177are about the same. Long series of chromosome defi ciencies have been created using
the gametocidal chromosomes, not only for wheat but also rye and barley chromo-
somes in wheat (Massoudi-Nejad et al. 2002 ; Endo 2003 ), but the author is not
aware of any agricultural signifi cance of the translocations that might have been
produced.
Given the predictably high amount of work needed to produce translocations via
chromosome pairing and recombination, many keep trying shortcuts, with irradia-
tion being the means of choice. At times it does succeed but the issue brings on an
interesting question: what is the minimum acceptable level of chromosome com-
pensation in agricultural practice? Incomplete compensation created by unbalanced
chromosome translocation may affect grain yield; what level of grain yield loss can
be accepted in exchange for a defi ned benefi t, such as resistance, tolerance or simi-
lar? It is this author’s experience from years of interactions with breeders that the
ultimate criterion deciding of cultivar release/acceptance is grain yield and tests are
usually performed in stress-free environments. As a general rule, breeders do not
acce pt yield reduction in exchange for a promise of greater yield stability in the
event of an infection, heat wave or similar calamity. This in itself is interesting as in
traditional agriculture yield stability was far more important to farmers than a prom-
ise of large yields under favorable conditions (Denison 2012 ).
7.6 Induced Homoeologous Recombination
To those who deal with interspecifi c introgression s it is obvious that only crossing
over has the capacity to deliver the desired segment of alien chromatin into its
proper position in the crop’s genome. No method based on chromosome fragmenta-
tion, physical or genetic, can reliably produce a genetically balanced introgression.
Given genome sizes and an almost infi nite number of possible break positions, it is
practically impossible to place the desired piece of alien chromatin in its correct
position in the wheat genome while removing the corresponding portion of the
wheat genome to avoid any gene dosage issues. All transfers made by random frag-
mentation are, by defi nition, non-compensating and differ only by the degree of
non-compensation. The bad ones are rejected outright; the few good ones with min-
imal adverse effects may be tolerated when the benefi t they offer outweigh prob-
lems. However, the fi rst alien chromosome engineering effort in wheat was done by
irradiation (that is, chromosome fragmentation ) and it was accepted in agriculture
to considerable benefi t (Sears 1956 ).
Introgression s via chromosome fragmentatio n seem quick, cheap and effective,
but most of the time they generate stocks unsuitable for production agriculture. On
the other hand, introgressions via homoeologous recombination are guaranteed
laborious and can be very time consuming but once certain assumptions are met,
they guarantee the desired outcome. In wheat, the approach via homoeologous
recombination requires a method of disabling the Ph1 s ystem which enforces strictly
homologous chromosome pairing, by imposing some criteria of chromosome affi nity.
7 Introgressions Between Wheat and Rye