A new stem rust resistance locus detected in wheat variety Yalta

To monitor evolution and pathogenic variability of wheat stem rust pathogen (Puccina graminis f. sp. tritici) in Australia, the Australian Cereal Rust Control program regularly conducts national annual surveys. Recently, we detected a new pathotype 34-1,2,5,7 (culture # 661) virulent on stem rust resistance genes Sr5, Sr6, Sr7b, Sr9g, Sr11, Sr15 and Sr17. Although virulent on Sr11, this pathotype produced a low infection type (IT 22+C/X) on the Sr11-differential genotype Yalta, indicating that Yalta carries an uncharacterised resistance (SrY) in addition to Sr11. To characterize SrY, we screened a RIL population Yalta/W2691 (104 lines) with two pathotypes: 21-0 (avirulent on Sr11 or AA) and the newly identified 34-1,2,5,7 (virulent on Sr11 but avirulent on SrY or BB). Yalta produced low infection types, "1C" and "22+C/X" with pathotypes 21-0 and 34-1,2,5,7, respectively, whereas W2691 was susceptible to both pathotypes. The population segregated for AA/aa (35 Res: 69 Sus) and BB/bb (36 Res: 68 Sus) loci with pathotypes 21-0 and 34-1,2,5,7, respectively. The observed segregation (AA/aa and BB/bb) however failed to fit with predicted single gene 1:1 model (P<0.05) with both pathotypes. Joint segregation analysis (AA/aa vs BB/bb) also significantly deviated (P<0.01) from 1:1:1:1 (AABB:AAbb:aaBB:aabb) genetic model. It appears that population is skewed towards susceptibility in each case either by chance or differential gametic transmission as reported previously in progenies derived from crosses involving variety Yalta. The segregation pattern (AABB and aabb) with two pathotypes was, however, highly coupled apart from 13 lines, of which, 6 lines (AAbb) were susceptible with 21-0 and resistant with 34-1,2,5,7, and 7 lines (aaBB) resistant with 21-0 and susceptible with 34-1,2,5,7, showing that the two loci are linked (?2 linkage = 76.9; P<0.001) and located very close to each other. If that is the case, it may imply that SrY is common in wheats carrying Sr11. Cultivar Charter has been used in India to differentiate pathotypes virulent for Sr11, suggesting that Charter also carries a second stem rust locus (SrC) possibly corresponding with SrY. Further studies and mapping work are underway to determine the genetic relationship between SrY, SrC and Sr11.