145The homologous synapsis that follows disassembly of SC between homoeolo-
gous is expected to be concluded either by extension of SC stretches initially formed
between homologues, or by additional interactions in sites apart from the already
synapsed region, or both. A number of ring bivalents at metaphase I similar to the
number of bivalents at pachytene in wild-type hexaploid and tetraploid wheats
(Martínez et al. 2001a , b ) indicates that each bivalent present at pachytene forms at
least one chiasma in each chromosome arm. Because chiasmata are distally located
in wheat, some recombinational events processed in the crossover pathway should
occur between homologous sites interacting after the synaptic multivalent correc-
tion. This supports that the recombination machinery may be loaded at different
times during the prophase I.
The loading of the recombination machinery has been studied in hexaploid wheat
and wheat– rye hybrids by immunolocalization of MLH1, a DNA mismatch protein
considered to mark interfering crossovers at diplotene (Martín et al. 2014 ). Bread
wheat yields a number of MLH1 foci which fi ts the number of chiasmata estimated
at metapahase I, and is not affected by the presence or absence of Ph1. Wheat– rye
hybrids show also a similar number of MLH1 sites both in the presence and absence
of Ph1. However, these numbers are much higher than those expected from the
number of MI chiasmata. Almost 70 % of MLH1 sites formed in the ph1b wheat × rye
hybrids, and 95 % in the wild type hybrids, are processed to produce noncrossover
products. Martín et al. ( 2014 ) suggest that these MLH1 sites, in which the crossover
pathway is aborted, are disassembled later in the hybrids than when chromosomes
hav e a homologous partner, and that this behavior is conditioned by Ph1.
In the absence of Ph1 , recombinational events involving homoeologous chromo-
somes are not impeded or aborted at zygotene but some of them progress through
the crossover pathway to culminate in the formation of homoeologous chiasmata.
Such a DSBs repairing option would facilitate the stability of the homoeologous SC
stretches and, therefore, the persistence of multivalent through the remaining pro-
phase I stages and metaphase I.
6.6 Additional Genetic Factors Regulating Wheat– Alien
Chromosome Pairing
6.6.1 Wheat Genotype
The diploid-like meiotic behavior of wheat is the result of a complex interaction
between two main loci, Ph1 and Ph2 , and other minor genes that either promote or
suppress homoeologous chromosome pairing (Sears 1976 ). The occurrence of natu-
ral allelic variation for genes r espons ible of the regular meiotic behavior in wheat is
supported by numerous studies that report a wheat genotype-dependent variation in
the level of homoeologous pairing in hybrids with related species. Driscoll and
6 The Mode and Regulation of Chromosome Pairing...