Comparison of the dosage effects and dominance of 10 C1 alleles

--Elizabeth E. O. Caldwell and Peter A. Peterson

Ten alleles of the C1 locus (Table 1) were assembled to represent the diversity of expressivity of this anthocyanin pathway gene. The C-I alleles (dominant inhibitors) produce a colorless kernel when homozygous and suppress the pigment expression of the C alleles. The C alleles produce colored kernels both when homozygous and in combination with the recessive colorless alleles. These recessive colorless alleles can be subdivided into mutable and stable classes. The mutable (c-ruq) alleles were created by the insertions of the transposable element Ruq into a C allele. A spotted phenotype is expressed in the presence of either Uq or Ac and is dominant over the stable recessive c allele. In the absence of Uq and Ac, the c-ruq alleles are colorless.

Table 1. Ten alleles of the C1 locus are listed with their source, type of allele and homozygous phenotype.
Allele Source Allele Type Phenotype
C-I(Cornell)   dominant color inhibitor colorless
C-I(K55) inbred K55 same colorless
C-I(A69) inbred A69 same colorless
C-S B. McClintock* dominant color producer colored
C-(hiloss) Hi Loss B line** same colored
C-(Cshbz) C sh bz wx line same colored
C-(lineC) color converted W22 same colored
c-ruq67 c-m816667 mutable (+Uq), colorless (no Uq) spotted, colorless
c-ruq3  c-m804531 same spotted, colorless
c c sh Bz wx line recessive colorless colorless
* via K. Cone, K

**from M. M. Rhoades, MM

To further describe the expressivity of these alleles, the 10 were intermated (Crosses 1 and 2) to establish their relative dominance and dosage effects. This evaluation of the alleles' relative potency was made using the original lines. All were homozygous dominant for A1, A2, C2, and R, but no attempt to control allelic variation at loci other than C1 was made.

              Female         Male         Embryo       Aleurone

Cross 1   colorless      colored      colorless     colorless
               colorless X    colored       colored       colorless

Cross 2   colored        colorless     colorless      colorless
                colored X     colorless      colored         colored

Crosses 1 and 2 reveal differences among both the dominant color inhibitors and the dominant color producing alleles. The dosage was controlled by the mating scheme. When C-S is present in one dose in the aleurone (Cross 1), only the suppression by C-I(Cornell) and C-I(K55) is complete. C-I(A69) allows limited pigment formation (light pale). When C-S is present in two doses (Cross 2), C-I(Cornell), and C-I(K55) only partially suppress C-S expression (dark pale). C-I(A69) allows the fully colored C-S phenotype. None of the other colorless alleles tested affect the C-S colored phenotype.

Except for the interaction with C-S, the C-I alleles are indistinguishable when examining progeny with two doses of C-I (Cross 1). With one dose of C-I (Cross 2), allelic differences emerge. C-I(A69) is not able to suppress any of the C alleles tested. C-I(Cornell) allows a weak pigmentation with C-(lineC). These observations allow the ordering of the C-I alleles based on their suppression ability with the strongest listed first: C-I(K55), C-I(Cornell), and C-I(A69). These observations also allow the ordering of the C-S, C-(hiloss), C-(lineC), and C-(Cshbz) in decreasing order of dominance after the C-I alleles.

For the recessive c and c-ruq alleles, one dose of the allele does not affect the expression of the C alleles. The one exception may be c in combination with C-(Cshbz) or C-(lineC). The progeny of these two crosses appear paler than the other crosses of this group. When the c-ruq and c alleles are present in two doses the expression of the C-(Cshbz) and C-(lineC) alleles is reduced. This reduction in color may be slightly less for c-ruq31 when compared with c-ruq67. Therefore, the c-ruq31 allele is either slightly more dominant than c-ruq67 or equally dominant. The c allele has the least dominance (most recessive) of the tested alleles (note: the c x C-(hiloss) progeny are not available.) The order of this allelic series is: C-I(K55), C-I(Cornell), C-I(A69), C-S, C-(hiloss), C-(lineC), C-(Cshbz), c-ruq31, c-ruq67, and c.

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

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