A silky-tassel (sit) mutant
--Ru-Ying Chang and Peter A. Peterson

In a line of the genotype, T2-3e~a-m(papu)//N a sh2 (translocation breakpoint linked to a-m(papu), a plant, designated 922036-1, showed a heavily silky tassel in the summer of 1992. The tassel was covered by very long, almost evenly distributed, silks. Pollen amount was reduced substantially and the pollen shed very poorly. The selfed top ear did not yield any seed set and thus only the outcrossed second ear was obtained. This mature ear was also covered by long silks which were difficult to remove. Normal kernels were interspersed with almost empty kernels (may be partly due to the translocation involved).

The F1 from this cross showed both normal tassels and normal ears, indicating that silky tassel is a recessive trait. F2 progeny were planted in 1993 summer. The segregation for silky tassels is listed in Table 1.

Table 1. Segregation ratios for sit mutants in the F2 generation of sit x normal or selfed progeny of a mutant plant (last row).
Entry Silky-tasseled Normal
1993 F2 13 111
1993g F2  3 15
1994 F2  4 13
1994 mutant selfed  3  8

The silky phenotype expressed in the F2 was less severe and showed a wide array of severity. The five light silky tassels (Table 1) showed only a few long silks. The segregation did show a clear-cut result. Deviation from 3:1 was highly significant (chi2 = 13.936), and that from 15:1 was approaching significance (chi2 = 3.794). Segregation in the 93 greenhouse (93g) and 94 summer approached 3:1 closely (15:3 for 93g and 11:4 for 94). However, selfed seed of plants with medium silky tassels (severely silky plants could not be successfully selfed) yielded less than 1/2 offspring with silky tassels.

The above observations show that the silky tassel phenotype is probably controlled by a single recessive gene whose expression is influenced by other factors such as environment, etc. Tests have shown that this phenotype is independent of either the a-m(papu) mutable allele (En) or the translocation feature (T2-3e) and is not dependent on the direction of the cross.

Three silky-ear mutants (si) were reported by Fraser in the 1930's (Fraser, J. Hered. 24:41-46, 1933). Ears of our silky tassel (sit) mutants showed a greater degree of severity than si1 reported by Fraser. sit ears closely resemble his si2 and si3 mutants, though are less severe. One distinct characteristic is that our sit mutant showed severe silky phenotype on tassels, which was reported to be a minor characteristic for Fraser's mutants. Allelism tests and mapping experiments have not been executed. 

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