169Whole chromosome substitutions can be generated in several different ways. If
disomic additions are available, they can be crossed as male to monosomics or
nullisomics of wheat, creating either double monosomics (one rye and one wheat
chromosome present in the targeted homoeologous group), or monosomic substi-
tutions of the alien chromosome. The latter are easier to work with as upon self-
pollination they quite readily generate disomic substitutions (about 25 %
frequency, consistent with the 25 % inclusion rate of monosomics in the gametes
and strong selection for euploid gametes on the male side, Sears 1954 ).
Unfortunately, only 12 nullisomics of wheat are available, and almost all of them
in cv. Chinese Spring. If double monosomics are created (one alien chromosome
and one wheat homoeologue), the issue of competition for fertilization between
the two chromosomes comes into play and with few exceptions (the so-called
cuckoo chromosomes), alien chromosomes tend to lose. No matter, this approach
to production of alien chromosome substitutions can be called “direct” as specifi c
chromosomes are placed into specifi c positions in one of the three genome s of
wheat, one chromosome at a time. A less direct approach is by intercrossing, say,
hexaploid triticale, genomic constitution AABBRR or nearby, with hexaploid
wheat, creating a hexaploid hybrid AABBDR. Upon self- pollination a series of
chromosome substitutions can be created, with both single and multiple substitu-
tions possible. The bulk of substitutions will be for the D-genome chromosomes,
but exceptions to this are possible and not infrequent. The success rate for any
individual rye chromosome depends on its compensating ability for the corre-
sponding D-genome chromosome. Sets of substitutions were developed in this
manner by Shchapova and Kravtsova ( 1982 ), Merker ( 1984 ), Friebe and Larter
( 1988 ), Alkhimova et al. ( 1999 ), and Silkova et al. ( 2006 ); more are probably
available but not published on. This process of producing substitutions could per-
haps be simplifi ed by backcrossing the F 1 as male to triticale, selection of plants
disomic for specifi c rye chromosomes followed by several generations of self-
pollination to eliminate monosomics. A somewhat hybrid approach is by inter-
crossing sets of monosomics/nullisomics of wheat with wheat–rye amphiploids
(say, an octoploid AABBDDRR) and sorting out chromosome constitutions after
several cycles of self-pollination. For any specifi c targeted substitution, the origi-
nal cross would create a double monosomic (one wheat and one rye chromosome)
and with suffi cient homoeology (compensating ability) of the rye chromosome, a
substitution is possible. Again, this approach demands reliable chromosome iden-
tifi cation and patience. An added advantage using monosomics/nullisomics is that
should any centric translocation s occur, they should involve wheat chromosomes
from the donor plant (here the octoploid) and not the nullisomic/monosomic par-
ent. In this sense, involvement of chromosomes of Chinese Spring, not the most
desirable genetic background in wheat breeding, can be easily avoided. The issue
is not trivial: once a centric translocatio n is formed, the proximal part of the wheat
chromosome arm will remain unchanged by crossing over, as crossing over in
wheat is limited to distal halves of the arms.
7 Introgressions Between Wheat and Rye