expected, segregated for ij phenotype. 2) the backcross progeny showed 1:1 segregation of RFLP markers from the Mu and the ij-ref parents. By using RFLP markers, we have constructed a linkage map of ij. With one marker 6 m.u. proximal to the iojap locus as a probe, the backcross progeny showed 7 of the 14 plants with the RFLP alleles of the Mu and the backcross female parent, and 7 with the RFLPs of ij-ref and the backcross parent.

Genetic evidence that the new ij mutant is allelic to ij-ref comes from an allelism test. In summer 1988, normal plants (ij-Mu/+ or +/+) among selfed progeny from the backcross plants were selfed and crossed to heterozygous ij-ref tester plants of 6 different inbred backgrounds (Oh51a, Tr, Mo17, Ky21, K55, and Wg). Our preliminary sandbench progeny tests confirmed that the new ij gene from Mu background is allelic to ij-ref from all the inbreds.

By using an internal sequence (pA/B5) of Mu1 as a probe, we also examined whether the plants have a high copy number of Mu. All plants have 30-60 copies of Mu, while ij-ref and Ky21 inbred (female parent of the backcross) showed 2-3 hybridizing bands on the EcoRI-digested DNA blot.

A new j2 mutant from Robertson's Mu background is being subjected to similar genetic tests.

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