149et al. 1988 ; Chen et al. 1992 ; Jauhar 1992 ). The availability of addition and substitu-
tion lines of the E and P genome chromosomes in hexaploid wheat has provided
further information on their chromosomal loc ation. Either Dvořák ( 1987 ) and
Charpentier et al. ( 1988 ) allocated strong promoters to chromosomes 3E and 5E of
A. elongatum , which suggests some homoeoallelic relationships to the Ph loci of
hexaploid wheat (Sears 1976 ). Additional promoters and suppressors seem to exist
in other E chromosomes. The meiotic behavior of hybrids between Chinese Spring
and addition lines of the P genome chromosomes, or chromosome arms, into wheat
has demonstrated that the Ph1 suppressor system of A. cristatum is polygenic, with
no major gene (Jubault et al. 2006 ). However, this Ph1 switching off system, though
effective in promoting synapsis among wheat A, B, and D homoeologous, is unable
to promote synapsis between wheat and the less related P genome chromosomes
(Yang et al. 2010 ).
The existence of Ph - suppress ors in Elymus species has been deduced from the
meiotic analysis of hybrids between durum or common wheat and Elytricum fertile ,
an amphiploid of wheat and Elymus sibiricus (2 n = 4× = 28, SSHH) (Motsny and
Simonenko 1996 ; Simonenko et al. 1998 ). As described for A. cristatum , this
homoeologous pairing promoter system seems too weak to induce pairing between
wheat and the distant related S/H genome homoeologous (Motsny and Simonenko
1996 ).
All studies mentioned above are based on the level of meiotic chromosome pair-
ing observed in interspecifi c hybrids or wheat and related species. However, the
existence of alien genotypes that interact with the meiotic control syste m of wheat
has also been inferred from analyses of karyotype or genome stability in specially
designed newly derived amphiploids. For instance, Zhang et al. ( 2013 ) have ana-
lyzed, by means of a FISH/GISH approach, the chromosomal constitution of artifi -
cial allotetraploids derived from crosses between diploid Triticum / Aegilops species
related to the progenitors of tetraploid and hexaploid wheats. The combinations Ae.
sharonensis – T. monococcum and Ae. longissima – T. urartu (comparable to tetra-
ploid wheat, AABB) showed extremely low incidence of chromosomal rearrange-
m ents compared to T. urartu – Ae. tauschii and Ae. bicornis – Ae. tauschii amphiploids
(AADD- and BBDD-like allotetraploids, respectively). Such a difference was pro-
posed to be due to the presence of Ph -suppressors in the parental accessions of Ae.
sharonensis and Ae. longissima. This interpretation is in close agreement with the
absence of extensive genomic changes in a wheat– Ae. speltoides amphiploid, and
derived lines, produced from an Ae. speltoides accession which does not induce
homoeologous pairing in hybrids with wheat (Kumar et al. 2010 ).
6.6.3 B-Chromosomes
B-chromosomes coexist with the basic chromosome complement (A chromosomes)
in Ae. mutica , Ae. speltoides , and S. cereale. The fi rst report on the infl uence of
B-chromosomes on the wheat meiotic control system was authored by Mochizuki
6 The Mode and Regulation of Chromosome Pairing...