1416.4 Unravelling the Structure of Loci Ph1 and Ph2
The ph1b deletion covers an intercalary region, in which, the Ph1 locus was con-
fi ned to a submicroscopic region of less than 3 Mb fl anked by the breakpoints of
deletions ph1c and 5BL-1 (Gill et al. 1993 ). The 5BL-1 deletion belongs to the dele-
tion stocks of wheat, a set of truncated chromosome lines generated using the game-
tocidal genes of Ae. cylindrica , Ae. triuncialis , or Ae. speltoides (Endo and Gill
1996 ).
A set of fi ve lines carrying specifi c deletions generated by fast-neutron irradiated
wheat were used to physically map the Ph1 region, which corresponds to a subre-
gion of 400 Kb of the rice genome (Roberts et al. 1999 ). Markers of this rice region,
and the equivalent region in Brachypodium sylvaticum , were used to saturate and
sequence a 2.5-Mb region of the chromosome of wheat co ntaining the Ph1 locus
and homoeologous regions of chromosomes 5A and 5D. The 5BL region includes
36 genes among which the Ph1 locus was assigned to a structure consisting of a
segment of subtelomeric heterochromatin from 3AL that inserted into a cluster of
seven cyclin-dependent kinase ( Cdk )-like genes, with a similar sequence to
mammalian Cdk2 (Griffi ths et al. 2006 ; Al-Kaff et al. 2008 ). Two other clusters
with fi ve and two copies of Cdk -like genes are present in chromosomes 5A and 5D,
respectively. The Cdk locus on 5B is dominant to loci on 5A and 5D in determining
the overall kinase activity. Among the seven Cdk -like genes of chromosome 5B,
5B2 was considered to be the wheat equivalent of the gene encoding the human
CDK2 kinase (Yousafzai et al. 2010 ). The level of phosphorylation measured in
specifi c sites of histone H1 was twofold higher in the absence than in the presence
of Ph1 (Greer et al. 2012 ). This result and the fact that mammalian CDK2 is required
for proper homologous pairing and recombination (Ortega et al. 2003 ; Viera et al.
2009 ) were considered to support the multi Cdk -like gene structure proposed for
Ph1 (Greer et al. 2012 ). Homoeologous recombination can be induced in hybrids of
wild-type wheat with rye by treatment prior to meiosis with okadaic acid, a potent
drug that activates CDKs causing a premature chromosome condensation. Then
such an effect was assumed to be a phenocopy of Ph1 (Knight et al. 2010 ).
However, Bhullar et al. ( 2014 ) concludes that Cdk2 - 4 is present in chromosome
deletion lines that show the meiotic mutant phenotype, and therefore, it is not a good
candidate for the Ph1 locus. Seven markers localized in the Ph1 region were used to
identify a 450-Kb segment of rice chromosome 9 including genes involved in chro-
matin reorganization, microtubule attachment, acetyltransferases, methyltransfer-
ases, DNA binding, and meiosis/anther specifi c proteins (Sidhu et al. 2008 ). Among
91genes present in this region, 26 were selected for further characterization by gene
silencing. Transient or stable silencing of one of these genes, called C - Ph1 (candi-
date Ph1 ), located on chromosome 5B, results the same meiotic phenotype as the
ph1b mutant. Homoeologous copies of C - Ph1 present in chromosomes 5A and 5D
show a dramatically different structure and expression pattern, and their inactivation
does not modify the wild-type meiotic phenotype (Bhullar et al. 2014 ). On the other
hand, silencing of the Cdk2 - 4 gene does not change the wild-type MI phenotype.
However, multivalents are formed and centromere clumping is observed at prophase
6 The Mode and Regulation of Chromosome Pairing...