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detected, no matter which wheat homoeologues were involved. Two recombinants
were recovered and confi rmed by probing with labeled genomic DNA but both were
associated with poor vigor of carrier plants, and the effort was abandoned. Both
failures were probably due to structural differences between wheat and rye chromo-
somes: the 4RL arm is composed of segments homoeologous to wheat 7S and 6S
groups while 6RL is composed of segments homoeologous to wheat homoeologous
groups 6L, 3L, and 7L (Devos et al. 1993 ). It is not known in which segments of the
4RL and 6RL arms the targeted loci were located. Perhaps terminal defi ciencies
such as those recovered by Dundas et al. ( 2001 ) and those produced by gametocidal
chromosomes (Endo 2003 ) can be used to recombine the carrier segment with its
corresponding homoeologous segment in the wheat genome, without interference
from segments constituting the structural difference and homoeologous to different
wheat chromosomes that very likely disturb meiotic pairing and make the transfer
impossible. This approach has never been tried, and it would be a complex one. Not
only proper defi cient chromosomes would have to be created, the transfer by
induced homoeologous recombination affected, but the normal structure of the
recipient chromosome would have to be restored at the end of the exercise. The
author once attempted to transfer a dwarfi ng gene from 4DS to 4AL (chromosome
4A is inverted relative to other group-4 chromosomes of wheat and on L it carries
segments originating from two other chromosomes) but the effort lost in competi-
tion for microscopy time with projects offering more immediate payoffs.
Once collections of recombined wheat–alien chromosomes are generated, usu-
ally with much effort, they can be used to transfer onto the alien segments, again by
crossing over, specifi c desired alleles from an alien species. In this fashion Cainong
et al. ( 2010 ) introduced an allele for Hessian fl y resistance into wheat using preex-
isting recombinant chromosomes 2BL-2RL. This is not necessarily as easy a pro-
cess as it appears, even though only homologous recombination is involved (between
the alien segment already translocated to a wheat chromosome and a donor alien
chromosome). The success rate, or, rathe r, the size of the progeny population needed
to recover the desired product, depends on the distance between the translocation
breakpoint and the locus of interest. Differences in chromosome confi gurations may
severely limit the crossover rate between such two chromosomes.
7.7 Introgression s from Wheat to rye
While rye has been a popular donor of specifi c loci to wheat, the reverse has never
been seriously considered. Of course, transfers from a diploid to polyploid can now
be called routine, no matter how complicated and labor intensive they may be; trans-
fers into diploids are inherently more diffi cult as diploids poorly tolerate some types
of chromosome aberrations. However, such transfers can be done, and much easier
at the tetraploid level than diploid. Probably the fi rst attempt at introgression from
wheat to rye was by Schlegel et al. ( 1991a ) who introgressed wheat chromosome 5B
into diploid rye. Lukaszewski (unpublished) introduced each of the B-genome
A.J. Lukaszewski