The arrival of Africanized honey bee Apis mellifera to the United States less than fifty years after its original release in Brazil is an interesting phenomenon. To better understand the population genetic aspects of this phenomenon, genetic variability and differentiation among Africanized honey bee populations were investigated. Mitochondrial and microsatellite DNA markers were used to compare the genetic composition of several populations of Africanized honey bees from Brazil, Central America, and Texas. Using mitochondrial DNA markers, three haplotypes were identified. One haplotype was European (C1) and the others were African (A1 and A4). A major shift in the frequencies of African mitochondrial DNA haplotypes A1 and A4 was detected across the range of Africanized honey bees. The A1 haplotype increased northward to Texas and largely replaced the A4 haplotype. The direction of the clinal distribution of A1-A4 in South America and perhaps even Africa appears opposite to the findings of the present study. It was reported that A1 was predominant in the more equatorial regions of Africa and South America, while the frequency of A4 increases in the more temperate (southern region). Selection along a temperature/climate gradient has been suggested as the reason for an observed A1-A4 cline in South America. However, in our study, the best explanation for the A1 northward incline would be selection for some geneotypes. The microsatellite DNA analysis revealed that the three populations displayed high genetic diversity with an average of expected heterozygosity per locus 0.82. Furthermore, private alleles found in the Texas population suggest ongoing hybridization with non-Africanized honey bees
