Selection of stables or consolidated spontaneous mutants of the c locus

In the system (Line X x Line Y) x Line Y (BC1), all lines homozygous of the genotype A1 A2 C R Pr (Materials were indicated in MNL 55:58, 1981), 40 BC1 progenies (13519 kernels) gave 256 mutants with aleurone colorless. These are only possible by complementary aleurone gene mutation or mutation of C to C-Im inhibitor of aleurone color. For that reason in selfing plants originating by selection of colorless kernels it is expected to obtain all aleurone colorless, or the ratio 3 colorless: 1 colored. However, 50 progenies did not segregate colorless and 80 progenies segregated different ratios. These unexpected ratios would indicate mutation and back mutation as the following tests show for C-Im sh/C Sh x C Sh/C Sh:

Table.

Crossing (the pollen) of the above BC1 with c-tester, all kernels had aleurone color, indicating that all C-Im back mutated to normal C gene during ontogeny.

Meanwhile in the selection of colorless aleurone progenies was obtained a line that segregates in a mendelian ratio, 3 colorless: 1 colored, during 3 generations: S1 195:75; S2 133:42; S3 104:35. This mutant is symbolized C-Im1. Backcrosses with Line Y (activators) gave a deficiency of colorless kernels in 8 progenies: 2:208; all color; 33:310; all color; 5:345; 25:280; 30:270; 20:260.

In another line selected, another dominant mutant inhibitor of aleurone was obtained, symbolized C-Im2, that segregates also in mendelian ratio when selfed (C-Im Sh/C sh):

Table.

In the back-cross with Line Y (activators) in 12 progenies there was not the deficient segregation for colorless as found with C-Im1:

Table.

Is even the gene C-Im2 unstable? Is C-Im1 different from C-Im2 or does the line that carries C-Im1 also carry a Ds-like element, and is C-Im2 free of Ds? The question is one of molecular genetics, but classical genetics will answer it if the gene is of mendelian inheritance and during how many generations, and in what genetic environment it is stable.

The stability of the C-Im gene was not instantaneous as a gene mutation by DNA deletion. Perhaps the new mutant C-Im2, by means of adequate insertion of nucleotides and protection by histones, becomes stable against the selective pressure of the internal cellular environment (Darwinism in intraorganic molecular level? or DNA and protein metabolism?).

In the system indicated above, (X x Y)x Y, the gene C also mutates to recessive c-xm. The classic c of c-tester is stable in the system, but c-xm originated by C mutation to c-xm. In mutagenic effect on its locus c-xm Sh/c sh x C sh/C sh gave 11 ears (2000 kernels) with 95% colorless kernels (bad expression of sh). The new line that carried c-xm crossed with line C sh (Activator Y) would be considered "a new system" of high mutagenic effects, starting from the mentioned unstable system (X x Y) x Y. Perhaps it is possible that by an evolutive process, many communications of a "new system" of instability have the same origin. For that reason it would be useful to indicate with precision (when it is possible) the ancestral material that was studied, in order to establish that diverse evolutionary lines arise from the same system of instability.

In MNL 40:62-63, 1966, I indicated that "CIP gen was originated in maize by teosinte". Such interpretation was originated because the cytoplasm of teosinte from Huixtla (south of Mexico) is a mutational activator of the X line: "homozygous A1A2CRPrBCl, 1877; Dr. Randolph 1933".

The homozygous genotype of the X line in teosinte cytoplasm shows high mosaicism in anther color (Figure 1).

Figure 1. (A) Pollen of red anthers sector; (B) Pollen of yellow anthers sector; (C) Mosaicism in anthers.

Luis B. Mazoti
 
 


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