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Study of the F2 generation of cross of resistant Acc 3749
of Ae. squarrosa with susceptible accession (Acc 3754) showed
that the resistant parent possesses one dominant and one recessive
gene for resistance to pathotype 77-1. Both of these genes were
individually effective against this pathotype. However, testing of
the F2 generation of Acc 3754 (S) x Acc 3749 (R) with
pathotype 77-4 in indicated that Acc 3749 possesses two dominant
genes for resistance where both genes are individually completely
effective against pathotype 77-4. It has been observed that dominance
or recessiveness of resistance genes is not absolute and that the
dominance relationship can change with pathogen isolate. The stem
rust resistance gene Sr6, which in most cases is dominant,
displays recessive inheritance with some pathogen cultures (Roelfs
1988). Therefore, it is possible that the two genes of Acc 3749
providing resistance to 77-1 are the same as those that provide
resistance to 77-4, and that only the dominance relationship of one
of the genes in Acc 3749 changed with change in rust pathotype.
However, it is difficult to prove or disprove this assumption with
the limited data available and, therefore, the presence of more than
two leaf rust resistance genes in Acc 3749 cannot be ruled out.
Study of the intraspecific cross of T. dicoccoides between Acc
4667 exhibiting intermediate reaction (; to 0N on first leaf and 0N
to 3-N reaction on second leaf of seedling) and Acc 13985 exhibiting
resistant reaction (; on both the leaves) to pathotype N of stripe
rust showed that the former accession possesses a dominant gene for
intermediate reaction and the latter possesses another dominant gene
conferring complete resistance (12 resistant : 3 intermediate : 1
susceptible ratio; chi2 = 1.02)
In the present study, examination of variability for resistance to
leaf rust and stripe rust by testing of different accessions of wild
Triticum and Aegilops species with individual isolates
possessing diverse pathogenicity showed that there is large
intraspecific variability for rust resistance within each of these
species. Existence of a number of rust reaction patterns among small
samples of accessions of each species showed that each species
possesses a number of rust resistance genes. If these accessions were
tested with even more number of pathotypes, further variability for
rust resistance genes among different accessions may be revealed.
These observations with multipathotype seedling tests were highly
supported by testing of F2 generation of the intraspecific
crosses with individual rust pathotypes. Inspite of the fact that all
accessions of T. urartu used in the present study were
collected from Turkey, F2 of all crosses between different
accessions of this species segregated for rust reaction (Table
5). This observation has an important implication in the
utilization of wild relatives of wheat as donors of rust resistance.
It suggests that when one or more genes transferred from a wild donor
species are overcome by new pathotypes due to directional selection,
the same donor species could still be a reservoir of a number of new
resistance genes that can be transferred and deployed in the future.
At least six leaf rust resistance genes (Lr21, Lr22a, Lr32, Lr41,
Lr42 and Lr43) have been transferred from Ae.
squarrosa (Cox et al. 1993; McIntosh 1998) and transfer of other
Lr genes is in progress. This suggests that, as source of
resistance, although Ae. squarrosa did not appear to be as
good as other Aegilops species with C, U and M genomes
(Dhaliwal et al. 1991, 1993; Harjit-Singh et al. 1998), still it
possesses impressive intraspecific genetic diversity for leaf rust
resistance.
Based on the higher proportion of accessions exhibiting resistance to
prevalent isolates, it is often concluded that a particular wild
related species is a better source of resistance than another with
lower proportion of resistant accessions. However, the latter may
still possess a number of genes for resistance that are not useful
against the present pathotypes but may be useful against emerging
pathotypes in future. Our observations at the Punjab Agricultural
University, Ludhiana showed that T. dicoccoides (AB) is highly
susceptible to leaf rust and stripe rust (Dhaliwal et al., 1993;
Harjit-Singh et al. 1998) but still we could find different useful
stripe rust resistance genes among the two accessions studied in the
present study. Similarly, van-Silfhout (1989) found that 850 samples
of T. dicoccoides collected from Israel possess at least
eleven stripe rust resistance genes. Also, he found that several
entries found to be susceptible to one or more isolates from Israel
proved resistant to eight of the main stripe rust pathotypes from
Netherland.
The significantly large intraspecific diversity revealed in the wild
Triticum and Aegilops species in the present study
suggests that these species shall continue to offer a number of novel
resistance genes, in spite of the fact that many of the resistance
genes contributed by these species have been overcome by new
pathotypes. The progenitor species like Ae.squarrosa (Cox et
al. 1993; McIntosh 1998), T. dicoccoides (van Silfhout 1989;
van Silfhont et al. 1989) and T. boeoticum (Gill et al. 1995)
continue to be promising sources of rust resistance for transfer and
deployment. Thus, the wild progenitor species should be
preferred as source of new genes for resistance against the prevalent
pathotypes over the non-progenitor and distantly related species due
to ease of transfer through recombination and reduced linkage drag
from the former.
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