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( 1964 ) who proposed that B-chromosomes of Ae. mutica suppress homoeologous
pairing in T. aestivum × Ae. mutica hyb rids lacking chromosome 5B. Vardi and
Dover ( 1972 ) reported a temperature-dependent signifi cant decreasing effect of the
B-chromosomes of Ae. mutica on the level of meiotic pa iring in wheat × Ae. mutica
hybrids, as well as some spindle irregularities linked to the presence of Ae. speltoi-
des and Ae. mutica Bs in their respective hybrids with wheat.
Jones ( 1991 ) and Jenkins and Jones ( 2004 ) reviewed the effect of rye
B-chromosomes in wheat × rye hybrids. Some studies failed to detect any signifi cant
infl uence of two rye B-chromosomes on meiotic pairing in wheat–rye hybrids
regardless chromosome 5B is present or absent (Roothan and Sybenga 1976 ; Lelley
1976b ) However, other studies report a homoeologous pairing suppressor effect of
the rye Bs in a wheat–rye ph mutant background (Cuadrado et al. 1988 ) or irrespec-
tive the presence or absence of chromosome 5B (Viegas 1980 ). A wheat genotype-
dependent infl uence of rye B-chromosomes in hybrids involving different durum
wheat vari eties has also been noted (Cuadrado et al. 1988 ). Romero and Lacadena
( 1980 ) showed that rye B-chromosomes decreased the level of homoeologous pair-
ing in wheat–rye hybrids lacking a wheat chromosome with a suppressor effect (3A,
3D, or 5B), whereas they increased the level of pairing when a chromosome with a
promoter effect (3B, 5A or 5D) was absent. Estepa et al. ( 1993 ), who analyzed
wheat– rye hybrids with a different number of Bs, reported no clear effect of
B-chromosomes on the level of chromosome pairing, although a greater variance in
the distribution of MI associations was obtained with odd numbers of B-chromosomes.
Similar results were described in diploid rye by Jones and Rees ( 1969 ). As stated by
Jenkins and Jones ( 2004 ), the only general conclusion that can be drawn is that the
rye Bs carry genes that interact with the pairing control genes of wheat under certain
circumstances.
In a recent study, Kousaka and Endo ( 2012 ) examine the effect of the rye
B-chromosome and two of its segments (namely B-9 and B-10) on homoeologous
pairing in hybrids between bread wheat and Ae. variabilis , either in the presence or
absence of chromosome 5B, and with one or two doses of chromosome 5D. When
Ph1 is absent, two rye B-chromosomes signifi cantly decrease the chiasma frequency
but do not compensate the absence of the suppressor activity of Ph1. A similar
effect was attributed to the B-9 segment. However, both B-9 and B-10 increase the
level of homoeologous pairing in hybrids carrying chromosome 5B. This study
demonstrates that the effect of the Bs on homoeologous pairing is not confi ned to a
unique region and furt her supports that its intensity is dose-dependent and condi-
tioned by the numeri cal balance between wheat pairing regulators genes on chro-
mosomes 5B and 5D.
Next-generation sequencing technologies have shown that B-chromosomes of
rye harbor pseudogenes originated from the A chromosomes and organellar (plastid
and mitochondrion) DNA sequences (Houben et al. 2013 ). It has been further dem-
onstrated that some of the A-derived sequences are transcribed in a genotype-
specifi c manner (Banaei-Moghaddam et al. 2013 ). This could explain the apparently
contradictory fi ndings and complex interactions pointed out on the effect of
B-chromosomes on homoeologous pairing in wheat–alien combinations.
T. Naranjo and E. Benavente