Introduction of alien variation into bread wheat through chromosome engineering

B.C. JOSHI and S.M.S.TOMAR

Biotechnology Centre and Division ot Genetics, Indian Agricultural Research Institute, New Delhi, India

Monosomic 5B(2n = 41) of cv. Chinese Spring was crossed with a rust resistant strain of rye R466 Acca (Secale cereale, 2n = 14) obtained from CIMMYT, Mexico. In the F₁ two types of hybrids were obtained 2n = 27 and 2n = 28, the former lacking chromosome 5B of Triticum aestivum and the latter having it. In the absence of chromosome 5B of T. aestivum, the hybrid with 27 chromosomes exhibited extensive chromosome pairing. The hybrid with 28 chromosomes had mostly univalents, because chromosome 5B of T. aestivum does not allow the homoelogous chromosomes to pair (SEARS & OKAMOTO 1958, RILEY & CHAPMAN 1958).

The wheat-rye hybrid with 27-chromosomes, was directly backcrossed to three hexaploid wheat cultivars; Sonalika, Chinese Spring and Lokrin. The BC₁-F₂ families of mono 5B x rye x wheats produced a spectrum of aneuploids (chromosome number ranging from 2n = 34 to 44). At the appropriate time the field population was infected with a total of about 40 races of stem, leaf and stripe rusts. In the segregating populations only rust resistant hybrid derivatives were selected for further analyses. In the off-season, the progenies of rust resistant plants were grown at the Regional Station of Indian Agricultural Research Institute, Wellington (Nilgiris), a 'hot spot' for rust and powdery mildew. Again rust resistant plants were selected and many possessed high tiller number, high spikelet number and more number of grains per spike. The enormous variability generated by the intergeneric cross of mono 5B x rye x wheats in the BC₁-F₃ generation is evidenced by Table 1.

The procedure of rigorously testing rust resistant progenies, having other desirable traits was continued, and in the BC₁-F₇ generation 93 single plants progenies were tested at Wellington, under natural and artificial epiphytotic conditions of infection. The rust reactions were recorded according to the modified Cobb's scale. To ensure good infection of powdery mildew the outstanding selections were tested in green house under optimal conditions of growth. The seedlings were scored for resistance on 0 - 4 scale (Table 2).

In the BC₁-F₇ generation about 100 rust resistant disome plants (2n = 42) have been identified cytologically. The variability maintained in the genetic stocks as exhibited by the disomes is given in Table 3.

Although the source of rust resistance used in the present programme is different than Petkus rye, the adult plant reaction to stem, leaf and stripe rusts and seedling reaction to Erysiphe graminis tritici exihibited by the derivatives is more or less similar to that of 1B/1R substitution and 1B/1R translocation lines such as Kaokaz and Burgas-2 which derive their resistance from Petkus rye. These cultivars possess linked genes Lr 26 (against leaf rust), Sr 31 (against stem rust), Yr 9 (against stripe rust) and Pm 8 (against powdery mildew). The genetics of adult plant and seedling resistance in the present wheat-rye derivatives to rusts and powdery mildew is under study.

References RILEY, R. and V. CHAPMAN. 1958. Genetic control of the cytologically diploid behaviour of hexaploid wheat. Nature, Lond. 182: 713-715. SEARS, E.R. and M. OKAMOTO. 1958. Intergenomic chromosome relationships in hexaploid wheat. Proc. Xth Int. Cong. Genet. Montreal 2: 258-259.