181from the engineered chromosome with Sec - 1 (Howell et al. 2014 ). Given that the
resistance loci are now of little value, the entire exercise which required cytological
screening of well over 25,000 plants, remedied the translocation problems but also
removed its main advantage. Now new primary recombinants have to be isolated to
fi nd a breakpoint between the Sec - 1 and the root biomass locus.
Induced homoeologous recombination exercises always require fair amounts of
labor; the amount depends on the desired level of precision and the affi nity between
the engineered chromosomes. However, at times it does not work no matter how
much effort is invested. Failed attempts are not routinely reported but several have
been published. Dundas et al. ( 2001 ) failed to recover any recombinant chromo-
somes in an attempt to transfer a cyst nematode resistance locus from the long arm
of rye chromosome 6R to a wheat homoeologue among 3786 progeny screened.
Screening was done using DNA markers so perhaps some recombinants involving
non-designated chromosomes were present but not identifi ed. Lukaszewski et al.
( 2001 ) screened 3563 progeny in an attempt to transfer from rye chromosome arm
4R L a locus for resistance to Russian wheat aphid. Screening was done using cyto-
logical markers in terminal positions so any single crossover event should have been
Fig. 7.4 A pair of engineered translocations 1RS.1BL of the Aurora/Kavkaz origin. Wheat chro-
matin labeled green /rye chromatin red. The 1RS arms of the translocation carry two wheat inserts:
the proximal one removes the Sec - 1 locus of rye; the distal one introduces wheat loci Gli - B1 and
Glu - B3. The rye segment separating the two inserts contains four relevant resistance loci
7 Introgressions Between Wheat and Rye