165introgressed rye chromosome arm also carries a locus affecting root biomass in
wheat (Ehdaie et al. 2003 ) and its presence positively affects yield, especially under
drought stress (Rajaram et al. 1983 ; Villareal et al. 1998 ). A similar yield effect was
observed in wheat with an introgression from an Agropyron species (Singh et al.
1998 ), where the responsible genes have already been identifi ed and cloned (Placido
et al. 2013 ).
The techniques for transfers of chromatin from related species into crops are not
new (see Jiang et al. 1994 ) but only crossing over is capable of producing compen-
sating chromosome translocation s with predictable regularity. It requires certain
minimum lengths and levels of DNA sequence homology so it is unlikely to take
place between unrelated chromosomes or in non-corresponding positions. However,
the effect of the Ph1 locus, or, rather, its absence when homoeologous recombina-
tion is induced, is the unknown and unquantifi ed factor. Crossing over in the absence
of Ph1 , especially that involving homoeologues, may be different not only quantita-
tively but also qualitatively from the very stringent exchanges taking place with Ph1
present. Be it as it may, crossing over still appears as the most reliable approach to
production of alien introgressions. E.R. Sears who invented the approach consid-
ered it the easiest (Sears 1972 ) but it might have been overly optimistic. The
approach is very demanding in terms of labor and resources. Because pairing and
recombination of alien chromosomes with their wheat homoeologues is infrequent,
and because linkage drag demands crossover points in specifi c, narrow intervals,
there is no way around l arge populations. Regardless of what technique is used,
whether it is cytogenetic or DNA marker, whether plants are bulked or analyzed
individually, thousands must be screened to assure proper precision. Only our
Chinese colleagues are lucky to fi nd perfect centric translocation from monosomic
addition s, and introgressions of small intercalary segments without ever manipulat-
ing chromosome pairing (Ren et al. 2009 ; Fu et al. 2010 ). For everybody else, the
prospect of sifting through large populations is intimidating and so over the years
many other options have been tried. These include random chromosome fragmenta-
tio n by irradiation or gametocidal chromosomes (Massoudi-Nejad et al. 2002 ) and
even karyotypic aberrations occurring in tissue culture (Lapitan et al. 1984 ).
Unfortunately, these almost always end up with predictable results: because the
points of breakage (and then fusion) are random the resulting translocation s are not
compensating. They may create pretty pictures but not particularly useful products
with acceptable agronomic value. There is hope that new molecular techniques will
alleviate the problem (Wulf and Moscou 2014 ), but this may be long in coming. For
the time being the GMO label is not welcome in many regions of the world and even
if and when it starts to be ignored, there will still be the issue of identifi cation, isola-
tion, and the ownership of specifi c genes in alien species (relative to wheat ).
Chromosome engineering is commonly associated with methods of cytology and
long hours over the microscope. In fact, all chromatin manipulations as described
here can be performed just as well, if not better, using the techniques of molecular
biology, but usually far more expensively. Until we learn how to manipulate cross-
ing over for practical purposes, population sizes will remain large regardless of
techniques used to screen them.
7 Introgressions Between Wheat and Rye