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Results and
discussion
Monosomic analysis
In the cross between Kalyansona mono-6A and Kite, out of 4
F1 plants, 3 were monosomics and 1 disomic. The
F2 progeny of disomic F1 consisted of 36
resistant and 33 susceptible plants. The F2 of monosomic
F1 had 35 resistant plants and only 2 susceptible plants
which appeared to be nullisomics with narrow leaves and thin culm.
These results clearly indicated that Sr26 gene of Kite was
located on chromosome 6A.
Test cross analysis
The test cross results are summarized in Table
1.
As a result of pairing failure between the entire 6A of Kite and
telo-6AL of Chinese Spring in the F1,a large number of
monosomics were scored in the test cross progeny and these were all
susceptible to stem rust. The resistant plants had the entire 6A of
F1 with the formation of 21 normal bivalents. The test
cross plants with a telocentric (20" + t1") were susceptible. Hence,
all the 123 test cross plants analyzed were non-crossovers. It can
thus be concluded that the Sr26 gene shows complete linkage
with the 6A centromere.
It was interesting to note that though there was no 6A-6AL
pairing in the Kite x ditelo-6AL F1 (20"+1'+t'), normal pairing with
the formation of a heteromorphic rod bivalent (20"+t1") was observed
in the monotelodisomic susceptible test cross plants. The
Agropyron segment did not permit any 6A-6AL pairing in the
F1. This indicates that most of the long arm of Kite 6A
consists of the Agropyron segment. Knott (1961) also observed
that when the Thatcher translocation line was backcrossed to
Thatcher, the F1 plants had 21" but there was always at
least one open bivalent. Prabhakara Rao (1977) transferred the
Sr26 gene to durum wheat but observed that plants heterozygous
for the Agropyron resistance gene always had 14 bivalents with
at least one open rod bivalent. Such a large structural alteration of
6A was too drastic for durum background with the result there
was no male transmission of the Agropyron resistance gene from
heterozygotes. Hence, homozygous resistant line could not be
recovered in the durum background.
The present study indicates
that the wheat-Agropyron translocation break-point on 6AL is
located very close to the centromere with the Agropyron
segment replacing most of the long arm of 6A. The Sr26 locus
may be located anywhere along the length of the Agropyron
segment. There is no way of determining its exact location since
pairing does not take place between this translocated segment and the
corresponding portion of wheat 6AL.
References
Knott DR (1961) The inheritance of rust resistance VI. The
transfer of stem rust resistance from Agropyron elongatum to
common wheat. Can J Plant Sci 41: 109-123.
Luig NH (1978) Mutation studies in Puccinia graminis tritici.
Proc 5th Intl Wheat Genet Symp, New Delhi: 533-539
McIntosh RA (1978) A catalogue of gene symbols for wheat. Proc 5th
Intl Wheat Genet Symp, New Delhi: 1299-1309.
Prabhakara Rao MV (1977) Transfer of genes for stem rust resistance
from Agropyron elongatum and Imperial rye to durum
wheat. In: Induced Mutations against Plant Diseases
(IAEA-SM-214/34) Vienna: 527-531.
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