Abstract
I. "FILAiMENT" A MUTANT IN DROSOPHILA.
(1). Filament forms the first dominant eggs mutation
to be reported in Drosophila. As yet, no satisfact -
ory means of determinigg its genotype from the study
of the external morphology of the fly has been dis -
covered. Like many known dominant mutations, it is
lethal when homozygous. When present in the hetero
zygous condition, it lowers the total fecundity of
the females and the size of the eggs they produce.
Its main visible effect however, is on the two filaments
- causing a variable amount of their fusion
and reduction in size. Hatchability tests show that
aging of the females lessens the viability of the
eggs produced - the older the females the less viable are
their eggs. When the effect of age is practically
nil, namely, during the first 3 or 4 days of laying,
the viability of the Filament eggs is conditioned by
the structure of the eggs. The more extreme the departure
of the egg structure from the normal, the lest
viable the eggs become. Viability of the eggs with
no filaments is zero.
(2). A comparison of the hatchability percentages with
the fertility (no. of adult flies obtained) percentages
as also the larger number of adult flies obtained
when Filament female was fertilised by wild type
male than when it is fertilised by Filament male, suggest
that possibly Filament is lethal when homozygous;
51
and that the lethal effect acts probably during the
larval or pupal stages.
(3). The shape of the egg is not necessarily determined
by the genetic possibilities of the embryo. filament
flies have been obtained from apparently .normal Filament
eggs. The gene for Filament is on the second
¡ chromosome.
(4). A study of the fecundity and ovarian rhythm in the
normal and Filament indicates the presence of a rhythm
(periods of high laying with periods of little or no
laying) in both; but fewer number of eggs per wave are
developed in Filament. The curve for the average number
of eggs produced per batch with successive waves is
one of first rapid increase followed by a gradual decrease,
the usual curve for egg -production. The curve
for the average time taken to lay these eggs in terns
of 12 -hr units with successive waves, follows on the
contrary, a linear trend. These results substantiate
s the assumption made by Donald and Lamy (1937) that
egg -production is due to the inter -action of, two
factors - one determining the number of egg- primordia
formed at the apices of the egg - strings, the other,
influencing the rate at which these priinordi a develop
into eggs ready for fertilisation and laying. The
different nature of the two curves show these factors
to be independent.
(5). The presence of the ovarian rhythm, lower number o
eggs produced per wave and the longer time taken to
lay them, point to the conclusion that the effect of
Filament on egg- production, is one of lowering the
normal activity of the ovary; in other words, reducing
the efficiency of the mechanism of egg -production
and laying. As this mechanism, as already shown, is
dependent on two factors, say factor A (determing the
number of eggs produced per wave) and factor B (deter
mining the rate of development into ripe eggs), a cor
responding repercussion in their normal rate of activity
is also expected. Thus there will be a reduction
in the rate of activity of the two factors. fieduction
in factor A activity will lead to the production
of lower number of eggs per wave; reduction in facto
B activity will lead to the lowering of the rate of laying-
lengthening the total period taken for laying.
This has been shown to be exactly the case in Filamen.
(6). The study of Filament has established at least 2
processes that went to evolve the number of filaments
in insects, esp., Drosophiladae. One is "total fus
and the other is "reduction" of the filaments.
II. THORAX - A MUTANT IN DROSOPHILA PSEUDO-OBSCURA.
A full morphological description of
a new autosomal gene in Drosophila pseudo - obscura -
thorax (th) - that partly overlaps wild type and
affects the thorax and its appendages, along with its
linkage data is given. It is found to be situated
on the third chromosome, 22.4 units to the right of
orange (or).
III. STERILITY MUTATIONS IN DROSOPHILA MELANOGASTER.
In an attempt to test Berg's hypothesis
namely that there are "much more numerous sterility
mutations in the X- chromosome than in the secon 1chromosome
and possibly more than in all the autosomes
taken together ", experiments were deviced to find out
the frequency of the male "specific" (producing sterility
of the males only, leaving females fertile)
and female "specific" (producing sterility in the
females only while leaving the males fertile) dominant
sterility mutations produced in the autosomes, as these
were not considered by Berg.
Our results show that such dominant sterility
mutations are either very rare or do not occur. Berg's
original conclusion therefore, holds good.
Localisation of ;S,,of the X- steriles
show 5 to be located between the garnet and forked
region, which lends support to Berg's suggestion that
there are found in the X- chromosome, greater number
of genes per unit of length of the active region,
affecting sex and is in agreement with the unpublish
ed data of Berg dealing with the location of 17 X-
steriles.