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such random introgressions was produced by E.R. Sears, to transfer a leaf rust
resistance locus from Aegilops umbellulata (Sears 1956 ). This translocation
offered substantial agronomic benefi t thereby setting the trend. Sears ( 1993 ) pre-
sented a complete protocol for irradiation-induced chromatin transfers into wheat;
unfortunately, many attempts that followed were not nearly as precise and as suc-
cessful as the original one of Sears ( 1956 ).
Two quite popular transfers were nominally by irradiation but ended up as ce n-
tric translocations. The fi rst was the already mentioned translocation 1RS.1AL in
cv. Amigo (Sebesta and Wood 1978 ) the other is a transfer of a segment of a
Haynaldia villosa chromosome to wheat by irradiation that started from a centric
translocation of an entire short arm of chromosome 6V (Chen et al. 1995 ). In both
cases it is more than likely that the translocations originated by centric misdivision;
perhaps irradiation was responsible in some degree for poor chromosome pairing
but it is unlikely that it produced the desired chromosome breaks. Follow-up irradia-
tion reduced the H. villosa segment to only a fraction of the arm (Chen et al. 2008 )
but the fi rst result s of a recombination study indicate that the remaining small frag-
ment of H. villosa chromatin, while residing on the correct wheat chromosome arm,
may not be in its correct position (Lukaszewski, unpublished data).
In most cases, however, transfers by random means are truly random and non-
compensating, and have little chance for acceptance in commercial agriculture.
Many such translocations have been produced in wheat; many are reviewed and
illustrated by Friebe et al. ( 1996 ). The irradiation approach is usually undertaken to
avoid large samples required for transfers via homoeologous recombination but the
savings may be illusory. E.R. Sears ( 1956 ) selected the Ae. umbellulata transfer
from among 6000 progeny of irradiated plants. The initial size of the progeny popu-
lation was not given for the Pm21 transfer from H. villosa but 100 resistant progeny
were screened cytologically and several with the centric translocation were ident i-
fi ed. For chromosomes with normal misdivision frequency the author usually
screens ca. 100 progeny to identify all possible centric fusion combinations and
almost always fi nds them, especially for such readily misdividing chromosomes as
1R or 6V. On the other hand, the 6VS arm recombines with 6A of wheat with ca. 3
% frequency and screening of ca. 1000 progeny of ph1b p lants yielded 32 recombi-
nants of the H. villosa arm. In this sense, standard cytogenetic approaches are not
necessarily more laborious.
An interesting translocation, also created by irradiation, was Transec (Driscoll
and Anderson 1967 ). It was created to introduce leaf rust and powdery mildew
resistance from rye into wheat but was also hoped to offer a system of hybrid wheat
production. Since the translocation removes chromosome arm 4BS carrying a locus
critical for male fertility, translocation homozygotes are male sterile. Unfortunately,
no reliable system of hybrid wheat production could ever be established based on
Transec.
Fragmentation of the donor alien chromosome can also be accomplished by the
so-called gametocidal chromosomes (Endo 2003 ). These chromosomes appear to cut
other chromosomes indiscriminately and in this sense they may be no different than
irradiation. Therefore, chances of recovery of compensating balanced translocations
A.J. Lukaszewski