| Unexpected chromosome numbers in backcross I generations
of F1 hybrids between Triticum aestivum and related alien
genera D. JEWELL and A. MUJEEB-KAZI CIMMYT, Apdo. Postal 6-641, Delg. Cuauhtemoc 06600, Mexico D.F. Self-sterile F1 hybrids within the Triticnae are often partially female fertile. This allows successive generations to be obtained by backcrossing the F1 hybrid to a common wheat parent with the eventual isolation of individual addition lines of the alien species as described by O'MARA (1940). The reason for the partial female fertility has been assumed to be due to the formation of unreduced female gametes. The expected chromosome number in the first backcross generation (BC1), based on the premise of unreduced eggs being the only functional gametes, has not been uniformly achieved. MUJEEB-KAZI & BERNARD (1982) found a variation in chromosome numbers in BC1 progenies involving Agropyron, Triticum aestivum, T. turgidum and Elymus species hybrids, as did PIENAAR for T. aestivum/A. distichum (1980). An explanation has been offered for only some of these unexpected chromosome numbers. For example, apomixis (MUJEEB-KAZI 1981), chromosome elimination of the alien genome, and spontaneous doubling have been used to explain unexpected numbers of exact multiples of the expected. The derivation of the many other observed chromosome numbers has not been explained, possibly due to complexities involved in determining chromosome identification. GERLACH (1977) demonstrated that 9 of the 21 wheat chromosomes could be identified using the N-banding technique. JEWELL (1979) confirmed the N-banding of wheat chromosomes and demonstrated that all 14 chromosomes of Aegilops variabilis also exhibited unique N-banding patterns which enabled them to be identified and differentiated from wheat chromosomes. The hybrid of T. aestivum cv. Chinese Spring and the tetraploid Ae. variabilis usually exhibits 35 univalents at metaphase 1 and there is less than 1 chiasmata per cell (DRISCOLL & QUINN, 1968). These plants are self-sterile. Therefore, the BC1 plants should provide excellent material for chromosome analysis. The technique used for N-banding analysis is described in detail by JEWELL (1981). However, for ongoing studies at CIMMYT, due to high altitude (above 2,225 m), it was necessary to adjust the technique because the normally used temperature of the acid buffer (92o + or - 1C) is not obtainable. At 87C + or - 1C it requires 46 to 50 minutes in 1M NaH2PO4 buffer to produce good bands. However, the chromosome morphology is adversely affected by this length of treatment. Hence, it was more satisfactory to use 2M NaH2PO4 buffer for 25-28 minutes at 87 + or - 1C. Apart from this, the technique was the same as previously described (JEWELL 1981). |
| The research on the backross plants of Chinese Spring by Aegilops variabilis was undertaken while the senior author was at the Waite Agricultural Research Institute, University of Adelaide, Australia. Reprints may be requested from A. Mujeeb-Kazi. |
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