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his ignorance. The history of the 1RS.1BL translocation includes a cultivar with the
1R(1B) substitution (Zeller and Hsam 1983 ) but apparently this was not good
enough to withstand the selection pressures in wheat breeding. This is perhaps
because substitution of the long arm of rye 1R for any of the group-1 long arms of
wheat reduces seed set and makes plants appear weaker, thinner, more slender with
clearly smaller grains and this effect extends to whole chromosome substitutions in
homoeologous group 1 (Kim et al. 2004 ).
Chromosome substitutions (Fig. 7.1 ) helped allocate individual chromosomes of
rye to wheat homoeologous groups. In most cases, a rye chromosome compensates
(that is, is homoeologous to) even if incompletely, for just one homoeologous group
of wheat, but within that group it may show more affi nity to one chromosome than
to another (Sears 1968 ).
Exceptions are rye chromosomes 4 and 7 where each one of them shows some
affi nity to both wheat ; homoeologous groups 4 and 7 (Zeller and Hsam 1983 ).
Chromosome rearrangements during rye evolution after the wheat–rye split differ-
entiated the rye genome from those of wheat to a considerable extent. DNA markers
made mapping of these rearrangements possible and a detailed map of individual
segments’ affi nities is available (Devos et al. 1993 ).
Fig. 7.1 Substitution of rye chromosome 2R for wheat chromosome 2B in hexaploid wheat. Here,
2R was reconstituted f rom two centric translocations, 2RS.2BL and 2BS.2RL. Rye chromatin
labeled green ; wheat chromatin red
A.J. Lukaszewski