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Results and Discussion
At metaphase I control plants regularly formed 21 bivalents
characteristic of diploid-like pairing reported inthis
species (RILEY 1974). Microsporocytes with ring bivalents
were in greater frequency than rod bivalents in all the
genotypes. Anaphase I in control plants revealed normal
segregation of 21 : 21 chromosomes on both the poles.
The cytological data (Fig.1)
indicate that all the genotypes displayed a regular trend
for the increase in the frequency of aberrations with every
increase in the EMS dose. It is evident from Fig.1
that EMS treatment of 0.6 percent at all the durations in
all the genotypes produced maximum aberrations ranging from
4.52 percent (Al-Samma) to 13.81 percent (Maaya). Similar
relationship between dose and chromoromal aberrations was
reported by several workers (SEARS 1957 ; MATSUMURA 1961 ;
LARIK 1975 ; LARIK et al. 1981, 1982). Multivalents
ranged from trivalents to quadrivalents. Although not
consistent there was a tendency for increased frequency of
multivalents with increasing doses of EMS. A marked
differences in the frequency of multivalents was apparent
between treatments within the same genotype. A high
incidence of ring bivalents in wheat may be attributed to
the large size of the chromosomes ; presumably large
segments of chromosomes were involved in interchanges. The
data have also suggested that there were genotypic
differences in the frequency of occurrence of EMS-induced
multivalents. Variety Maaya displayed maximum frequency of
multivalents whereas, variety Al-Samma showed minimum
frequency of multivalents. In few cases chromosomes at MI
appeared to be clumped. This can be attributed to the
straight and narrow nature of the spindle (LARIK et
al. 1981).
Anaphase I and later stages were accompanied by various
types of anomalies. Anaphase bridges appeared in various
configurations, the most common being sticky bridges and
bridges due to delayed separation of chromosomes. The
occurrence of various forms of anaphase bridges suggest that
their formation depends upon the event of crossing over,
which occurs regularly at meiosis. The delayed separation of
some chromosome associations was probably due to delayed
chiasma terminalization. However, REES & THOMPSON (1955)
and LEWIS & JOHN (1966) have shown that similar
configuration could arise from spontaneous breakages.
The univalents accumulated in the equatorial region during
MI become the lagging chromosomes at AI in PMC's. This was
reported by several workers (KIHARA 1930 ; LARIK et
al. 1981,1982) and is also confirmed by the prresent
study. In few cases at AI a 20 : 22 chromosome disjunction
was observed. If the unbalanced gametes are functional as
has been reported by SEARS (1954) in Triticum
aestivum and LARIK (1978 a, b 1981) in Avena
sativa, it may be possible to build up an array of
aneuploids, which might be helpful in genetic study of our
wheat cultivars.
Pollen fertility, as judged by stainability decrease with
the increase of duration of EMS dose (Fig.1).
Similar results were reported by GAUL (1964). Low fertility
may be ascribed at least partly due to irregular disjunction
of chromosomes at anaphase resulting from the formation of
interchange multivalents. On the contrary, the production of
gametes with duplications and deficiencies for a certain
chromosome sections add to pollen sterility. While
recognising the role of meiotic anomalies in causing
sterility, the role of genetic factors affecting meiosis
cannot be ruled out in this polyploid. Orientation of
chromosomes at MI is another factor influencing sterility
(KUMAR & DAS 1973). Crossing over in the interstitial
segments also affects sterility (RANA 1965).
The difference in chiasma frequency between varieties and
interactions in treated populations were highly significant
(Table 1). The continuous nature
of the variation in chiasma frequency among genotypes,
expressed by the Fig. 1, indicate
that formation of chiasmata is controlled by polygenes and
it has a profound effect on the distribution of the various
chromosome configurations at meiosis (LARIK et al.
1982).
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