Mutability of ij-ref --Ed Coe and Chang-deok Han We have found high mutability of the reference allele of ij when particular background constitutions are brought together. The mutability is much higher (i.e., more frequent) than the occasional green sectors on iojap plants reported previously (Coe et al., Amer. J. Bot. 75:634, 1988). Some explanation of the requirements for recognizing mutability, and some prior observations pertinent to the new observations, may be desirable. Because the common iojap phenotype in many backgrounds includes white striping on the green leaf field, green sectors can be recognized only on plants that display a uniform pale field of expression, namely the pale leaf color termed "grainy" that is typical of iojap plants in the background of inbreds Tr and Ky21 (see Coe et al., 1988). In Tr background (but not Ky21), clonal green sectors are seen on 10% or more of plants, typically only one sector on a plant. In two cases in Tr, in fact, a green sector entered the tassel and back-mutations of ij to + have been recovered. In Ky21, where green sectors would be recognized, none have been seen. In another background in which green sectors might have been recognizable, Oh51a, most seedlings are pure white or nearly so and do not usually survive; if green sectors occur on Oh51a iojap seedlings, they are infrequent.

The high mutability appeared in experiments intended to study the inheritance of the background-dependent expression of iojap. Several backgrounds carrying iojap (+/ij x +/ij) were intercrossed and the F1 plants were backcrossed with parent stocks carrying ij (male parents either +/ij or ij ij). Surprisingly, among the iojap plants from backcrosses of Oh51a/Tr x Oh51a most of the segregating "grainy" plants showed high mutability. In leaves at the flowering stage hundreds of small sectors (1-10 cm long and 0.5-4 mm wide), were evident. Close study of F1 iojap plants of Oh51a/Tr and its reciprocal, Tr/Oh51a, also revealed mutability at a lower frequency. When the backcross had been made with Tr instead, low mutability was recognizable but confident classification was not possible.

We assume that in the cross and backcross there has been an activation of a mobile element, inasmuch as neither inbred background itself shows mutability. It is reasonable, in the absence of germinal products, to assume that the sectors are back mutations at the locus like those in Tr background, from which germinal reversions have been isolated. Tests of iojap plants for activation of receptors of Ac and Spm, carried out in 1980, were negative. Tests of these and other systems in the "activated" material have not yet been done.

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