1376.3 Meiotic Phenotype of Ph Mutants
The fi rst attempt to isolate meiotic mutants in hexaploid wheat was carried out by
treatment of plants ditelocentric 5BL with ethyl methanesulfonate (EMS) in order
to produce mutations in the Ph1 locus. The occurrence of mutations was checked in
the meiosis of hybrids of the EMS-treated plants with rye. A mutant called 10/13
was later fi xed in homozygosis both in the 5BL ditelocentric and in euploid condi-
tion. Although the mutant genotype formed regularly 21 bivalents, its hybrids with
rye showed a higher level of homoeologous pairing than those of the wild-type
genotype (Wall et al. 1971a , b ). This mutation was associated to the 5BL arm and
segregated independently of the centromere (Wall et al. 1971a , b ). Another mutant
consisting in a deletion of the Ph1 locus was obtained after normal pollen irradia-
tion with X-rays and pollination of plants monosomic-5B (Sears 1977 ). Wheat
plants homozygous for this mutation showed multivalents at metaphase I denoting
the occurrence of homoeologous pairing. This mutant was called ph1b since it was
isolated later than the 10/13 ( ph1a ) mutation.
In the search for the ph1b mutation, Sears isolated a second X-ray induced muta-
tion, termed ph2a , which conditions an intermediate level of pairing in hybrids with
Ae. kotschyi. This mutation is located on chromosome arm 3D S and was suggested
to be a defi ciency that includes the Ph2 locus (Sears 1982 ). Complementation tests
demonstrated that mutations ph1b and 10/13 are not allelic and segregate indepen-
dently, while ph2a and 10/13 are allelic. Then the 10/13 mutation was redesignated
as ph2b. The ph1b and ph2b mutations map 0.9 cM from the centromere of 5B and
2.3 cM from the centromere of 3D, respectively (Sears 1984 ).
A Ph − ( ph1c ) mutant was also isolated in cv. Capelli of T. turgidum after irradia-
tion of seeds with neutrons (Giorgi 1978 , 1983 ). This mutation consists of an inter-
calary deletion in the middle of 5BL, which includes the Ph1 locus, and originated
together with a tandem duplication involving the same region (Dvořák et al. 1984 ).
Deletion and duplication are assumed to be produced by the same event, an unequal
interchange between sister chromatids or homologous chromosomes. Intergeneric
hybrids and haploids of mutant ph1c show a high level of chromosome pairing
(Giorgi and Cuozzo 1980 ; Giorgi and Barbera 1981a , b ; Jauhar et al. 1999 ; Cifuentes
et al. 2006 ; Cifuentes and Benavente 2009a ).
Meiotic phenotypes of wild type and mutants of tetraploid and hexaploid wheat
have been described at prophase I and metaphase I. Ult rastructural analysis of SC
formation in fully traced spread nuclei at zygotene and pachytene denoted the for-
mation of multivalent associations involving homoeologous chromosomes in wild
type and Ph mutants of both hexaploid and tetraploid wheat (Holm 1986 ; Holm and
Wang 1988 ; Martínez et al. 2001a , b ). On average 28 % (12/42) of chromosomes of
hexaploid wheat, and 39 % (11/28) of tetraploid wheat, are involved in multivalent
SCs at mid zygotene both in wild type and Ph1 mutants. In the ph2b mutant of
hexaploid wheat, only 22 % of the chromosomes are involved in multivalent SCs.
While synapsis progresses to become completed in most pachytene nuclei, multiva-
lent SCs are corrected and transformed into bivalents except in the case of ph1b and
6 The Mode and Regulation of Chromosome Pairing...