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genes carrying agronomically important traits, such as rust resistance from diverse
Aegilops species (Aghaee-Sarbarzeh et al. 2002 ; Chhuneja et al. 2008 ), or salt toler-
ance from the less related species Thinopyrum junceum (Wang et al. 2003 ). The
great practical advantage of Ph I^ strains, over Ph1 − or ph1b mutant stocks, for obtain-
ing alien introgression into wheat by meiotic recombination, is that Ph I^ has a domi-
nant effect capable of inducing homoeologous pa iring in the wheat–alien F1 hybrid
itself as well as in the following backcross generations (Marais et al. 2010 ; Li et al.
2011 ).
6.6.2.2 Ph -Like Genes in Other Wheat Relatives
Inhibition by alien species genes of the wheat Ph system has mostly been character-
ized in Ae. speltoides. However, many studies demonstrate that genes affecting the
wheat diploidizing control system exist in several other species of Aegilops , Secale ,
Agropyron , or Elymus.
The presence of homoeologous pairing promoting genes in the diploids Ae. lon-
gissima and Ae. mutica was evidenced as early as reported for Ae. speltoides (Riley
1966b ; Mello-Sampayo 1971b ). Mello-Sampayo ( 1971b ) observed that the amount
of MI pairing in wheat × Ae. longissima hybrids was signifi cantly higher than in
hybrids with Ae. sharonensis , though lower than in hybrids with Ae. speltoides. The
segregation for various levels of homoeologous pairing found in hybrids of distinct
Ae. mutica accessions with Chinese Spring led to suggest allelic variation at two Ae.
mutica loci for the high and low pairing phenotypes (Dover and Riley 1972a ). The
expression of those ph -like genes was only evidenced when at least one chromo-
some 5B was present but seemed to be unaffected by the Ph1 dosage or by the pres-
ence of Ae. mutica B chromosomes (Dover and Riley 1972a , b ).
A promoting effect on wheat × alien homoeologous pairing was also reported in
some lines of Ae. variabilis (syn. Ae. peregrina ) (Farooq et al. 1990a , b ; Fernández-
Calvín and Orellana 1991 ) and Ae. geniculata (syn. Ae. ovata ) (McGuire and
Dvořák 1982 ; Farooq et al. 1996 ). Between-lines variation for Ae. variabilis ph -like
genes concerns not only their presence or absence but also their interaction with the
Ph1 gene. To this regard, the effect of some promoting genotypes is only detectable
in Ph1 hybrids (Fernández-Calvín and Orellana 1991 ), whereas the pairing pro-
moter genes present in other genotypes additively interact with the ph1b mutation
(Farooq et al. 1990b ). Paring regulators infl uencing the meiotic behavior of wheat–
alien hybrids are su spected in other allotetraploid Aegilops species (McGuire and
Dvořák 1982 ; see also Jenczewski and Alix 2004 ).
Lelley ( 1976a ) conducted MI pairing analyses of hybrids between the seven
wheat– rye cv. ‘Imperial’ addition lines and the Secale species S. cereale and S.
montanum. This study evidenced the presence of alleles suppressing the wheat Ph
control on chromosome 3R.
Within the genus Agropyron , gene s interfering the homoeologous pairing restrict-
ing system of wheat have been identifi ed in diploid and autotetraploid lines of A.
elongatum (E genome) and A. cristatum (P genome) (Dvořák 1987 ; Charpentier
T. Naranjo and E. Benavente