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GENETICS OF NATURAL POPULATIONS. XVIII. EXPERIMENTS ON CHROMOSOMES OF DROSOPHILA PSEUDOOBSCURA FROM DIFFERENT GEOGRAPHIC REGIONS

1948; Oxford University Press; Volume: 33; Issue: 6 Linguagem: Inglês

10.1093/genetics/33.6.588

1943-2631

Theodosius Dobzhansky,

Insect Resistance and Genetics

THE PROBLEMSixteen different gene arrangements have been recorded in the third chromosomes of natural populations of Drosophila pseudoobscura, and eight in the third chromosomes of the related species Drosophila persimilis.Representatives of the same species which carry different types of third chromosomes have different adaptive values in some environments.If artificial populations with certain proportions of the chromosomal types are set up in population cages, these proportions may undergo rapid changes.Analysis of these chahges leads to the inference that the adaptive values of individuals carrying two chromosomes of different types (structural heterozygotes) are higher than those of individuals with two similar chromosomes (structural homozygotes).The superior fitness of the heterozygotes results in populations reaching certain equilibrium states, at which the chromosomal types are present in definite proportions (WRIGHT and DOBZHANSKY 1946).Differential survival which favors structural heterozygotes relative to the homozygotes has been further ascertained through observations on deviations from the HARDY-WEINBERG equilibrium ratios of hetero-and homozygotes among flies which developed in population cages (DOBZHANSKY 1947a).I n the experiments previously reported, the experimental populations were made from flies the ancestors of which had been collected in a single locality-Pition Flats, on Mount San Jacinto, in Southern California.The gene arrangements found in the third chromosomes of the Piiion Flats population occur, however, in populations of the Pacific Coast from British Columbia to Lower California.The problem naturally arises whether the adaptive properties of flies with the same chromosomes are alike wherever a given chromosomal type occurs, or whether the gene contents of these chromosomes are geographically differentiated.The flies used in the experiments to be reported in the present publication are descendants of wild flies collected in three different localities: Piiion Flats and Keen Camp, both on Mount San Jacinto in southern California, and Mathe;, in the Sierra Nevada of central California.It will be shown that chromosomes with the same gene arrangement found in geographically distinct populations have different adaptive properties.MATERIAL Progenitors of the flies that served as material for the experiments were collected a t Piiion Flats and Keen Camp in April 1945 by MR.ALEXANDER SOKOLOFF, and a t Mather during the summers of 1945 and 1946 by the writer.