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Quinn ( 1970 ) and Farooq et al. ( 1990a ) compared the number of MI chromosome
association in hybrids between A. variabilis (2 n = 4× = 28, UUSS) and a series of
bread wheat varieties. Both analyses evidenced signifi cant differences in the extent
of homoeologous pairing between hybrids and concluded on the existence of wheat
varieties with a relatively higher pro moter pairing effect than the canonical variety
Chinese Spring. Similar results have been reported by Farooq et al. ( 1996 ) in hybrids
with Ae. geniculata (syn. Ae. ovata ; 2 n = 4× = 28, UUMM). Interestingly, the level
of synapsis in prophase-I nuclei and the frequency of chiasma at metaphase I are
higher in haploids of wheat varieties Thatcher and Chris than in haploids of Chinese
Spring (Martinez et al. 2005 ).
Up to date, the phKL gene of the Chinese landrace Kaixian-luohanmai (KL) is
the best well-characterized ph -like gene described in a wheat variety other than
Chinese Spring. This gene, assigned to chromosome 6A, shows an effect similar to
that of inactivity for the Ph2 locus in hybrid combinations with Ae. variabilis and
rye (Liu et al. 1997 ; cited by Liu et al. 2003 ). Liu and coworkers developed a
KL-derived line, which lacked 3DS, and demonstrated the additive effect of phKL
and the absence of Ph2 on promoting homoeologous pairing in wheat–alien combi-
nations (Liu et al. 2003 ). A further comparative analysis of the effects of the phKL
gene and the ph1b , ph2a , and ph2b mutations on homoeologous pairing in ABDUS
and ABDR hybrids, concluded that phKL has an intermediate effect between t he
mutations of the Ph1 and Ph2 genes (Xiang et al. 2005 ; cited by Hao et al. 2011 ).
However, phKL seems to have an homoeologous pairing promoting effect even
stronger than ph1b in hybrids with the less related species Psathyrostachys huashan-
ica (2 n = 2× = 14, NsNs) (Kang et al. 2008 ). By using GISH to discriminate wheat
from rye partners, Hao et al. ( 2011 ) showed a similar amount of wheat–rye MI pair-
ing in ABDR hybrids derived from phKL and ph1b wheat lines, the hig her overall
level of pairing in the ph1b hybrids being attributed to a higher amount of wheat–
wheat chromosome pairing. While its molecular characterization is in progress (Dr.
DC Liu, personal communication), all suggests that the lower the relatedness
between homoeologous genomes, the higher the promoting effect of the phKL gene.
Allelic variation for the control of homoeologous pairing has also been demon-
strated in crop and wild tetraploid wheats. Dhaliwal ( 1977 ) found that some lines of
emmer and timopheevii wheats compensated for the absence of chromosome 5B
while Ozkan and Feldman ( 2001 ) reported frequencies of chiasmata per cell rang-
ing from 0.56 to 6.32 in hybrids b etween a line of Ae. peregrina and several geno-
types of T. turgidum ssp. dicoccoides or T. timopheevii. Substitution of chromosome
5B of T. aestivum cv. Chinese Spring by 5B of T. turgidum ssp. dicoccoides or 5G
of T. timopheevii yielded also some homoeologous pairing promotion in the i nter-
specifi c hybrids, which suggested the presence of weak Ph1 alleles in these species.
On the other hand, a GISH analysis of MI association in durum wheat × Ae. genicu-
lata hybrids (ABUM) supports the presence of a ph allele with a lower promoting
effect than ph1c in the cultivated variety Ardente (Cifuentes et al. 2006 ).
T. Naranjo and E. Benavente