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7.4 Centr ic Translocation s
A note on terminology: it has been proposed to describe centric translocations in
wheat by listing the recipient arm fi rst and the donor second, separated by a “.”.
Following this rule, the most common wheat–rye translocation in wheat should be
denoted as T1BL.1RS, and often is. This system of notation is background depen-
dent: when a wheat–rye translocation is transferred from wheat to triticale or to rye ,
the background changes and the order of arms should be fl ipped, to T1RS.1BL. In
other words, the notation depends not on the arm composition of the translocation
but on the genetic background in which it is present. Since the author works with
both wheat and triticale and encountered this problem numerous times, in this chap-
ter the notation will list the short arms fi rst, followed by the long arms that is,
1RS.1BL. This still leaves unresolved the issue of the homoeo-isochromosomes,
such as, say, 1BL.1RL.
The poster child of rye introgression s into wheat is the 1RS.1BL translocation. It
involves entire chromosome arms, 1RS of rye and 1BL of wheat. With the break-
point in the centromere region it must have formed by misdivision of univalents and
fusion of telocentric chromosomes. Its history has been presented by Zeller and
Hsam ( 1983 ) and recently reviewed by Mujeeb-Kazi et al. ( 2013 ) with heavy
emphasis on the contribution from CIMMYT, so it will be outlined here only briefl y.
The translocation appeared among progenies of hexaploid triticale × wheat hybrids
selected for “triticale-like resistance to wheat diseases.” It involves a B-genome
chromosome of wheat and not 1D, as the pedigree would dictate, but such shifts
have been noted and may be a consequence of reduced chromosome pairing in
hybrids. The rye chromosome arm (1RS) carried into wheat four loci for resistance
to wheat fungal diseases: Lr26 , Yr9m , Sr31 , and Pm9. The translocation has spread
to breeding programs and commercial wheats all over the world (Rabinovich 1998 ).
In compendium assembled by Schlegel ( http://www.rye-gene-map.de/rye-
introgression /index.html ) among lines released since 2000 (not counting germplasm
releases and experimental lines), ca. 30 % carry the 1RS.1BL translocation. Among
US wheat tested in recent nurseries, about 12–13 % carry the 1RS.1BL transloca-
tion ( http://www.ars.usda.gov/Research/docs.htm?docid=11932 ). This is almost 50
% increase from 7.1 % of such lines in wheat yield test nurseries in 1989
(Lukaszewski 1990 ). It is interesting that most of the spread of the translocation
occurred when, or after, the rye resistance genes have broken down and contributed
little value in breeding. Measurable yield effects have been associated with this
translocation, e ven in absence of disease pressure (Rajaram et al. 1983 ) but the
results of individual trials, in different environments and in different sets of wheats
were at times confusing, with some experiments showing a clear grain yield benefi t
of the translocation (Carver and Rayburn 1994 ; Villareal et al. 1997 , 1998 ; Kumlay
et al. 2003 ) while others showed no such effect (McKendry et al. 1996 ; Bullrich
et al. 1998 ; Singh et al. 1998 ; Espita-Rangel et al. 1999a , b ; Lelley et al. 2004 ).
Eventually, it was discovered that the rye chromosome arm 1RS carries a locus for
increased root biomass in wheat (Ehdaie et al. 2003 ; Waines and Ehdaie 2005 ,
A.J. Lukaszewski