University of California

Kn2 and Rs*-1025 are putative alleles of Rs1-O

--Philip Becraft and Michael Freeling

Rough sheath 1-O (Rs1-O ) is a dominant mutation of spontaneous origin, which causes disruption of the ligular region, as well as reducing plant stature and causing the plant to bend over (Khadzhinov, Bull. Appl. Bot. Genet. Plant Breeding. Series 2: 247, 1937). Rs1-O leaves contain convoluted, sheath-like tissue around the ligular region and blade base, which appears to result from continued growth of this tissue beyond normal. The ligule can be discontinuous, displaced or missing. The vascular anastomosis which normally occurs in the ligular region as the basipetal veins enter the sheath is greatly disrupted, with veins often crossing each other several times before finally joining. In a B73 background, the Rs1-O phenotype is usually apparent from about leaf 9 on.

Knotted 2 (Kn2 ) and Rough sheath-1025 (Rs*-1025 ) are also dominant mutations which cause phenotypes similar to Rs1-O. Kn2 arose spontaneously in a commercial hybrid, and was originally designated K-4 (Zuber, MNL 49:135, 1975). Kn2 is not linked to Kn1 (Freeling and Hake, Genetics 111:617-637,1985). Rs*-1025 arose in a Robertson's Mutator family. The Rs*-1025 phenotype is very similar to Rs1-O, although it tends to be less severe, and is more prone to genetic background effects. Kn2 affects the ligular region much like Rs1-O, but the phenotype tends to affect the lower portion of the plant, with normal leaves appearing after about leaf 14, in a B73 background.

To test for linkage, Kn2 and Rs*-1025 were crossed to Rs1-O, and the double mutants were outcrossed to a wild-type tester. From the Kn2;Rs1-O testcross, 23/23 plants displayed a mutant phenotype. In the Rs*-1025;Rs1-O testcross, 15/15 progeny were mutant. This demonstrates that Kn2 and Rs*1025 are both linked to Rs1-O. The genetic linkage and similarity in phenotypes suggest that these mutations are allelic.

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

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