14 4
centromere–telomere axis, and homology of both the telomeric and centromeric
regions (Dubcovsky et al. 1995 ; Luo et al. 1996 ). Thus, Ph1 confers to the DSBs
repairing mechanism the ability of discriminating between homologous and
homoeologous chromosomes thus restricting the progression through the crossover
pathway to homologous interactions.
How this Ph1 role is accomplished remains an unresolved question. The pres-
ence of Cdk2 -like genes in wheat deletion lines showing the Ph1 mutant meiotic
phenotype (Bhullar et al. 2014 ) questions that the Cdk2 -like gene cluster structure
has any functional role on the Ph1 activity (Greer et al. 2012 ). A Ph1 gene encoding
a product capable of detecting some degree of divergence (homoeology) among
DNA sequences involved in the recombinational machinery, and excluding them
from the crossover pathway, seems now feasible in the light of the discovery of the
C - Ph1 gene. The fact that mutations in genes, such as Asy1 , mimic the meiotic phe-
notype of the ph1b mutant (Boden et al. 2009 ) might seem contradictory with this
model. [ Asy1 encodes a protein of the SC lateral element, which promotes DMC1-
mediated interhomologue recombination in Arabidopsis (Sánchez-Morán et al.
2007 )]. However, there is a substantial difference between Asy1 and the C - Ph1
orthologue in Arabidopsis. The asy1 mutant of Arabidopsis shows a synapsis and a
reduced number of chiasmata between homologous chromosomes, while silencing
of the C - Ph1 orthologue results in the formation of multivalents and therefore, in
nonhomologous interactions (Bhullar et al. 2014 ). Furthermore, the clumping of
centromeres observed in the silenced plants of Arabidopsis denotes a different
arrangement o f chromatin. This may be in agreement with some conformational
changes of chromatin produced in the absence of Ph1 (Upadhya and Swaminathan
1967 ; Mikhailova et al. 1998 ; Maestra et al. 2002 ; Greer et al. 2012 ).
The involvement of Ph1 in resolving SC multivalents into bivalents has also been
explored. Many polyploid species form SC multivalent confi gurations at zygotene.
In most cases, multiple associations decrease in number during prophase I through
late zygotene to pachytene (Grandont et al. 2013 ). Irrespective of the nature, homol-
ogous or homoeologous, of the chromosomes involved, the initiation of recombina-
tion is thought to be necessary for synapsis. Accordingly, early recombination
nodules associate with the SC in wheat synaptic multivalents (Hobolth 1981 ). The
transformation of each multivalent in two or more bivalents, in wild-type wheat,
implies that the early homoeologous recombination events abort and are redirected
into intersister or noncrossover pathways at the stage of late zygote and pachytene.
This is accompanied by disassembly of homoeologous SC stretches, which are
resynapsed completing the homologous pattern (Holm 1986 ; Holm and Wang 1988 ;
Martínez et al. 2001a , b ). Separation of initially synapsed homoeologous segments
may be facilitated by chromosome movements leading to disorganization of the
bouquet as occurs in heterozygotes for an inversion involving most of the 1RL chro-
mosome arm. The subtelomeric region of the normal chromosome, which associ-
ates with the subtelomeric region of the inverted arm at the bouquet stage, synapses
with the proximal region of the inverted arm once the bouquet disorganization starts
(Valenzuela et al. 2012 ).
T. Naranjo and E. Benavente