AN EXPLORATORY INVESTIGATION OF THE SELECTIVE VALUE OF CERTAIN GENES AND THEIR COMBINATIONS IN DROSOPHILA

¹ From the University of Michigan, Ann Arbor, Michigan

1. A total of 203,159 flies (offspring) were counted in this investigation. Of this number, 190,318 were the basis of the selective index calculations. (The remaining 12,841 offspring were in families which were not comparable with the others for several reasons.)

2. In sixteen experiments a total of 647 females were compared, in groups of about forty, in respect to longevity, fecundity, and rapidity of production.

3. A wild-type female in Experiment 7 had the longest life span of all (57.75 days), and the longest mean duration of life of any one group was exhibited by the five wild females of that same experiment (42.25 days). Other groups which ranked comparatively high in longevity were the yellow cut⁶ females of Experiment 7, the eosin ones of Experiment 6, and the yellow white ones of Experiment 1a.

4. The highest fertility record (taking into account only adult offspring) was exhibited by one of the eosin females in Experiment 6 (1434 offspring). The highest mean number of offspring of any group was found in the group of which this female was a member, the five eosin females of Experiment 6 (1046 offspring). The yellow purple females of Experiment 5 also showed high fecundity.

5. Selective index, the value of an individual in selection, is calculated by dividing the mean number of offspring by the mean number of days between generations. This is logically a better criterion of value to a race than is either longevity or fecundity.

6. Nineteen genes and their combinations, mostly in groups of three, were compared in respect to selective index.

7. In general, the addition of a mutant gene to another mutant gene or to a combination of mutant genes may (1) raise the selective value of the final combination above the value of either of the component mutations or combinations taken singly; (2) result in a selective value intermediate between those of the component mutations or combinations; (3) yield a selective value lower than the lowest of the components; or (4) make the selective value practically equal to that of one of the components. Some of the more striking examples of these reactions are mentioned below.

8. Flies which carried eosin, in combination with practically any other gene tested, had greater survival value than flies with the same genetic composition except for eosin.

9. Purple and scarlet genes are likewise advantageous, but less consistently so than eosin.

10. The genes cut⁶, black, miniature, sooty, and vestigial have been demonstrated to produce a weakening effect when combined with several other genes separately.

11. It has been shown that females go on producing offspring at a normal rate for at least two days after the removal of their mates. Thereafter the number of offspring falls off rapidly.

12. Variability of females of the same genotype in production, longevity, vitality, and the time of development of their offspring renders uncertain the application of the conclusions of this investigation to Drosophila females in general.