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caused by the defi ciency for chromosome 5B, or about twice as high as the pairing
produced by the lack of 3DS (Mello-Sampayo and Canas 1973 ). Because of the
redundant functional activity of these two minor suppressors and their location on
homoeologous arms (3AS and 3DS) they were proposed to be homoeologous loci.
The level of homoeologous pairing in the double mutant Ph1 − / Ph2 − is not quite dif-
ferent from that of the single mutant Ph1 − (Ceoloni and Donini 1993 ). This suggests
no additive cooperation between both pairing suppressor genes. The absence of Ph1
seems to induce the maximum possible level of homoeologous pairing in wheat.
Two additional minor suppressors with a similar effect to that of 3AS were reported
on chromosome 4D (Driscoll 1973 ) and chromosome arm 2DL (Ceoloni et al.
1986 ).
Genes that promote pairing are located on group 2, 3, and 5 chromosomes. All
three homoeologous arms 2AS, 2BS and 2DS carry a pairing promoter gene with a
minor effect (Sears 1954 ; Ceoloni et al. 1986 ). A relatively strong promoter gene,
whose presence is necessary for normal synapsis and chiasma formation, is located
on 3BL (Sears 1954 ; Kempana and Riley 1962 ). Its effect is more intense than the
effect of promoters on group 2 chromosomes. The arms 3AL and 3DL carry also
pairing promoters somewhat weaker than 3BL (Mello-Sampayo 1971a ; Driscoll
1972 ; Mello-Sampayo and Canas 1973 ).
The 5BS arm carries a gene that promotes pairing (Riley and Chapman 1967 ;
Feldman and Mello-Sampayo 1967 ). However, the intensity of its promoter activity
is less than the suppressor effect of Ph1. This pairing promoter appears to reside in
the distal third of 5BS as homozygo sity for a deletion of this segment results in
chiasma frequency similar to that found in wheat lacking the entire 5BS arm (Kota
and Dvořák 1986 ). Chromosomes 5D and 5A carry also pairing promoters since
chiasma frequency decreases in plants nulli-5D, while four doses of chromosome
5A compensate for the lack of 5D and restore the number of chiasmata produced in
euploid plants (Feldman 1966 ). Chromosome 5D has been shown to carry a gene or
genes, called ltp (low temperature pairing) that stabilize the number of chiasmata
formed at high and low temperatures. Chiasma frequency in nullisomic-5D between
19 and 29 °C resembles that in euploids. Above and below these temperatures chi-
asma frequency is sharply reduced in 5D-defi cient plants but little changed in
euploids (Riley 1966a ; Riley et al. 1966 ). Reduced chiasma frequency at low and
high temperatures is accompanied by failure of synapsis during zygotene (Bayliss
and Riley 1972a ; Morais et al. 1992 ). The temperature sensitive stage was located
in the premeiotic interphase prior to DNA synthesis (Bayliss and Riley 1972b ). A
weak stabilizing effect on chiasma formation at low temperature is derived from the
presence of four doses of chromosome 5A in plants nulli 5D-tetra 5A (Riley et al.
1966 ). By contrast, tetraploid wheat, which has only the A and B genomes, shows
no reaction to temperature changes (Riley et al. 1966 ). This suggests that the
homoeoallele present in T. turgidum , most likely on chromosome 5A, is of greater
effect than that in T. aestivum.
T. Naranjo and E. Benavente