The following experiments with Drosophila melanogaster were planned to study the effects of four environmental factors on a mixed population. Varying the environmental conditions under which the larvae developed would be expected to show to what'extent these factors affect the total yield and also the ratio of the types to each other.
METHODSAll experiments were carried out in incubators at approximately 25°C. The food was the customary banana-agar mixture in the proportion of 500 grams of banana, 500 cc. of water, and 10 grams of agar-agar. The bananas were always overripe (the skins dark brown or black), but not decayed. The amount of yeast used varied with the a r e a -o n e drop of a yeast suspension (one cake of fresh yeast mixed with 100 cc. of water) was used in vials, two drops in 4-ounce bottles, four drops in half-pint and pint milk bottles, and six drops in 250-cc. Erlenmeyer flasks.Virgin wild females of a particular inbred stock (wildtype flies from the Star Dichaete stock) were crossed to vestigial males. Virgin F, females were backcrossed to vestigial males. The use of the backcross assures equal numbers of two kinds of eggs being deposited in each culture. A
It has been known for some time (Roberts, '18) that high temperatures lengthen the wings of the vestigial mutant of Drosophila melanogaster. I n connection with an experiment on the effect of temperature on selection and population, it was thought advisable to make exact measurements of the vestigial wings at the different temperatures to determine whether this lengthening effect was a gradual one increasing with the temperature o r a sudden chahge at a critical temperature.
METHODSSince a wild-vestigial backcross was used, the vestigial offspring were expected to constitute approximately half of the progeny of the backcross generation. The parents and the F, heterozygous females and the vestigial males to which they were backcrossed were raised at 25°C. The F, females were forty-eight hours old ahd had been mated for twenty-four hours before they were put in the culture bottles and placed in the incubators at the various temperatures. These F, individuals were offspring of the same mating. They were placed simultaneously in incubators set at 18.3", 26", 28", 29", 30", and 31°C. The variation in temperature within any incubator was +O.O5"C. Standardized thermometers were used in the experiments.
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