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I when the C - Ph1 orthologue is silenced in Arabidopsis , which supports C - Ph1 as
an excellent ca ndidate for the Ph1 locus (Bhullar et al. 2014 ). This further suggests
that okadaic acid might phenocopy a suppressor gene different than Ph1.
The Ph2 locus has not yet been identifi ed although some candidate genes have
been proposed. Based on the conserved regions of the bacterial mismatch repair
gene MutS and its homologues ( MSH ) in yeast and human, Dong et al. ( 2002 ) iden-
tifi ed three MSH7 homoeoloci located on chromosome arms 3AS, 3BS, and 3DS
with highest similarity to maize and Arabidopsis MSH7. The copy on 3DS is
included in the region deleted in the ph2a mutant, and is a good candidate for Ph2.
MSH7 genes of wheat are expressed in tissues associated with a high level of mitotic
and meiotic activity, with maximum expression in the reproductive organs of young
fl ower spikes. A reduction of the MSH7 expression is observed in the wheat deletion
mutant ph2a while the ph2b mutant shows a similar transcription level than the wild
type. This suggests that ph2b is a point mutation that has affected the activity of the
protein rather than the expression of the gene (Dong et al. 2002 ). In fact, two SNPs
(single-nucleotide polymorphisms) leading to amino acid changes in the ph2b
MSH7 protein were rep orted, although it remains to be established whether such
changes affect the protein function (Lloyd et al. 2007 ). The reduction of fertility in
plants of barley transformed with a MSH7 RNAi knockdown construct is consistent
with some role of MSH7 on meiotic recombination in cereals (Lloyd et al. 2007 ).
The mismatch repair system plays indeed an important role in reducing meiotic
recombination between divergent sequences in Arabidopsis (Li et al. 2006 ), which
has been specifi cally associated to MSH7 in tomato (Tam et al. 2011 ).
Based on syntenic relationships between rice and wheat a length of 80 Mb was
estimated for the region deleted in ph2a (Sutton et al. 2003 ). A 173-Kb DNA
sequence localized within this region contains the Wheat Meiosis 1 ( WM1 ) gene
family, which is composed of seven members. These genes are predominantly
expressed in fl oral tissues at early meiosis and encode mainly membrane-anchored
leucine-rich repeat-like receptor proteins with possible roles on disease response
and fl oral development (Whitford et al. 2007 ). In addition to 3DS, the WM1 gene
cluster is present on barley 3HS but missing from the A and B genomes of hexa-
ploid wheat. There is yet, no evidence supporting that the WM1 gene fami ly or any
individual gene family member represents the Ph2 gene.
6.5 The Mode of Action of Ph1
From the discovery of Ph1 a number of hypotheses to explain its mode of action
have been advanced. Some rely on assumptions concerning the presynaptic orga-
nization of chromosomes. Feldman ( 1966 ) studied meiosis in hexaploid wheat car-
rying four (diisosomic 5BL) or six doses (triisosomic 5BL) of Ph1 , respectively.
Extradoses of Ph1 caused partial a synapsis of homologues, pairing between
homoeologous, and interlocking of bivalents. Similar effects were observed by
T. Naranjo and E. Benavente