The recessive intensifier of plant color (a3)

The original description of a3 in 1934 by E. W. Lindstrom (MNL 8:10) identifies "a new recessive anthocyan gene" with data indicating 22% recombination with golden and 40% with R. Further description apparently is not in the literature, but the a3 stock supplied from the Stock Center is described as a recessive intense plant color. C. R. Burnham and R. V. Kowles (MNL 43:113) found no satisfactory evidence of linkage of a3 with R, sr2, or g. The recessive factor is uncovered by TB-3La (MNL 47:147). The data below indicate that a3 is a recessive gene whose expression requires an intermediate-level (barred) B allele. The barred allele in these experiments is B-a3, one of the low-intensity alleles at this locus (MNL 51:61); since this allele has been extracted from the a3 stocks, by intercrossing and selfing, as a homozygous strain that is only barred (i.e., not intense), the intense a3 expression is controlled by an additional gene. Presumably the additional requirement is the recessive factor on 3L which, when hemizygous in the presence of B-a3, intensifies the plant color. F2 progenies from B-a3/b, a3/+ (F1 plants were barred):
 
R constitution, seed color Barred Intense* Green
R-g/r-r, colored class 50 25 22
B-g/r-r, colorless 71 25 20
R-g/R-g 55 16 12
R-g/r-g, colored class 40 8 18
R-g/r-g, colorless 42 13 15
Total 258 87 87
Expected for 9:3:4 243 81 108

c2 = 5.45 ns
F2 progenies from B-a3/B, a3/+ (F1 plants were purple):
 
R constitution, seed color Purple Intense* Barred
r-r/r-r 71 8 4
R-g/R-g 61 4 18
R-g/r-g, colored class 27 2 7
R-g/r-g, colorless 28 2 7
Total 187 16 36
Expected for 12:1:3 179.2 14.9 44.8

c2 = 2.15 ns
*Purple plants with green auricle, somewhat less darkly pigmented than B Pl, with faintly pigmented cobs.

Intense individuals in progenies segregating for green and intense (as in the first of the above F2 progenies) have always given barred progeny from crosses with A3 b, demonstrating that intense plants must be a3 B-a3 (i.e., never a3 b). Similarly r-r, R-g and (in a few tests) R-r do not appear to duplicate the effect of B-a3, since intense individuals in segregating progenies have always given barred progeny in the same test cross (i.e., they were never a3 b r-r, for example).

The above observations demonstrate that a3 is a recessive intensifier of the plant color conferred by B-a3 (and presumably other B alleles). Chromosome 3 linkage tests are in preparation.

E. H. Coe, Jr.


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