Wild O chromosomes of Drosophila subobscura were isolated from three populations in southern Finland. The populations are on the northern margin of the distribution of the species. Lethal frequencies and viability effects of wild chromosomes were studied in homozygous and random heterozygous combinations. The percentage of chromosomes with lethals was 9.2% and with semilethals 10.5%. These are the lightest loads reported for the species over its range of distribution; the central populations may have loads about two and a half times heavier. The presence of a light load in a marginal population contrasts to the high amount of enzyme polymorphism but agrees with the low amount of inversion polymorphism. Exact measurements of the load are important at the present stage of environmental pollution.
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