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At times, even pairing of homologous chromosomes, such as those in inter- cultivar
hybrids, may be restricted by Ph1 (Dvorak and McGuire 1981 ).
The Ph1 system is disabled either by its removal, with two options available, or
by its suppression, by specifi c chromosomes of Aegilops speltoides (Feldman and
Mello-Sampayo 1967 ; Dvorak 1972 ; Chen et al. 1994 ). The Ph1 removal options
are either nullisomy for chromosome 5B (ditelosomy 5BS would be even better
except that it has not yet been produced) or by the ph1b mutation (Sears 1975 ,
1984 ), which in fact is an intercalary deletion of less than 3 Mb of DNA including
the Ph1 locus (Gill et al. 1993 ). The other locus of the system, Ph2 on 3DS, has a
minor effect on chromosome pairing and a combination of two mutations, ph1b and
ph2b , does not appear to improve the frequency of rye homoeologous paring in
wheat (Ceoloni and Donini 1993 ), so it does not increase the chances of success
while adding an additional degree of complication to the system.
Once the Ph system is disabled, homoeologous chromosomes are free to pair and
recombine. Their pairing frequencies appear to depend on two factors: the level of
affi nity and their structural similarity. Chromosomes 1, 3 and 5 from T. monococ-
cum pair and recombine with their wheat homoeologous (1A, 3A, 5A, respectively)
only rarely in the presence of Ph1 , b ut behave like normal homologues in its absence
(Luo et al. 1996 , 2000 ). Chromosomes from more distantly related species, such as
S. cereale rye, do not pair with wheat homoeologues at all when the Ph system is
present and operational; in its absence individual chromosome arms show surpris-
ingly large differences in pairing (and recombination), ranging from as high as ca.
12–13 % for 1RL or 2RL to essentially zero for chromosome 4R (Naranjo and
Fernandez-Rueda 1996 ). Of the 14 rye chromosome arms, only fi ve (1RS, 1RL,
2RL, 3RS, and 5RS) (Naranjo and Fernandez-Rueda 1991 ; Devos et al. 1993 ) are
colinear with wheat; the rest are translocated. Presence of such structural changes
must severely hinder pairing and recombination to the point where no recombinant
chromosomes can be recovered (Dundas et al. 2001 ; Lukaszewski et al. 2001 ). Even
relatively minor structural differences, such as the presence/absence of telomeric
C-bands on rye chromosomes, reduce pairing frequencies (Naranjo and Lacadena
1980 ; Naranjo and Fernandez-Rueda 1991 ). It would be an interesting exercise in
cytogenetics to test if removal of the translocated segment of an alien homoeologue
would permit pairing with its homoeologous segment of a wheat chromosome, so
that structural chromosome differences would no longer hinder chromosome engi-
neering in wheat. Such removal could be accomplished by chromosome fragmenta-
tion using the gametocidal chromosomes (Endo 2003 ). The original chromosome
structure would then have to be restored by recombination with a normal chromo-
some, however diffi cult it could be given the effect of heterozygosity for defi cien-
cies on MI pairing (Curtis et al. 1991 ).
The system of interspecifi c introgression by homoeologous recombination
was created by E.R. Sears ( 1981 ) and is used to this day, however sparingly. The
donor and recipient chromosomes are set up as monosomic and Ph1 i s disabled.
Recombination of the two chromosomes creates recombinants in two confi gura-
tions: wheat chromosomes with terminal segments of the alien chromosome; and the
alien chromosome with terminal segments of the wheat chromosome. Homoeologous
A.J. Lukaszewski