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Many other centric wheat–rye translocations have been produced in wheat but
none seems to have made it into commercial production even when they offer a
good set of disease resistance genes, such as translocations of the long arm of chro-
mosome 2R (Friebe et al. 1990 ; Hysing et al. 2007 ). The author himself has all
possible centric translocation of rye 1R (both arms, all wheat homoeologues, Fig.
7.2 ), both 2B-2R and 2AS.2RL (the latter obtained from ER Sears), both 3R-3B,
7DL.4RL, 5BS.5RL, 6BS.6RL and two of 7RL presumably to 7D. Several other
translocations, including those with similar structure to the above but from unre-
lated sources have been described in the literature (Friebe et al. 1996 ). The old
2AS.2RL translocation with the long arm originating from Imperial rye is an inter-
esting one, as it was shown to reduce yield reduction under water stress (Lahsaiezadeh
et al. 1983 ).
The two widespread wheat–rye translocations, 1RS.1BL and 1RS.1AL are cen-
tric, that is, they were formed by misdivision of centromeres and fusion of two
chromosome arms broken at the centromere. Both were lucky occurrences; they
were not planned or designed. By design, centric translocations can be produced
from any two chromosomes as long as each one has a tendency to break at the cen-
tromere when present as a univalent in meiosis. The process does not require homol-
ogy, homoeology, affi nity, similarity. Any arm combination can be produced from
any two chromosomes; the process of arm fusion is random and so compensating
translocations (short arm-long arm, S.L, and long-arm-short arm, L.S) are produced
as frequently as the non-compensating combinations (S.S or L.L). Of course, only
compensating translocations have a chance of surviving breeding pressures; others
can be maintained for experimental purposes, such as homoeo-isochromosomes
tested for the mode of action of the Ph1 locus in wheat (Dvorak and Lukaszewski
2000 ). Because of sister chromatid cohesion in the fi rst meiotic anaphase, break-
points are not always located in the centromere itself (understood as the region of
the chromosome underlying the kinetochore) but may occur in the regions fl anking
the centromere (Lukaszewski 2010 ). Cytogenetic exercises with un-translocating
and re-translocating chromosome arms in centric translocation led to introduction
of rye centromeres into wheat chromosomes (Zhang et al. 2001 ); these do not
appear to have any detectable effect on the behavior of the chromosomes or plants
carrying them.
The two widespread translocatio ns in wheat, 1RS.1BL and 1RS.1AL, occurred
in progenies of triticale × wheat hybrids but both appear to be spontaneous events.
Both involve wheat chromosomes that should not have been univalent to misdivide.
Counting on such occurrences may be disappointing so luck usually needs a bit of
help. Centric translocations are relatively easy to produce, if suitable aneuploid
lines are available and targeted chromosomes misdivide. It is not clear at all what
makes a chromosome susceptible to misdivision, and considerable variation in sus-
ceptibility is present. Among wheat group-1 homoeologues in cv. Pavon 76, chro-
mosome 1B misdivides the most frequently; chromosome 1D the least, hinting that
chromosome length may be a factor. Chromosomes reconstructed from centric
translocations tended to misdivide more often than their original counterparts
(Lukaszewski 1997 ) and over 20 % of screened progeny carried centric misdivision
A.J. Lukaszewski