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We have determined the linkage of gene cn-A1 and the gene coding for enlarged
glume Egl and located in 7AL (Arbuzova et al. 1996). BC9 NIL
ANK-30A (Koval 1997, 1999) was used as a parent. The segregation of the F2
population of the cross between ANK-32A (chlorina, short glume) and
ANK-30A (green, enlarged glume) is shown in Table 2.
Among 271 F2 plants, 55 displayed chlorina phenotype, while 216 were
green (χ2 = 3.20). The segregation with regard to the glume
length was 228: 43 (χ2 = 12.06). The linkage between these genes amounted
to 36.1 plus or minus 3.8%, χ2 = 6.72 being for the segregation classes
9 : 3 : 3 : 1.
In the crossing combination (ANK-32A x ANK-32B), all the F1 plants
displayed chlorina phenotype. The F2 plants segregated into following
four classes ; green, chlorina, pale chlorina (semilethal) and xantha (lethal)(Table
3). The green and chlorina-type plants exhibited normal fertility. The number
of xantha plants corresponded to a dihybrid segregation pattern and represented
a phenotypic manifestation of recessive homozygotes for both loci. The progeny
of all the F2 plants were grown to examine the further segregation.
Analysis of the F3 progeny has demonstrated that the group of F2
green plants contained heterozygotes which segregate chlorina at a ratio of 3
: 1 (χ2 = 0 .09); the group of F2 chlorina contained
plants double heterozygous, segregating xantha at the ratio 15: 1 (χ2
= 4.46). All the F2 plants belonging to pale chlorina group were heterozygous
for either of loci (3 : 1, χ2 = 3 .98). The phenotypic classes
that we observed in the (cn-A1 x cn-D1) cross suggested a
dose effect and a complementary interaction between the two loci, cn-A1c and
cn-D1c.
The chlorophyll deficiency is increased by excess doses of the
gene also in case they belong to one and the same locus. One plant
displaying pale chlorina phenotype emerged spontaneously in F3
progeny of the hybrid (mono7A x ANK-32A). The progeny of this
plant segregated as 4 chlorina plants (2n=42 chromosomes) : 25 pale
chlorina plants (2n=43 chromosomes): 3 xantha plants (2n=44
chromosomes). Thus, the emergence of pale chlorina and xantha
phenotypes depends not on the interaction of alleles from different
loci, but exclusively on excess doses, of the mutant allele.
Electron microscopic examination has demonstrated that the increase
in the gene dose causes the decrease in the volume of photosynthetic
membrane structures and the number of thylakoids per granum
(Table 4). The chloroplasts of
Novosibirskaya 67 display a typical structure: they are of oval shape
and their medium density matrix contains numerous, ribosomes and well
developed photosynthetic membranes (Fig. 1
a). Occurrence of starch grains in stroma is common as well as
certain amount of plastoglobuli (osmophilic globules). In the
chloroplasts of the NILs ANK-32A. and ANK-32B (two doses of the
mutant gene), the volumes o f the membrane structures and the number
of thylakoids per granum are equally decreased; however, they yet
contain starch grains and plastoglobuli (Fig.
1b, c). The volume of the photosynthetic membranes and number of
thylakoids continue to decrease in pale chlorina plants (three doses
of the gene; Fig. 1d), whereas complete
degradation of the granum structures occurred in xantha plants (four
doses of the gene; Fig. 1e).
Nullisomics of soft wheat display green color; therefore, the
chlorina phenotype is not a result of either deletion or halt in gene
function. An increase in the number of cn alleles, either of
both loci or of the same one, in the genotype enhances the phenotypic
manifestation of the trait. Similar effect is observed in case of a
decrease in the number of the corresponding normal loci as a result
of aneuploidy involving other chromosomes (Pettigrew and Driscoll
1970). One may suggest that cn alleles produce a defect
protein, which compete with the products of the wild type alleles at
the game metabolic stage.
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