16 4
of the art when it comes to chromosome manipulations. The body of literature is
very large and this chapter cannot possibly aim to mention every contribution ever
made. It attempts, however, to present the most important points.
Specifi c potential advantages of rye in wheat breeding, allocated to individual
chromosomes and even chromosome arms, were well reviewed and summarized by
Zeller and Hsam ( 1983 ). Since then the number of identifi ed characteristics useful
in wheat breeding must have doubled, many of those were allocated to chromo-
somes, chromosome arms, and even chromosome segments. The list is now long
and it includes resistance to various diseases and pests such as leaf, stem, and yel-
low rust, powdery mildew (Ren et al. 2009 ; Yang et al. 2009 ; An et al. 2013 ) barley
yellow dwarf virus (Nkongolo et al. 1992 ), nematodes (Asiedu et al. 1990 ), several
destructive aphid species (Lukaszewski et al. 2001 ; Crespo-Herrera et al. 2013 ),
Hessian fl y (Friebe et al. 1990 ; Mukai et al. 1993 ), better zinc and copper effi ciency
(Schlegel et al. 1991b , 1998 ), restoration of male fertility in timopheevi cytoplasm
(Curtis and Lukaszewski 1993 ), and many more, even including potential weed
control (Bertholdsson et al. 2012 ). A very specifi c class of newly identifi ed param-
eters are the characteristics of the root system, such as those encoded by loci on rye
chromosome arms 1RS and 2RS (Ehdaie et al. 2003 ; Waines and Ehdaie 2007 ;
Sharma et al. 2011 ). The 1RS arm has already made an immense contribution to
wheat production worldwide, even though it has only recently been recognized as
such (Waines and Ehdaie 2005 ).
In most cases, the term “alien introgression ,” as awkward as it is, is reserved for
chromatin transfers into cultivated crops from its wild relatives. Depending on the
genetic distance and the isolation barriers separating the donor and the recipient, the
issue may be as simple as the cross-backcross approach combined with selection for
the desired allele, or as complicated and tedious as enforcement of crossing over
between distantly related chromosomes, selection of translocation breakpoints in
the desired intervals and assembly of intercalary alien segments in recipient wheat
chromosomes. Because donors are wild, undomesticated species, the issue of the
so-called linkage drag is always present; the transfers have to involve as little chro-
matin as possible to exclude any fl anking loci with potential negative effects such as
weediness. In this sense, rye , a cultivated species itself, would appear to be an easier
source, as domestication has already removed most of its undesirable wild charac-
teristics and one could perhaps assume that the precision required for the introgres-
sions would be less. Unfortunately, experience so far shows that this is not always
the case.
While it is generally believed that the transfers should be as small as possible to
minimize linkage drag , linkage drag is not always bad, and unexpected benefi ts may
appear from larger introgression s. At times, unknown and unrecognized alien loci
linked to the targeted ones, in the background of cultivated wheat may produce
astounding effects. It is more than likely that the poster child of alien introgressions
into a crop, the wheat–rye translocation 1RS.1BL in wheat , appeared under selec-
tion pressure for disease resistance (three cereal rust and one powdery mildew resis-
tance loci on the 1RS arm). However, the translocation really took off globally well
after some or all these resistance genes had broken down. We now know that the
A.J. Lukaszewski