45of homoeologues in allopolyploid Triticum species while allowing regular pairing of
homologous chromosomes (Riley 1960 ; Sears 1976 ).
The suppressive effect of Ph1 on homoeologous pairing in inter-specifi c and
inter-generic Triticum hybrids is absolute. In contrast, its effect on homoeologous
pairing in bread wheat itself might not be indispensible as plants defi cient for this
gene exhibit relatively little homoeologous pairing (less than one multivalent per
cell resulting from inter-genomic pairing (Sears 1976 ). Interestingly, and in accord
with the above, Ph -like gene(s) have not been found in any of the allopolyploid
species of the closely related Aegilops genus (Sears 1976 ). Nevertheless, these
species also exhibit exclusive bivalent pairing of fully homologous chromosomes
presumably due to the sequence-elimination system.
The mechanism controlling the Ph1 mode of action is still unclear. Six extra doses
of the Ph1 -containing arm of chromosome 5B of hexaploid wheat caused partial
asynapsis of homologues at meiosis, concomitantly allowing some pairing of
homoeologous chromosomes and inducing a high frequency of interlocking biva-
lents (Feldman 1966 ). Similar effects were observed by pre-meiotic treatment with
colchicine (Driscoll et al. 1967 ; Feldman and Avivi 1988 ). These effects were
explained by the assumption that the hexaploid nucleus still maintains some organi-
zational aspects of the individual ancestral genome s, i.e., each genome occupies a
separate region in the nucleus, which in turn, is recognized by Ph1 (Feldman 1993 ,
and references therein). A model was thus proposed whereby Ph1 exerts its effect at
pre-meiotic stages, before the commencement of synapsis , where it affects the pre-
meiotic alignment of homologous and homoeologous chromosomes and as a result
controls the regularity and pattern of pairing (Feldman 1993 ). In euploid common
wheat, two doses of Ph1, while scarcely affecting homologous chromosomes, keep
the homoeologues apart, thereby ensuring exclusive homologous pairing in meiosis.
In the absence of Ph1 , the three genomes are mingled together in the nucleus and
consequently, the homoeologues can also pair to a small extent with each other. Six
doses of Ph1 or pre-meiotic treatment with colchicine induces separation of chromo-
somal sets and random distribution of chromosomes in the pre-meiotic nucleus, lead-
ing to an increased distance between homologues. This results in partial asynapsis of
homologues that are relatively distant, and some pairing of homoeologues that
happen to lie close to one another. An interlocking of bivalents can occur with their
chromosomal constituents coming to pair from a relative distance and catching other
chromosomes between them (Feldman 1993 , and references therein). This model is
supported by the fact that while extra six doses of chromosome arm 5BL, as well as
pre-meiotic treatment with colchicine, caused partial asynapsis of homologues
including the inter-chromosomal pairing of isochromosomes, they did not affect the
intra-chromosomal pairing of isochromosomes (Feldman and Avivi 1988 ). This indi-
cates that when chromosomes are close to one another, as the two homologous arms
in an isochromosome that are connected to each other by the common centromere,
extra dose of 5BL as well as colchicine do not suppress their pairing at meiosis.
Two deletions that include the Ph1 locus were induced by x-irradiation: ph1b
(Sears 1977 ), induced in the bread wheat cultivar Chinese Spring (CS), is a submi-
croscopic interstitial deletion, about 3 Mb in size (Gill and Gill 1991 ; Gill et al.
1993 ), and ph1c (Giorgi 1978 , 1983 ), induced in the durum cultivar Cappelli, is a
2 Origin and Evolution of Wheat and Related Triticeae Species