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Data in Table 1, indicate that
fast neutrons were more efficient in producing aberrations.
This may be interpreted to mean that more than one
ionization was neccessary to produce a break (LEA, 1955).
However, a more critical examination of chromosomal
anomalies (Table 1) suggests that
differences amongst various treatments of gamma rays and
fast neutrons were not significant (Table
2). This may be due to the fact that the doses employed
in the present experiment were selected on the basis of
previous experience which aimed at the induction of an array
of useful mutations (LARIK 1978b; LARIK et al 1980b).
Selection of higher doses of gamma rays as well as fast
neutrons are likely to create highly significant differences
among treatments.
The results (Table 2) however,
suggest that the differences amongst different categories of
chromosomal anomalies were highly significant (P>=.01).
Category 1 (21:21) was significantly different from all
other categories at l% level of significance. This is
understandable as this category represents normal chromosome
pairing and distribution. Category 2(20-20) was also
significantly (P>=.05) different from all other
categories (Table 1). The
differences among other categories (from 3 to 7) gradually
became diluted and consequently reached a non-significant
level. The present analysis is perhaps the first critical
analysis of chromosomal anomalies. Previous workers (NILAN
et al 1962; LARIK 1975) have also reported similar
anomalies but their experimental data were not subjected to
proper statistical analysis and therefore were not amenable
to meaningful inferences.
The occurrence of dicentric bridges accompanied by free
acentric fragments were observed in many treated plants,
suggesting that most of the individuals were heterozygous
for paracentric inversions. First meiotic anaphase bridge
results from dicentrics and should be lethal (DUBININ 1964).
Laggards and unequal segregation at AI will give rise to
aneuploid spores of uncertain viability (LARIK 1978a; LARIK
& THOMAS 1979). Lagging chromosomes arise from acentric
fragments.
The occurrence of fragments in cells carrying bridges were
considered to be due either to the smallest of the fragments
or its disappearance in earlier division or its origin from
the achromatic portions. Multipolar anaphase could have
caused misdivision of centromeres, leading to the formation
of chromosome fragments (FEDAK 1979, 1980). Another
interesting point was the absence or rare presence of
micronuclei which normally originated from acentric
fragments. The association of bridges and fragments with the
meiotic sequence suggests that their formation depends upon
an event which occur regularly at meiosis. The most obvious
event of this kind is crossing over. Presence of fragments
in Nf 600 RADS of Pak-70 at MI points towards the process of
breakages and reunion is similar to one considered by REES
(1962) in rye. Structural differences between chromosomes
could also be inferred from the present study of AI and AII
where bridges accompanied by fragments were observed.
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