Introduction The X_a/x_a gene of barley, which is localized in the long arm of chromosome 1 (7H), presents semidominant expression, being the homozygous seedlings for the mutant allele (x_ax_a) yellow and lethal, while the heterozygotes are viable and light green (Konishi, 1972). The easy distinction among genotypes, even in chimerical conditions, makes this mutant very suitable for genetic studies (Prina and Favret, 1988; Prina et al, 1995).

Two chlorophyll mutants segregating in association with X_a/x_a gene We previously reported (Prina et al, 1995) the localization of two chlorophyll mutants. They were obtained after EMS treatments applied on one thousand seeds coming from heterozygotes X_ax_a3 plants. The selection was carried out at the greenhouse in the M₂ generation, after analyzing the progenies of 708 spikes (corresponding to 177 M₁ plants). Two chlorophyll mutants were observed to segregate in association with X_a/x_a gene. Both mutant alleles had recessive expression. One of the mutants had a yellow color, lighter than x_ax_a homozygotes, and died at second leaf stage. We named it clear xantha (xc). The other mutant, denominated virido-albina (va), presented positional variegated leaf, with green top and albino bottom. Homozygous vava plants were weak and only occasionally survived at field conditions. The new mutants were significantly associated to X_a/x_a gene and were localized at both sides of it (Table 1).

Other deviations in the segregation of X_a/x_a gene From the same material mentioned above, those families presenting deviations in the segregation of X_a/x_a gene without showing other visible mutants, were selected assuming putative early zygotic or gametic lethals linked to X_a/x_a. One of them showed a drastic defect in normal seedlings which persisted in the M₃ and the M₄ generations. The progenies from selfing showed very significant deviations from 1:2:1 ratio, but fitted with 1:1 ratio (chlorina: yellow), which was waited for the case of a gametic lethal linked in repulsion with X_a/x_a (Table 2). The presence of a few normal seedlings can be interpreted as a consequence of recombination or due to incomplete penetrance of the lethality.

Confirmation of the gametic lethal Double heterozygous plants for X_a/x_a gene and for the putative gametic lethal above mentioned, Gm/gm, were crossed as females with double heterozygotes XcxcVava. The progenies from selfing normal plants revealed that the proportions of normal vs clear xantha seedlings corresponded to 1:1 ratio, without showing any case of recombinants after analyzing 407 seedlings coming from 16 families (Table 3). In the case of Va/va gene the proportion between normal and mutants fitted with 2:1 ratio, after analyzing 1097 seedlings coming from 59 families (Table 4). This last result can be explained as a consequence of recombinants and agree with the low association found between Xc/xc and Va/va genes.

Conclusions A new gametic lethal has been induced and localized in the long arm of chromosome 1 (7H). The close linkage between the gametic lethal Gm/gm and Xc/xc gene for clear xantha mutant could be used as a balanced lethal system adequate for the maintenance of both mutants alleles.

References

Konishi, T., 1972. An incomplete dominant chlorophyll mutation on chromosome 1. Barley Genetics Newsletter 2: 43-45.

Prina, A. R. and Favret, E.A., 1988. Influence of marker genes on the expression of somatic mutations in barley. J. Heredity 79: 371-376.

Prina, A. R., Arias, M. C. and de la Fuente, M. C., 1995. A new mutant allele for X_a/x_a gene and its use for location of newly induced mutants in the long arm of barley´s chromosome 1. BGN 25: 31-33.

Table 1. Recombination percentages and their standard deviation. (Prina et al, 1995).

Table 2. Phenotypic frecuencies in selfed progenies of heterozygous plants producing segregation distortion of X_a/x_a gene.

Table 3. Phenotypic frequencies in selfed progenies of double heterozygous plants Xcxc-Gmgm.

Table 4. Phenotypic frecuencies in selfed progenies of double heterozygous plants Vava-Gmgm.