| الملخص: |
I. "Filament" : 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. |
