By means of selection from the cross between floury-a and normal lines, it is possible to obtain a floury-a line with completely flint phenotype. Likewise among the offspring of other crosses later on, between lines carrying floury-a gene and normal lines, a relatively fast conversion to normal phenotype through selection was observed. This phenomenon led to the opinion that possibly the suppressor action on the floury-a phenotypic expression was conditioned by few genes.
The flint floury-a line has the floury nucleus of the kernel in greater proportion than the normal lines, however, all the periphery of the kernel has a thick portion of hard endosperm. Crosses between modified floury-a and normal lines were carried out, but only crosses with DY normal line are reported in this paper.
Ears with F2 kernels were obtained and parallel backcrosses were carried out with modified floury-a, with the purpose of studying the genetic condition of the floury-a phenotypic suppression.
The F2 grains and the backcrosses show endosperm structure segregation. The kernels have been visually classified in two types: (1) those of flint, hard structure, which are vitreous and translucent and are regarded as having normal phenotype, and (2) those of floury structure, which are soft and opaque, of a floury phenotype conditioned by the floury-a gene (recessive in two doses). In general the phenotypic expression of floury-a is good, not presenting major difficulties in the classification of floury and flint kernels.
The experimental results obtained from the first crossing (modified floury-a x normal DY) are indicated in Table 1. It may be observed that in the 15 ears analyzed the number of F2 kernels with flint phenotype differs significantly from the 3:1 ratio. The results show an evident excess of kernels with normal phenotype, which presupposes the phenotypic suppression of the floury-a gene.
Table 1. Cross 1 (modified floury-a
x normal DY)
|Obs.||Exp. (3:1)||Exp. (13:3)|
The segregation observed between flint and floury kernels suggests a 13 flint: 3 floury ratio. This type of segregation may be conditioned by a recessive suppressor in two doses, or by two complementary recessive genes dominating in two doses. The expected values were calculated both for a segregation of 3:1 and for one of 13:3. As may be seen in cross 1, the 13:3 ratio fits better than the 3:1 ratio, but also, in most cases, the hypothesis of a 3:1 segregation can not be accepted.
In order to establish whether the suppressor effect is conditioned by only one recessive gene in two doses, or by two complementary recessive genes dominating in two doses, the backcross of the F1 by the modified floury-a was carried out. If one were dealing with only one recessive gene in two doses, the backcross should show a proportion of 3 flint kernels to one floury kernel, whereas if it were due to two complementary recessive genes dominating in two doses the proportion would be 5 flint kernels for every 3 floury kernels. The experimental results strongly suggest that the nearest proportion is that of 5:3, with which the hypothesis of two complementary genes dominating in two doses is accepted (see Table 2).
Table 2. Backcross 1 (modified floury-a x normal
DY) x modified floury-a
|78-07503 x -07549(1)||154||98||157.5||94.5||0.2073||0.50-0.70|
|78-07503 x -07549(2)||131||85||135||81||0.3160||0.50-0.70|
|78-07503 x -07550(1)||99||66||103.1||61.9||0.4400||0.50-0.70|
Control experiments were carried out by crossing floury-a line with floury phenotype with DY normal line, and also backcrosses were made (see Tables 3 and 4). In these crosses the 3:1 ratio occurs invariably between F2 kernels and 1:1 ratio in the backcrosses, that is, the phenotypic expression of the floury-a gene in this genetic background is good and behaves as a recessive in two doses.
Table 3. Cross 2 (normal DY x floury-a)
||Exp. (3:1)||Exp. (13:3)|
Table 4. Backcross 2 (normal DY x floury-a) x floury-a
|76-7722 x -7554(1)||217||232||224.5||224.5||0.5011||0.30-0.50|
The results suggest that the suppression of the floury-a expression in the material tested is conditioned by two complementary recessive genes dominating in two doses, which are referred to as suppressor-1 floury-a (sfa) and suppressor-2 floury-a (sfa2).
Jorge Luis Magoja
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