PHILADELPHIA, PENNSYLVANIA

University of Pennsylvania

gl15 is a heterochronic mutation

--Matthew M. S. Evans and Scott Poethig

We have been searching for mutations that accelerate the transition from juvenile to adult growth by screening for mutations that have an abbreviated expression of juvenile traits, such as epicuticular wax, and precociously express adult traits, such as epidermal hairs. A mutation with this phenotype has recently been identified in a group of glossy mutations obtained from Dr. Sprague. We have confirmed his observation that this mutation is an allele of gl15 and have agreed to call this new allele gl15-2.

In families segregating for gl15-2, wild type plants have at least some wax on the first 5 or 6 leaves whereas their mutant siblings only possess epicuticular wax on the first 3 or 4 leaves. In both wild type and mutant plants, the disappearance of epicuticular wax is correlated with the appearance of epidermal hairs. Thus, mutant plants express epidermal hairs as early as leaf 3, in contrast to their wild type siblings, which do not produce epidermal hairs until leaf 5 or 6. The distribution of epicuticular wax and epidermal hairs on gl15-2 juvenile/adult transition leaves is similar to that on wild type transition leaves. Histological analysis demonstrates that gl15-2 also accelerates the change in epidermal cell shape that normally accompanies the transition from juvenile to adult growth. However, at least two other phase-specific traits -- root production and cuticle thickness -- do not seem to be affected by this mutation. In segregating families, wild type and gl15-2 plants are about the same size and have the same tassel morphology, ear position, and number of leaves. It is difficult to compare the expression of gl15-2 with the first allele of this locus (gl15-1) because these mutations are in different genetic backgrounds. Available stocks of gl15-1 have a phenotype similar to that of gl15-2 , but weaker.

The phenotype of these alleles of gl15 suggests that this gene regulates some aspect of phase change in maize. Because the character of transition leaves in mutant plants is similar to that of wild type transition leaves, we conclude that phase change occurs in the same manner in the mutant as the wild type, only earlier. gl15 maps to the long arm of chromosome 9 and is therefore not a recessive allele of Corngrass or the Teopod mutations. Because gl15 only affects a subset of the traits affected by these other heterochronic mutations, we believe that gl15 may act downstream of the Corngrass and Teopod genes.

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