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Meiotic consequences of induced
chromosomal anomalies in Triticum aestivum L.
A.S. LARIK, H.M.I. HAFIZ* and N.N. ANSARI
Department of plant Breeding and Genetics Sind Agricultural
University, Tandojam, Pakistan
Investigations on the mechanism of chromosome breakages,
types of aberrations and their genetic consequences form an
integral part of the most of the studies on radiation
genetics (BROCK 1977; KONZAK et al. 1977; LARIK 1975;
SEARS 1977; SHARMA & FORSBEGR 1977), covering a wide
range of plants belonging to both wild and cultivated
species. Mutations due to deficiency of genes with a
dominant or epistatic effect occur in very high frequency
(MAC KEY 1968) because the well buffered genomes of
polyploids can tolerate losses of large chromosome segments
and even of entire chromosomes (LARIK 1978a; LARIK &
THOMAS 1979; LARIK et al. 1980a).
Extensive investigations on the effect of physical and
chemical mutagens on the cytological behaviour of wheat and
other plants have already been reported (GAUL 1977).
However, cytological studies on the M2 and
M3 populations are very limited (LARIK et
al. 1980a). An attempt has been made in the present work
to extend these studies. This paper presents an analysis of
meiotic anomalies in M3 populations of bread
wheat and discusses their significance with reference to
genetics and plant breeding.
Materials and Methods
M3 populations of two cultivars (C-591 and
Pak-70) of bread wheat Triticum aestivum L. em Thell
(2n=6x=42=AABBDD) derived from four irradiation treatments
(gamma rays CO60; 10 and 20 kR and fast neutrons;
300 and 600 RADS) were sown through dibbling single seed per
hole at 30.5 cm in randomized block design with five
treatments, two varieties and four replications were
critically examined for their meiotic behaviour. Seventy
five seeds were planted in each plot with 15 seeds per row
while plant to plant distance within a row was kept 10
cm.
Immature spikes from each irradiated treatments and control
were fixed in Carnoy's (6:3:1) solution. Anthers were
stained in alcoholic-hydrochloric acid carmine stain for 2 h
at 60C (SNOW 1963). Analysis of different stages of meiosis
was on done at MI and AI.
Results and Discussion
Cytological observation in control plants
At metaphase I control plants regularly formed 21 bivalents,
characteristic of diploidlike pairing reported in this
species (RILEY 1974). Microsporocytes with two ring and rod
bivalents were of normal occurrence. Pollen mother cells
(PMCs) carrying chains and rings of four chromosomes were
never encountered. Anaphase I in control plants revealed
normal disjunction of 21:21 chromosomes on both the
poles.
Cytological observation in radiated plants
All the mutated populations revealed disturbed meiosis. We
observed extensive irregular pairing (Table
1). Reciprocal translocations were either rings or
chains of four chromosomes. They invariabily resulted in
adjacent type of segregation. The disjunction of ring
quadrivalents was usually symmetrical i.e. one half goning
to one pole, the other half going to other, but there were
few cells showing irregular separation as well. The most
extreme types of associations were chain of
6-chromosomes+1IV(ring)+7II+18I
and 1VI+18II. The occurrence of such
associations indicates homology between more than four
chromosome arms and the genotypes may thus be either doubly
aneuploid or heterozygous for interchange. Other reason for
such unusual type of associations may be allosyndetic
pairing or heterozygosity for several translocations (KOUL
1964). In few cases chromosomes at MI appeared to be
clumped. This can be attributed to the straight and narrow
nature of the spindle.
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