143premeiotic treatment with colchicine (Driscoll et al. 1967 ; Feldman and Avivi
1988 ). It was therefore assumed that the hexaploid nucleus still maintains some
organizational aspects of the individual ancestral genomes, i.e., each genome
occupies a separate region in the nucleus, which in turn, is recognized by Ph1. A
model was thus proposed whereby Ph1 affects the premeiotic alignment of homol-
ogous and homoeologous chromosomes (Feldman 1993 ). In the absence of Ph1 ,
the three genomes are mingled together in the nucleus and, consequently, both
homologues and homoeologous compete in pairing. In euhexaploid wheat, two
doses of Ph1 keep the homoeologous apart, thereby leading to exclusive homolo-
gous pairing in meiosis. Six doses of Ph1 or premeiotic treatment with colchicine
induce random distribution in the premeiotic nuclei leading to separation of homo-
logues, which results in overall reduction of synapsis, some level of synapsis
between homoeologous and interlocking of bivalents. Based upon the observation
of complex structures formed by multiple centromeres at leptotene, a later hypoth-
esis proposed that Ph1 exert its effect through the control of presynaptic centro-
mere association (Martínez-Pérez et al. 1999 , 2001 , 2003 ). In wild type wheat,
homologous centromeres coalescing in such structures become associated, and
then the entire chromosomes collocate prior to synapse, whereas the recognition of
homologous centromere s is not operative in Ph1 mutants.
However, these models are contradictory with the synaptic pattern observed in
wild-type wheat, which failed to reveal a large scale alignment of homologues in the
leptotene–zygotene transition stage (Holm 1986 ), as well as with the absence of
presynaptic association of two homologous chromosomes of rye, or their centro-
meres, in wheat– rye addition lines both in the presence and the absence of Ph1
(Maestra et al. 2002 ; Corredor et al. 2007 ). Furthermore, two homoeologous arms
physically connected to the same centromere, in a homoeoisochromosome, are
bound at metaphase I in the absence of Ph1 but not in its presence (Dvořák and
Lukaszewski 2000 ) while two homologous arms can recombine even if they are
linked to homoeologous centromeres (Corredor et al. 2007 ). Premeiotic colchicine
treatment does not affect premeiotic centromere association but inhibits bouquet
formation, i.e., the meiotic structure that brings together the majority of homolo-
gous pairs that normally occupy separated territories in premeiotic nuclei (Corredor
and Naranjo 2007 ). Disturbed organization of the bouquet is the main reason of the
failure of both synapsis (Corredor and Naranjo 2007 ) and resolution of interlock-
ings (Feldman and Avivi 1988 ).
The meiotic phenotype of Ph1 mutants indicates that Ph1 exerts its action on two
main steps of bivalent formation in polyploid wheat: fi rst, on the suppression of
recombination between homoeologous chromosomes in favor of that between
homologues and, second, on the correction of SC multivalents formed d uring zygo-
tene, which are transformed into bivalents.
In chromosomes of hexaploid wheat composed of homologous and homoeolo-
gous segments, recombination is absent from homoeologous segments while occurs
normally in juxtaposed homologous segments in the presence of Ph1. However,
extensive homoeologous rec ombination takes place in the absence of Ph1. This hap-
pens irrespective of the length of the homoeologous segment, its location on the
6 The Mode and Regulation of Chromosome Pairing...