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Another set of derivatives (Table 3) possessing resistance to leaf rust from Ae. triuncialis were obtained among the backcross progenies. Genomic in situ hybridization showed that these progenies possess a pair of translocated chromosomes where the break point is on the centromere and the short arm of the translocated chromosome is alien (Fig. 2b). These progenies possess 44 chromosomes as there is disomic addition of an acrocentric chromosome (Fig. 3). Occasionally, a trivalent (mean frequency from 0.04 to 0. 13) or a quadrivalent (mean range of 0.02 to 0. 16) was observed. Also 0 to 4 univalents (mean ranging from 0.20 to 1.12) were observed in these derivatives.

The derivatives carrying the homozygous translocation and disomic addition of acrocentric chromosome were derived from the BC2 plant No.18 that had exhibited 1 to 2- seedling response to leaf rust pathotype 77A-1. The selfed progeny of the plant No.BC218-13-2- 13 from which the derivatives presented in Table 3 were derived, also showed seedling resistance (Table 4) to pathotypes 77-2 as well as 77-4 (Avirulence/virulence formula: Lr9, Lr20, Lr26/Lr1, LrlO, Lr13, Lr15, Lr23). Progenies of two plants from this progeny exhibited uniform seedling resistance to pathotype 77-5 as well. So this set of derivatives possesses seedling resistance to leaf rust that is effective at adult plant stage. The slight shift in adult plant response to leaf rust (from 0 to tR/5MR) may be attributed to some shift in racial structure of pathogen population (Nayar et al. pers comm).

The recurrent parent is highly susceptible to Karnal bunt. It had disease incidence ranging from 22.5 to 99.0 percent whereas Ae. triuncialis remained free from Karnal bunt under artificial conditions. Out of the three interspecific derivatives (Table 3) tested, two remained completely free from Karnal bunt. In the third progeny, a few plants had disease incidence up to 4.5 percent. In case of Karnal bunt, entries having less than 5, percent disease incidence are classified as resistant (Fuentes-Davila 1996). So this progeny also falls into resistant category. This set of derivatives also exhibited moderate to high seedling resistance to the Keylong (Indian) isolate of powdery mildew.

These observations suggested that the small alien arm translocated to the wheat chromosome or the alien acrocentric chromosome is carrying gene(s) for resistance to the three diseases viz., leaf rust, Karnal bunt and powdery mildew. There is equal possibility that gene(s) for resistance to one or two diseases are located on the alien translocated arm and the gene(s) for resistance to the rest of the disease(s) are located on the acrocentric chromosome. Further investigations are needed to clarify this point.

The observations presented here show that the wild tetraploid non-progenitor species, Ae. triuncialis (UUCC), could be a good source of resistance to wheat pathogens. This is the first report of transfer of useful resistance genes from this non-progenitor species though similar transfers have been made from other non- progenitor tetraploid species like Ae.triaristata (Bai et al. 1994) and Ae. ovata (Harjit-Singh and Dhaliwal 1996). The lack of transfer of useful genes from Ae. triuncialis may be due to the presence of chromosome with gametocidal genes reported in this species which is preferentially transmitted and kills the gametes without it (TsuJimoto and Tsunewaki 1985). The work to use these alien substitution/ translocation and addition lines for precise transfer of the alien resistance genes to wheat chromosomes through induced homoeologous pairing and use of molecular markers is in progress.


Acknowledgement

This research has been financed in part by a grant made by the United States Department of Agriculture under US-lndia Fund.

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