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6.1 Introduction
Effi cient production of balanced gametes in diploid organisms depends on the regu-
lar bivalent formation at the prophase of the fi rst meiotic division. Homologous
chromosomes fi nd each other and interact at early mei osis to become physically
connected by chiasmata and form bivalents. Clustering of telomeres in the meiotic
bouquet at the leptotene–zygotene transition facilitates the approximation of termi-
nal regions of homologous chromosomes and their recognition (Naranjo and
Corredor 2008 ). Homologous interactions are initiated after the production of a
programmed set of double strand DNA breaks (DSBs) catalyzed by the topoisomer-
ase II related enzyme SPO11 at the initiation of meiosis (Keeney et al. 1997 ). DSBs
are immediately immersed in a repairing process, which is controlled to ensure that
each homologous pair receives at least the obligate crossover necessary for accurate
segregation. Concomitant with the bouquet organization, a linear protein axis is
formed along each homolog. Some of the proteins that make up the axis, for exam-
ple, ASY1 in Arabidopsis , play a key role by infl uencing DSB repair through inter-
homologous recombination (Sánchez-Morán et al. 2007 ). Then, homologous
chromosomes become aligned, at least in their distal region, and undergo synapsis
through the formation of a proteinaceous structure, the synaptonemal complexes
(SC) that maintains the homologous chromosomes intimately juxtaposed along
their length. Repair of DSBs starts with the invasion of the double strand DNA in
any of the chromatids of the homologous chromosome. A crossover and a noncross-
over (non-reciprocal exchange) represent the two possible outcomes in the pathway
that the homologous recombination machinery follows to repair one DSB. The
majority of DSBs are destined to become noncrossover products; the few that are
cross overs show a distal location in many plant species (Higgins et al. 2014 ) and
create chiasmata, which maintain homologues bound after the SC dissolution at
diplotene. Chiasmata enable the homologues to correctly orientate on the meiotic
spindle and segregate at anaphase I. Four haploid gametes are produced after the
second meiotic division.
Aneuploids and polyploids, with more than two potential partners for each chro-
mosome, generate deviations of regular bivalent pairing at metaphase I (MI).
Polyploidy is widespread in plants and is considered to be a central feature of plant
diversifi cation (Grandont et al. 2013 ). Most polyploid plant species, including
important crops like wheat, are allopolyploids that arose after hybridization between
related diploid progenitors. The polyploid condition confers some advantages such
as heterosis or gene redundancy but implies disadvantages such as the propensity to
produce aneuploid meiotic products that reduce fertility (Comai 2005 ). Many poly-
ploid species have evolved genetic regulatory systems that ensure a diploid-like
behavior with effi cient disjunction of homologous chromosomes at t he fi rst meiotic
division (Jenczewski and Alix 2004 ). The best-studied example is common bread
wheat, Triticum aestivum , most likely because of the outstanding importance of this
species as a crop, but also because its meiotic pairing regulatory system is effective
T. Naranjo and E. Benavente