Abstract:

The short arm of Haynaldia villosa chromosome 6V confers Pm21 that conditions immunity from or high resistance to almost all known powdery mildew races. In the middle of the 1990s, Cytogenetic Institute, Nanjing Agricultural University (CINAU) has obtained the T6VS•6AL translocation line by which some wheat cultivars with high yield and powdery mildew resistance have been developed. However, this translocation obviously has a large amount of redundant alien genes. To develop terminal and/or intercalary translocations with as short as possible alien chromosomal segments conferring Pm21 is more important in wheat breeding. In addition, the more translocations with different breakpoint positions at the length of 6VS are, the higher resolution of physical mapping of Pm21 is. In this research, our objective is to induce new translocations involving H. villosa chromosome 6V through pollen irradiation. Pollen of Triticum aestivum-Haynaldia villosa 6V monosomic addition lines (2n = 43), treated with 1 000 rad ⁶⁰Co-γ-rays, was pollinated to T. aestivum cv. ‘Chinese Spring’. Pollen mother cells (PMCs) of M1 plants at meiotic metaphase Ⅰ were used for GISH analysis. A trivalent observed was formed by two T. aestivum-H. villosa translocation chromosomes and a wheat chromosome in a M1 plant, indicating that the translocation chromosomes might be resulted from a reciprocal translocation event. The translocation involving the large alien segment was called ‘large alien segment translocation’ (LAST) while the translocation involving the small alien segment was called ‘small alien segment translocation’ (SAST). Sequential C-banding and GISH technique were used to identify a homozygous M2 plant (LAST’’+SAST’’, 2n = 44). The results showed that the reciprocal translocation involved T. aestivum chromosome 7B and H. villosa chromosome 6V with the translocation breakpoints at about FL0.60 of 7BS and about FL0.70 of 6VS, respectively. The LAST chromosome was identified as T7BS-6VS•6VL while the SAST chromosome was identified as T6VS-7BS•7BL. Seven chromosome constitution types were observed in the M₂ population with the ratio of 3(LAST’’+SAST’’):20(LAST’+SAST’):2(LAST’’+SAST’):1(LAST’+SAST’’):1LAST’:2SAST’:22(none alien). The LAST chromosome occurred accompanied by the SAST chromosome at most times. Resistance analysis showed that the plant with a single LAST chromosome was resistant to powdery mildew. Then, Pm21 was located at the proximal region of FL0.70 of 6VS. Meiotic analysis of M2 plants with the genome of 20ⅡW+Ⅰ7B+ⅠT7BS-6VS•6VL+ⅠT6VS-7BS•7BL (2n = 43) showed that T7BS-6VS•6VL and T6VS-7BS•7BL co-segregated at meiotic anaphase Ⅰ or telophase Ⅰ in 88.5% of PMCs. The LAST chromosome T7BS-6VS•6VL stayed late or centric-misdivided in several PMCs at anaphase Ⅰ. An intercalary translocation, T7BL•6VS-7BS, arising from the centric breakage-fusion of 7B and T7BS-6VS•6VL, was then detected in a progeny of a plant with the genome of 20ⅡW+Ⅰ7B+ⅠT7BS-6VS•6VL. It provided a new strategy for inducing intercalary translocations with alien chromosomal segments conferring resistance genes. Homozygous T7BS-6VS•6VL and T6VS-7BS•7BL translocation lines were obtained in M3 generation.

Key words: Triticum aestivum, Haynaldia villosa, 6V, Reciprocal translocation, Large alien segment translocation, Small alien segment translocation, Intercalary translocation, Powdery mildew resistance