135both in wheat and interspeci fi c hybrids, which has implications in the transfer to
wheat of alien genes controlling important agronomical traits. In this chapter we
review the advances produced in the identifi cation of genes that control meiotic
pairing in wheat and in hybrids of wheat with related species, with special reference
to the homoeologous pairing suppressor genes. Genes in other species with effect
on the control of homoeologous pairing are also concerned. In the strict sense, the
term chromosome pairing refers to the alignment of homologous chromosomes
prior to synapsis. However, the terms homologous pairing and homoeologous pair-
ing, which appear repetitively along the text, have been employed in the wide sense
that they had at the time of the discovery of the pairing regulating genes, i.e., the
association of homologous or homoeologous chromosomes at metaphase I.
6.2 Genes That Control Chromosome Pairing in Wheat
Common bread wheat is an allohexaploid species (2 n = 6× = 42) with three genomes ,
A, B, and D, from three related diploid species, that arose after two hybridization
events. The fi rst hybridization event involved T. urartu and Aegilops speltoides , the
donors of the A and B genomes, respectively, and originated the emmer tetraploid
wheat ( T. turgidum ), and the second hybridization involved T. turgidum and Ae.
tauschii , the donor of the D genome (Petersen et al. 2006 ). Tetraploid wheat is con-
sidered to have arisen 500,000 years ago while hexaploid wheat appeared within
modern agriculture, 8000 years ago (Huang et al. 2002 ). In spite of the genetic
synteny between homoeologous chromosomes, bread wheat forms 21 bivalents at
diakinesis and metaphase I of meiosis. Obviously, in addition to genes that control
m eiosis in diploid species, polyploid wheats have developed genetic systems that
restrict to homologues the formation of bivalent associations at metaphase I. The
large size (17 gigabase) and the repetitive nature of the wheat genome have hindered
and delayed the generation of a genome reference sequence (The International
Wheat Genome Sequencing Consortium (IWGSC) 201 4). Despite the lack of the
molecular genomic tools necessary for gene identifi cation and isolation, the number
and diversity of aneuploid stocks available in wheat have permitted to identify and
locate on chromosomes some of the genes involved in the control of meiotic events.
Two types of genes, promoters and suppressors, with effect on meiotic chromo-
some pairing in wheat itself or in hybrids of wheat and related species, were identi-
fi ed (Sears 1976 ). The Ph1 ( Pairing homoeologous 1 ) locus, located on the long arm
of chromosome 5B (5BL), is the most effective and acts by suppressing homoeolo-
gous pairing (Okamoto 1957 ; Sears and Okamoto 1958 ; Riley and Chapman 1958 ,
1964 ; Riley and Kempana 1963 ). Ph2 , another suppressor located on the short arm
of chromosome 3D (3DS), shows an intermediate effect (Mello-Sampayo 1971a ).
Besides Ph1 and Ph2 , a third suppressor, even less effective than Ph2 , was located
on the short arm of chromosome 3A (Driscoll 1972 ; Mello-Sampayo and Canas
1973 ). Defi ciency for both 3AS and 3DS results in a level of pairing similar to that
6 The Mode and Regulation of Chromosome Pairing...