1) The problem has been investigated as to whether
single breaks induced by irradiation in the chromosomes
of mature sperm of Drosophih /Tazy produce the
loss of whole chromosomes by formation of dicentric
chromosomes originated from lateral fusion of
broken sister chromatids at the point of breakage..
The two methods adopted for detecting losses
of X and Y chromosomes and another for detecting
losses of the autosomes are described in detail.
2) A dosage experiment (0, 1000, 4000 r) showed that
losses of X and Y are actually produced by irradiation
of the mature sperm, that they are, in part
at least, non lethal to the zygote and that they
occur with a frequency proportional to the first
power of the dosage.
3) An experiment in which the frequency of induced
losses was investigated with X and Y chromosomes
of' different cytological length and of different
shape (V shaped and ring shaped) , showed that this
frequency is correlated with the breakage frequency
of the X and Y chromosomes used, and that a ring-shaped
(Xc₂) X-chromosome undergoes loss with a
much higher frequency than a V- or rod-shaped X-chromosome
of the same length.
4) Losses of X or Y chromatids, giving origin to
"fractional" (mosaic) flies, in proportion to
chromatid deletions or minute rearrangements,
are less frequent than the corresponding losses of
chromosomes in proportion to chromosome deletions.
5) An experiment based on the use of triploids in
which induced losses of the two major autosomes
were investigated; showed that losses of the latter
also, are, under suitable conditions, non-lethal to
the zygote . Their frequency, when losses of one
autosome are compared with that of the other and
also with losses of X and Y, is again suggestive
of a correlation with breakage frequency.
6) In the preceding experiment , and in an additional
one made with the purpose of definitely settling
this question, no fly developed in which the loss
of both autosomes or of an X (or Y) and an autosome,
could be attributed to a dicentric translocation
between the chromosomes involved. The
conclusion is drawn that dicentric translocations
are generally, perhaps always, lethal to the zygote
even when, as in the present case, the elimination
of the dicentric would have left the zygote with
a normal diploid set.
7) The evidence found is considered as in agreement
with a loss mechanism of the kind suggested in 1).
It is concluded that losses are produced by single
breaks induced by single ionizations along the
chromonema leading to formation of a sister -
chromatid dicentric which either is lost immediately
at first mitotic anaphase or undergoes a
process of repeated breakage which eventually
leads to complete loss.
8) To explain why dicentrics of the kind described
in 7) can be eliminated by the zygote in such a way
as to allow /it to survive, while dicentrics formed by fusion
of different chromosomes can not, it is pointed
out that the symmetrical structure of the former as
compared with the asymmetrical of the latter may
account for a different behaviour in mitotic anaphase,
perhaps facilitating, in the latter, the
inclusion of the whole dicentric in one daughter
nucleus instead of its immediate loss through
lagging or repeated breakage.
9) It is emphasised that ti-n results found here are
in strong support of the "breakage first" theory
for structural chromosome changes as there is a
direct proportionality to dosage for "single break"
losses in contrast with the proportionality to
about the 1.5 power of the dosage found, within
the range of dosages used here, for gross rearrangements.