Rate of Development of Viability Mutants in Drosophila melanogaster

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Abstract
1. 21 wild type strains of Drosophila melanogaster which were coisogenic for chromosomes 1 and 3 and homozygous for different chromosomes 2 (which in 18 of the strains were previously irradiated) were tested with regard to viability by making the cross Cy L/+ × Cy L/+. The per cent of wild type flies in the F3-progeny, the "survival rate," was used as a measure of viability. 2. Experiments were performed with these strains on rate of development in 25⚬ C. Cy L/+ females were allowed to oviposit in periods of 1 hour and eclosion was checked with 2 hour intervals. 3. When arranging the strains in order according to their survival rates and grouping them into two halves, the upper half including strains with higher survival rates and the lower half including strains with lower survival rates, it was found that in the lower half the variances (estimated by the mean squares) in rate of development were significantly larger for wild type (homozygous) than for Cy L (heterozygous) flies. This was, however, not found in the upper half of strains. 4. For several reasons, which are discussed, one must be cautious when drawing conclusions from these findings. Still they indicate that, with regard to the character studied, flies with less viability vary more than those with higher viability. This is in accordance with recent studies on developmental homeostasis. Flies of less viability are found among the homozygotes in some of the strains. But as there are also strains with high viability of the homozygotes and as the homozygotes in these strains do not vary more than do the heterozygotes, there is no indication from the present study that it is homozygosity and heterozygosity per se which regulate the homeostatic forces. When comparing this with results of other investigators it must be kept in mind that heterozygosity in the present study means heterozygosity with regard to one wild type and one Cy L chromosome, in contradistinction to many other investigations where the heterozygosity concerns two different wild type chromosomes. 5. A rather strong correlation is found between the viability of the homozygotes and their rate of development. It is concluded that if one did not know anything about the viabilities of the different strains but selected slow growers, then one would preferentially draw less viable strains. It is stressed that if it were valid to extrapolate this to other animals in which there is no, or a limited, access to marker genes, it would be possible to find and isolate subvitals from selecting slow growers occurring in crosses producing homozygotes. With regard to questions on human populations and the importance of studying induced viability mutations (from radiation or from other sources) in mammals it would seem to be worth while to make experiments with e.g. mice along these lines.

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