| The monotelocentric addition line was crossed with Ae. speltoides.
Hybrid seedlings were obtained by means of embryo culture and plants having
28 + I chromosomes were selected. The plants were transferred into a growth
chamber prior to meiosis and were grown under the following conditions:
10 hours of full light at 26C and 39% relative humidity (RH), 4 hours at
22C and 50% RH, 6 hours of darkness at 13C and 85% RH and 4 hours again
at 22C and 50% RH. Thus the plants received 16 hours of light and 8 hours
of darkness each day. Spikes for cytological analysis were collected between
2 and 3 hours after the start of full light. The affinity of the Agropyron
telocentric for either wheat or Ae. speltoides chromosomes was estimated
by scoring the frequency of PMC's in which the telocentric was found to
be paired. Chiasma frequency was scored as a measure of the uniformity of
conditions. Chinese Spring ditelo 7A was crossed with Ae. speltoides and pairing of the telocentric 7A with its homoeologues was scored in the same way. Results The 29-chromosome plants from the cross with Ae. speltoides were of two types (Table 1). In two plants PMC's showed about 15 chiasmata per cell and the Agropyron telocentric was paired in 12.9% of the cells. Surprisingly, one plant had only 6.7 chiasmata per cell and the Agropyron telocentric was not paired in any of 200 cells. Since the wheat parent had been backcrossed 7 times to Thatcher and should have been essentially homozygous, it appears that the variability must have come from Ae. speltoides. Presumably the Ae. speltoides parent was segregating for at least two genotypes having different effects in suppressing the 5B diploidizing system in wheat. Further data on the suppression of the diploidizing system in wheat by the activity of Ae. speltoides suppressor genes will be published later. By comparison, in 28-chromosome plants from the cross, Chinese Spring ditelo 7A x Ae. speltoides, the telocentric paired in 77% of the PMC's (Table 1). In similar crosses JOHNSON and KIMBER (1967) found 56.0 and 59.0% Pairing for telocentrics 7B and 7D, respectively. Present data indicate that the Agrus chromosome arm carrying the gene for leaf rust resistance shows sufficient pairing with wheat chromosomes that the gene can be transferred by means of homoeologous recombination. Pairing affinity with wheat chromosomes has been estimated also for three different telocentrics of diploid Ag. elongatum chromosomes (DVORAK 1971b). These telocentrics were also found to pair with their wheat homoeologues. The chromosome homoeology between Agropyron and wheat genomes appears to be generally high enough to make gene transfer by means of homoeologous recombination practical. Literature cited CALDWELL, R. W., J. F. SCHEFER, L. E. COMPTON and F. L. PATTERSON 1956. Resistance to leaf rust derived from Agropyron elongatum. Rept. of the 3rd Intern. Wheat Rust Conf. Mexico City, pp. 95-96. DVORAK, J. 1971a. Hybrids between a diploid Agropyron elongatum and Aegilops squarrosa. Can. J. Genet.Cytol. 13: 90-94. DVORAK, J. 1971b. A study of homoeology between the chromosomes of diploid Agropyron elongatum HOST. and wheat, Triticum aestivum L. (Abstract.) Can. J. Genet. Cytol. 13: 630. JOHNSON, R. and G. KIMBER 1967. Homoeologous pairing of a chromosome from Agropyron elongatum with those of Triticum aestivum and Aegilops speltoides. Genet. Res. Camb. 10: 63-61. KNOTT, D. R.1961. The inheritance of rust resistance. VI. The transfer of stem rust resistance from Agropyron elongatum to common wheat. Can. J. Plant Sci. 41: 109-123. KNOTT, D. R. 1968. Agropyrons as a source of rust resistance in wheat breeding. Proc. 3rd Intern. Wheat Genet. Symp., Canberra, pp. 204-212. NANDA, J. S. 1968. Cytogenetic studies on a number of Triticum-Agropyron derivatives. Ph. D. Thesis, University of Saskatchewan, 58 pp. QUINN, C. J. and C. J. DRISCOLL 1967. Relationships of the chromosomes of common wheat and related genera. Crop Sci. 7: 74-75. RILEY, R. 1958. Chromosome pairing and haploids in wheat. Proc. 10th Intern. Congr. Genet., Mont- real, 2: 234-235. RILEY, R. 1960. The diploidization of polyploid wheat. Heredity 15: 407-429. RILEY, R.,V. CHAPMAN and R. JOHNSON 1968. Introduction of yellow rust resistance of Aegilops comosa into wheat by genetically induced homoeologous recombination. Nature 217: 383-384. SEARS, E. R. 1956. The transfer of leaf rust resistance from Aegilops umbellulata to wheat. In Genetics in plant breeding. Brookhaven Symp. in Biol. 9: 1-21. SEARS, E. R. and M. OKAMOTO. 1958. Intergenomic chromosome relationships in hexaploid wheat. Proc. 10th Intern. Congr. Genet., Montreal 2: 258-259. (Received July 28, 1971) |
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