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Materials and methods
LYB (an ear branching selection selected from a common wheat
cultivar, Liying No. 3) stably showed abnormal spikes with additional
spikelets on extended rachilla. The Chinese Spring monosomic series
with normal spikes were used for monosomic analysis of LYB's SS
character. Three-five monosomic plants were selected from each the 21
monosomic line, and were artificially pollinated with LYB. At the
same time, euploid Chinese Spring was crossed with LYB. A part of
F1 monosomic plants and F1 disomic plants was
selfed to obtain F2 seeds, other F1 monosomic
plants and F1 disomic plants were backcrossed to LYB to
obtain corresponding BC1 seeds.
On 3 Nov. 1995, the F1 and F2 seedlings were
separately planted. The seeds of BC1 of each combination were sowed
in the field. At harvest, all the spikes were observed carefully to
determine whether they were SS spikes or normal spikes, and the
plants were classified into SS spike type (with at least one SS
spike) and normal spike type (with normal spike only). The
F2 population which derived from F1 disomic
plants was taken as a control in F2 monosomic
analysis.
Result and discussion
The F2 monosomic analysis showed that 2A, 2D and 4A
F2 populations significantly surpassed the control
population in frequency of SS spike plants (Table
1). The progenies
which derived from backcrossing F1 hybrids monosomic for
2A, 2D or 4A to LYB exhibited higher frequency of SS spike plants
than other backcross combinations (Table
2).
These results mean that chromosomes 2A, 2D and 4A were responsible
for SS character.
Koric (1973) found that two genes promoting development of SS and a
dominant epistatic inhibitor of SS (Nr) exist in common wheat,
and that these genes were independently inherited. The F2
progenies which derived from F1 monosomic plants were
homozygous (disomic) or hemizygous (monosomic) corresponding
chromosome (s) of LYB. Thus, if a chromosome of LYB carries the gene
promoting development of SS, the plants in its corresponding
F2 population would be homozygous (disomic) or hemizygous
(monosomic) for the gene promoting development of SS. However,
according to Koric's conclusion (1973), these plants show SS only
when the dominant
Nr
gene on other
chromosome is absent in their genome. So, the segregation for spike
type in the corresponding F2 population would be identical
with single gene
(Nr)
segregation, and the
expected segregation ratio would be 1:3 (SS:normal). Only the
F2 population which did not possess Nr may show more than
1/4 SS plants. Therefore, to determine whether Nr gene exists or not,
chi-square analysis was used to test for the goodness of fit to the
segregation ratio of 1:3 in
F2
populations. The
result showed that F2 progenies of mono-2A and AA fitted
the single -gene segregation ratio well (Table
1), and the SS
character was recessive. The segregation of monosomics and disomics
in 2A and 4A F2 populations could not account for the
segregation of spike types, because both disomic and monosomic plants
segregated for spike type in these two F2 populations.
As more than two chromosomes were responsible for the SS character in
this cross, the segregation ratio of 1:3 (SS:normal) in 2A and 4A
F2 populations could not be interpreted as the act of two
recessive complementary genes. The most acceptable explanation is
that chromosomes 2A and 4A carry genes promoting development of SS,
and a dominant strong inhibitor of SS (Nr) exists on other
chromosome. Chromosome 2D should be the location of Nr gene,
because only it was responsible for the SS character except
chromosomes 2A and 4A. The fact that only 2D F2 population
showed more than 1/4 SS spike plants supported this conclusion.
Obviously, LYB itself could not carry the dominant allele of
Nr gene, because it stably showed SS phenotype. Chinese Spring
must carry the dominant allele of Nr gene, while LYB should
carry its recessive allele(nr) or delete this locus.
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