175products. In arm-fl ip exercises with chromosomes 2R and 2B, 35 % of progeny
from double monosomics carried at least one misdivision product (Brunell et al.
1999 ) while for 3R and 3B, this frequency was 6.1 % and only 11 centric transloca-
tions (including all possible arm combinations) were recovered from among 1891
plants screened. Chromosome 3B misdivided three times less frequently than chro-
mosome 3R (Lukaszewski unpublished). On the far end of the spectrum are several
wheat chromosomes that either do not misdivide in any detectable frequency, or one
of their misdivision products cannot be recovered. The same chromosomes from
other sources misdivide with usual frequencies. For this reason, E.R. Sears was
forced to import into Chinese Spring several telocentrics which could not be pro-
duced from Chinese Spring chromosomes (Sears and Sears 1978 ). Lukaszewski
attempted to generate original Chinese Spring telocentrics for these imports but so
far, only 6AS was created. Not a single misdivision product was observed among
over 500 pro geny of a 5D monosomic screened but when 5D from a different source
was tested, both telocentrics and isochromosomes of both arms were recovered
from a much smaller sample.
7.5 Translocations Produced by Chromosome
Fragmentatio n
If the targeted alien (rye?) chromosome arms are syntenic with their wheat counter-
parts, and introgression s are produced by controlled means such as centric breakage-
fusion of designated chromosomes or by homoeologous recombination, the resulting
translocations are genetically balanced and offer a chance of agronomic benefi t.
Unfortunately, cytogenetic stocks suitable for controlled production of centric
translocations are available only in very few genetic backgrounds and these are not
necessarily much desired in wheat breeding. Homoeologous recombination, which
assures proper placement of alien chromatin in the wheat genome, is even more dif-
fi cult to handle and always requires screening large populations. For this reason, in
times of need, many people resort to random processes. Their advantage is the ease
of use; the disadvantage is randomness. While there is a statistical chance of placing
the desired segment of alien chromatin in its proper position in the wheat genome,
this chance is low. In most cases, the targeted segment is inserted in a random posi-
tion in the wheat genome, it is non-compensating, and the degree of non-compensa-
tion depends on the length of the inserted segment as well as the effect of the
absence, if any, of a part of the wheat genome.
Random insertions are produced by fragmentation of the wheat genome
together with the alien donor chromosome; DNA repair mechanisms stitch broken
pieces together, almost never in the proper order. Therefore, random transloca-
tions can only differ in their degree of non-compensation. If done on a large
enough scale, there is a chance that the transferred segment will be small and
placed in a position in the wheat genome where little harm is done. As a result, the
benefi t of the introgression may outweigh its possible problems. The fi rst one of
7 Introgressions Between Wheat and Rye