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If more variation is introduced into the crosses by making double crosses
(F1 hexaploid triticale x F1 bread wheat) there should be a better chance
to obtain different chromosome combinations, since all gametes forming
the hybrids have a different genetic content and there is a higher chance
of introducing structural differences. The use of amphidiploids with wild
rye could increase the chromosomal variation, but has the disadvantage
of introducing agronomically inferior characters. Also from a recombination point of view the double cross is preferable in this case. The F1 between hexaploid triticle and bread wheat has the genomic constitution AABBDR. Only the A and B genomes have the opportunity to pair in meiosis and exchange alleles. The chromosomes of the single D and R genomes are inherited without recombination. This decreases the over all recombination by one third. If a double cross is made, all genomes have the possibility to take part in recombination, which results in a wider variation in segregating generations. In octoploid x hexaploid triticale crosses it is also advisable to use double or three way (octoploid F1 x hexaploid) crosses to increase recombination and the chances of substitution. In this type of cross the D genome has no opportunity to recombine in a single cross. This is of importance since chromosomes from this genome can replace rye chromosomes in substitutions. In crosses between octoploid and hexaploid triticale, and in crosses between hexaploid triticale and bread wheat the F1 and the early generations have an unbalanced chromosome composition resulting in meiotic disturbances and aneuploid chromosome numbers. This causes an increased frequency of inferior and more or less sterile plants. By natural selection and by increasing homozygosity in later generations chromosomal balance is restored. This means that a bulk propagation in earlier generations and selection of individual plants in later generations is the most suitable breeding method for this type of crosses. In intercrosses between hexaploid triticales the F1 also has an unbalanced chromosome constitution if the crossed lines have differences in their chromosome composition. This results in chromosomal disturbances, however not as severe as in the crosses discussed earlier. In this case it should be possible to rely on single crosses and make selections in early generations. Literature Cited GUSTAFSSON, J.P. and F.J. ZILLINSKY 1973. Identification of D-genome chromosomes from hexaploid wheat in a 42 chromosome triticale. - 4th Int, Wheat Genet. Symp. columbia, Missouri 1973. JENKINS, B.C. 1969. History of the development of some presently promissing hexaploid triticales. - Wheat Inf. serv. 28 : 18-20. KISS, A. 1966. Neue Richtung in der Triticale-Zuchtung. - Z. pflanzenzucht. 55 : 309-329. KRANZ, A.R. 1963. Beitrage zur cytologischen und gentischen E-volutionsforschung an dem Roggen. - Z. pflanzenzticht. 50 : 44-58. LARSEN, J. 1973. The role of chromosomal interchanges in the evolution of wheat. Triticum aestivum. 4th Int. Wheat Genet. Symp. Columbia, Missouri 1973. LARTER, E.N. and S.L.K. HSAM 1973. Performance of hexaploid triticale as influenced by source of wheat cytoplasm. - 4th Int. Wheat Genet. Symp. Columbia. Missouri 1973. MERKER, A. 1973. A Giemsa technique for rapid identification of chromosomes m Triticale.- Hereditas 75 : 280-282. MERKER, A. 1975. Chromosome composition of hexaploid triticale. - Hereditas 80 (in press). MUNTZING, A. 1972. Experiences from work with octoploid and hexaploid ryewheat (Triticale). - Biol. Zentralbl. 91 : 69-80. PISSAREV. V. 1963. Different approaches in Triticale breeding. - 2nd Int. Wheat Genet. Symp. Hereditas Suppl. 2 : 279-290. THOMAS, J.B. and P.J. KALTSIKES 1972. Genotypic and cytological influences on the meiosis of hexaploid triticale. - Can. J. Genet. Cytol. 14 : 889-898. ZILLINSKY, F.J. and N.E. BORLAUG 1971. Progress in developing. triticale as an economic crop. - CIMMYT Res. Bull. 17 : 1-27. Communciations from the Swedish Seed Association No. 424. (Received May 15, 1975) |
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