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ph1c , in which half of them persists until metaphase I. Pairing correction concerns
also the resolution of interlocking produced as a result of synapsis. In plants of
hexaploid wheat lacking chromosome 5B or with four of six doses of chromosome
arm 5BL, synapsis is arrested before completion, most likely due to the absence of
the 5BS ar m. The frequency of interlocked bivalent decreases with the level of syn-
apsis achieved in the different genotypes, which suggests that completion of synap-
sis is required for the interlocking correction (Holm and Wang 1988 ).
Meiotic phenotypes at metaphase I (Table 6.1 ) are relatively similar in the wild
type and ph2b genotypes of hexaploid wheat. None of them show multivalents
although the number of ring bivalents is slightly lower in the ph2b mutant. However,
the ph1b mutant shows multivalent confi gurations (Fig. 6.1 ) that identify the absence
of the Ph1 function. The same happens in the ph1c mutant of tetraploid wheat. As a
result of homoeologous recombination , the ph1b mutant line accumulates extensive
chromosome rearrangements and eventually becomes infertile (Sánchez-Morán
et al. 2001 ). Gross genome changes produced in the absence of Ph1 emphasize the
key role of the Ph1 locus in maintaining the genome integrity and ensuring the fer-
tility of this important crop.
In hybrids of hexaploid wheat with related species, all three genotypes show
homoeologous chromosome associations at metaphase I with low, intermediate, and
high pairing levels in the wild type, ph2b , and ph1b hybrid genotypes, respectively
(Fig. 6.2 ). The average number of the different types of confi guration produced in
hybrids of wheat with, rye, Ae longissima , Ae. sharonensis , and Ae speltoides are
shown in Table 6.2. MI pairing frequencies are markedly lower in wheat– rye hybrids
than in wheat– Aegilops hybrids mainly due to their different level of interspecifi c,
wheat– rye or wheat– Aegilops , homoeologous pairing. This variation is related to
the evolutionary history of the Triticeae : rye diverged 7,000,000 years ago in the
timeline of wheat evolution while the clade including the diploid species of the
Triticum – Aegilops complex radiated 2.5–4.5 million years ago (Huang et al. 2002 ).
Thus, Aegilops chromosomes are more closely related to wheat than rye chromo-
somes. Identifi cation of individual chromosomes demonstrated that homoeologous
wheat–wheat associations are unevenly distributed between genomes. Associations
of the A–D type are the most frequent among wheat–wheat associations in all
Table 6.1 Meiotic phenotype at metaphase I of wild type and ph mutants of hexaploid and
tetraploid wheat
Genotype I IIrod IIring Multiv Bonds/cell No. of cells
Wild type (6×) a 0.02 1.48 19.50 0 40.49 90
ph2b a 0.48 2.95 17.78 0 38.57 120
ph1b a 2.76 4.76 14.05 0.77 34.22 120
Wild type (4×) b 0.04 0.34 13.64 0 27.62 50
ph1c b 0.94 3.69 9.46 0.19 23.16 100
I, univalent s; IIrod, rod bivalents; IIring, ring bivalents; Multiv, multivalents
a Data from Martínez et al. ( 2001a )
b Data from Martínez et al. ( 2001b )
T. Naranjo and E. Benavente