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7.2 Addition Lines
If introgression is understood as insertion of alien chromatin into the genome of
another species, here in a crop such as wheat, addition lines do not entirely fi t into
the picture. Addition lines, as the name implies, are sets of lines where single chro-
mosomes of a donor are ADDED to the genome of the recipient. Addition lines can
be monosomic or disomic, depending on the dosage of the added chromosome.
Added chromosomes in disomic additions are transmitted to progeny with suffi cient
fi delity for easy maintenance, but insuffi cient fi delity to be of any agronomic impor-
tance. In research, they are routinely tested for chromosome numbers or chromo-
some constitution to make sure that the alien pair is indeed present; in large scale
grow-outs they have a tendency to quickly lose added chromosomes.
That addition of alien chromosomes to the wheat genome is technically possible
was fi rst demonstrated by O’Mara ( 1940 ) who created several disomic additions of
cv. ‘Imperial’ rye to wheat. This set was completed only many years later (Driscoll
and Sears 1971 ) and it became a standard tool in wheat and rye genetics. A wonder-
ful tool it is indeed: it separates the entire genetic contents of a donor species into
individual linkage groups (chromosomes). Using it, chromosome location of spe-
cifi c loci can be determined and specifi c individual chromosomes can then be
manipulated to generate more precise transfers into the genome s of crops. Once the
chromosome location of a locus of interest is determined, the addition lines are a
convenient starting point for the introgression work, to create substitution , translo-
cation or recombinant lines. So, while addition lines are not introgressions in the
true sense, they are often the fi rst step in the process of creating introgressions and
hence appear worthy of some space here. In a feat surely never anticipated by
O’Mara or Sears, such added chromosomes can now be sorted by fl ow cytometry,
creating large samples of DNA from specifi c chromosomes or chromosome arms.
In the standard O’Mara approach, addition lines are created by backcrossing a
wheat–alien species hybrid (F 1 or an amphiploid) to wheat to create a heptaploid
where the entire alien genome is present in a single dose. For a wheat–rye situation
the crossing scheme is: AABBDD × RR = ABDR, chromosome doubling to
AABBDDRR, backcross to wheat to create a heptaploid AABBDDR. This hepta-
ploid is backcrossed to wheat again, plants with 42 + 1 chromosomes (single rye
chromosomes present) selected and self-pollinated to create disomic additions.
There are two diffi cult steps in this procedure, one of which appears to have been
solved by technology. The fi rst is identifi cation of 42 + 1 plants with each of the
seven rye chromosome present at least once. It used to be a very demanding task; it
was simplifi ed once various chromosome identifi cation techniques entered the fi eld
(cytological or DNA markers). The second diffi cult step is selection of disomic addi-
tions among progenies of monosomic additions. In wheat, monosomic (unpaired)
chromosomes are included into gametes with ca. 25 % frequency (Sears 1954 ).
Competition for fertilization on the male side strongly favors balanced, 21-chromo-
some gametes. As a consequence, the recovery rate of disomics is only in the range
of 1 %. For a 95 % probability of recovering a disomic, ca. 300 progeny have to be
screened for each chromosome, or 2100 for the entire set (for a seven- chromosome
A.J. Lukaszewski