True resistance to BYDV is not naturally present in wheat, although different degrees of tolerance can be found. The more common and current source of BYDV resistance in wheat is Bdv2, this gene was transferred from Thinopyrum intermedium, a hexaploid species with genome composition: EcEc EbEb StSt. There are two main families of Bdv2 donor lines used within the IFAFS project: the TC lines and the P29-derived lines. BYDV resistance in P29 and TC lines derives from different 7E chromosomes of T. intermedium. Additional resistance genes could be located on chromosomes 1E and 2E, but further characterization is needed (1-4).
Origin of TC lines: TAF46 is a partial amphiploid (2n = 8x = 56) between wheat and T. intermedium, it carries all the chromosomes of wheat and seven pairs of T. intermedium chromosomes. After backcrossing of TAF46 to wheat several addition lines were obtained, one of these was L1, which carries stem rust and BYDV resistance in chromosome 7Ai-1. L1 was crossed with "Sunstar" and "Millewa" wheat and immature embryos or spikes were extracted from the F1 plants for cell culture and plant regeneration to induce translocations between 7Ai-1 and wheat chromosomes. The regenerated plants were backcrossed to wheat and different BYDV resistant families were obtained: TC5, TC6, TC7, TC8, TC9, TC10 and TC14. The translocations are located on chromosome arm 7DL (except TC7) (1, 4, 5). TC14 has the smallest translocation, and it was incorporated into different breeding programs, some of them at CIMMYT. TC14/2*Spear is one of the TC14-derived lines that showed the lowest virus titers when infected with isolates PAV-Mex and MAV-MEx, but all of the TC lines showed symptoms of infection in the field tests at CIMMYT locations in Mexico.
Origin of P29: The susceptible SRW wheat cultivar Abe was crossed with BYDV-resistant T. intermedium, the self sterile F1 BYDV-resistant hybrids were backcrossed first to Compton, followed by backcrosses of the resistant progeny to the line IN81401A1-43 (a selection of the cross Arthur/Cadwell). Then, a resistant plant with 2n = 49 chromosomes obtained at BC2F1 was backcrossed (BC3) to Cadwell. Finally, a self fertile BC3F2 plant from the Cadwell backcrosses with 43 chromosomes was allowed to self- pollinate. The initial seed stock of P29 is the harvest of four F4 plants from the selected BC3F3 plant which was BYDV-resistant and had 42 chromosomes. P29 is a substitution line in which chromosome 7D of wheat was replaced by a chromosome of T. intermedium. Several translocation lines derived from P29 are available, they were produced by gamma irradiation of seeds and backcrossing into wheat (2, 3, 4, 6).
The telomeric repetitive sequence pAW161 specifically hybridizes to the 7EL telomere in P29-derived wheat - T. intermedium introgression lines. This sequence is not present in the resistant TC14 line, one of the commonly used TC lines for breeding purposes. Specific PCR primers to the pAW161 clone were designed and a more user-friendly PCR method was developed to detect the 350-bp telomeric repetitive sequence (4, 6) (see the methods section).
Ayala et al. (7) determined that the microsatellite marker gwm37 was polymorphic for the T. intermedium translocation into wheat and validated the marker in TC or TC-derived lines. This a co-dominant and tightly linked marker. Using this marker it could be shown that Bdv2 has a dosis-dependant response: homozygous plants for the resistance show lower virus titers than heterozygous. However, scoring Xgwm37 can be diifficult, Ohm and Kong developed a new microsatellite marker that is easier to use and can detect both Bdv2 and Bdv3. (see the methods section). Stoutjesdijk et al. (8) developed a sequence characterized amplified region (SCAR) marker for the same family of translocations, the marker is dominant for the translocation: it cannot be used to discriminate between homozygous and heterozygous, but allowed the design of a high-throughput solid-phase PCR assay (9) (see the methods section).
P29 is a soft red winter wheat, it is a substitution line in which chromosome 7D of wheat was replaced by a chromosome of T. intermedium carrying the resistance to BYDV. Gamma irradiation on P29 seeds and backcrossing with wheat was used to induce small translocations of T. intermedium chromosomes onto wheat chromosomes and to obtain new lines with smaller amounts of Thinopyrum material.
P29 showed a yield similar tothat of Cadwell, one of its progenitors, in field tests at Lafayette, IN and is resistant to the P-PAV and MAV isolates of BYDV (3).
Lines of the TC14 group were tested by CIMMYT at Mexican locations. The plants showed symptoms of disease, although in greenhouse trails they had lower virus titers than non-resistant control lines. The current strategy at CIMMYT is to combine resistance (as present in TC14) with tolerance in good agronomic backgrounds (See the CIMMYT report).
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