Genetic load in northern populations of Drosophila subobscura.

O-chromosomes were sampled from two natural populations in Finland, one population in Sweden and one population from Spain. Viability was assessed using the Va/Ba balanced lethal strain. The genetic load for lethals and semilethals was 0.165/0.065 in Helsinki and 0.151/0.101 in Tvärminne, Finland; 0.309/0.118 in Gävle, Sweden and 0.351/0.112 in Barcelona, Spain. With the exception of the Gävle population, the results confirm the general picture that marginal populations are characterized by light loads in comparison with the central ones. Homozygotes for normal or almost normal chromosomes from the Gävle population were less viable than corresponding homozygotes from other marginal populations. Random combinations of wild chromosomes from the Spanish and Finnish populations did not show increased viability over random Spanish heterozygotes.