A test for allelism between Cg2 and Cg
--N. V. Krivov

Ninety various loci which affect morphogenesis have been identified and described in maize (Sheridan, WF, Annu. Rev. Genet. 22:353-85, 1988), among them two semidominant mutations, Cg at 3S-35 (Singleton, Amer. Nat. 85:88-96, 1951; Galinat, W, MNL26:51, 1952; Poethig, RS; MNL62:98, 1988) and Tp3 on 3S (Poethig, Genet. 119:959-973, 1988), that have an extremely strong pleiotropic effect. Atavistic changes in the shoot vegetative morphology occur in the carriers of these mutations, and it is surprising that they are both located on the 3rd chromosome.

Is Cg2 the allele of Cg or Tp3? A complete genetical affinity could not occur. Firstly, because as a rule, any mutational event in any locus is an unique phenomenon and reverses, as a rule, do not exactly restore the primary function of this locus. Secondly, because of the origin, and mainly due to the fact that the genetic background was not similar. The Cg2 macromutation originated among many M3 plants as a single plant after the exposure of the line VIR-44 pollen to irradiation with gamma-rays from Co-60 at a dose of 1500 rads, and pollination with a mixture of pollen from different neighbouring maize lines. The Cg macromutation is known to originate spontaneously in plantings of Lincoln sweet corn hybrid (Singleton, MNL21:6, 1947). And finally, while the expression of Tp1 is suppressed by increasing doses of its wildtype allele, the expression of Tp2 is essentially indifferent to the wildtype gene dose. These response patterns suggest that Tp1 has an antimorphic function, while Tp2 has a neomorphic one. Although this does not necessarily mean that these genes are functionally distinct, it seems unlikely that antimorphic and neomorphic mutations of the same function would have phenotypes as similar as those of Tp1 and Tp2 (Poethig, 1988).

To test for allelism the subline isolated in the progeny of the macromutation of corngrass, and Cg from the All-Union Institute of Plant Growing (St. Petersburg) collection (Cg2/Cg2 x Cg/+), was used. Thus, in case of no allelism of these two macromutations in F1 the heterozygote of two types Cg2 +/+ Cg and Cg2 +/+ + should be expected. In fact, in F1 of 113 maize plants 56 had a strong mutant corngrass phenotype and resembled Cg, while the rest had a weakened mutant Cg2 phenotype, as they didn't tiller and differed from the normal maize in tassel (side branchlets in the tassel were absent).

In F2 it was expected on self-pollination that the plants with a strong mutant corngrass phenotype (Cg2 +/+ Cg), if Cg and Cg2 are allelic, would have progeny represented only by the carriers of the corngrass macromutation and if they are not allelic, corngrass and phenotypically normal plants will appear in the ratio approximately 15:1, and that self-pollination of plants with a weak mutant phenotype (Cg2 +/+ +) would give the segregation of Cg2 to normal of 3:1 respectively. In fact, the plant progeny with a strong mutant expression has segregated in the ratio 3:1 (Table 1).

Table 1. Segregating progeny resulting from selfing plants (Cg2 +/+ Cg) with strong mutant corngrass phenotype.
 
Family
Norm m(+) Cg Cg2 m(Cg2) Total X2 1:3
286 15 15 6 36 5.3*
287 7 40 3 50 3.2
288 10 2 15 1 1 29 4.1*
289 5 5 28 9 1 48 0.4
290 10 2 21 4 3 40 0.5
291 15 2 37 5 5 64 0.1
292 21 4 23 6 9 63 7.2*
Total 83 15 179 28 25 330 3.9*
*The deviations are reliable at the significant level of 0.05%.

On the self-pollination of Cg2 +/+ Cg the appearance of phenotypically normal plants in a quantity larger than was expected theoretically suggests that the suppressor is more probably connected with the recessive gene (or genes), which contains Cg/+ in the genome, as both F1 and F2 plants with a strong mutant corngrass phenotype resemble Cg. These two phenotypically similar mutations located in one chromosome are likely to have different functions and the mutant "corngrass" phenotype is displayed only in those genotypes where Cg is present at least in one dose. The variation of gene suppressor dose, as well as Cg2 or Cg macromutations tested, probably resulted in the fact that in nearly half of the families the part of the phenotypically normal plants was significantly higher than can be expected (Table 2). Undoubtedly, the F1 phenotypical discreteness and the appearance of the phenotypically normal plants on selfing heterozygotes Cg2 +/+ Cg suggest that Cg2 and Cg macromutations are not allelic to each other.

Table 2. Segregating progeny resulting from selfing plants (Cg2 +/+ +) with weak mutant corngrass phenotype.
 
Family Norm m(+) Cg Cg2 m(Cg2) Total X2 1:3
296 22 4 27 7 60 10.7*
297 29 6 24 11 70 23.3*
298 42 5 14 6 67 72.8*
299 17 50 4 71 0.04
300 34 2 36 2 74 22.0*
301 35 7 52 7 101 5.0*
302 19 1 45 6 71 0.1
303 6 1 20 8 35 1.1
304 11 1 24 1 37 0.4
305 10 2 24 2 38 0.9
306 9 1 25 2 37 0.1
307 14 31 1 46 0.7
308 12 2 32 1 47 0.6
309 13 2 33 3 51 0.01
310 13 1 22 2 38 2.8
311 6 1 9 1 17 2.4
312 2 9 5 16 1.3
313 21 18 1 40 16.1*
314 25 19 44 23.7*
315 13 13 26 8.7*
316 8 10 1 19 3.0
317 29 17 3 49 30.5*
318 27 23 4 54 18.0*
319 24 3 12 5 44 20.5*
Total 441 24 15 589 83 1152 154.0*
*The deviations are reliable at the significant level of 0.05%.

Note: As M. D. Golubovsky suggests, plant class m(+), in which most shoots are normal phenotypically, at the time of segregation estimation were referred to as "norm" as distinguished from m(Cg2), in which most shoots have a mutant phenotype and were referred to as "corngrass". 


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