In the western honey bee (Apis mellifera) populations, cytosolic malate dehydrogenase (cMDH) allele frequencies are correlated with the environmental temperatures. The Slow and Fast alleles predominate in hot climates; the Medium allele predominates in cold climates. We wondered whether natural selection has had anything to do with the Medium allele in the imported exotic A. mellifera population in Thailand. We genotyped workers (n = 1032) from 86 commercial colonies from three regions. Three

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... leles: Fast, Medium, and Slow, were detected. Over 96% of the Thai A. mellifera are either homozygous for the Slow or heterozygous for the Fast allele; and the Medium allele has the lowest frequency (c.a. 10%) in all sampled population. This might be indicative of selection against the Medium allele. However, as the cMDH allele frequencies in the Thai A. mellifera are similar to those seen in the source populations in the United States and Northern Italy, it might also be that the observed frequencies are reflective of frequent imports from the United States, and that natural selection is unable to overcome the effects of migration.