The expansion of Africanized honeybees from South America to the southwestern United States in Ͻ50 years is considered one of the most spectacular biological invasions yet documented. In the American tropics, it has been shown that during their expansion Africanized honeybees have low levels of introgressed alleles from resident European populations. In the United States, it has been speculated, but not shown, that Africanized honeybees would hybridize extensively with European honeybees. Here we report a continuous 11-year study investigating temporal changes in the genetic structure of a feral population from the southern United States undergoing Africanization. Our microsatellite data showed that (1) the process of Africanization involved both maternal and paternal bidirectional gene flow between European and Africanized honeybees and (2) the panmitic European population was replaced by panmitic mixtures of A. m. scutellata and European genes within 5 years after Africanization. The post-Africanization gene pool (1998)(1999)(2000)(2001) was composed of a diverse array of recombinant classes with a substantial European genetic contribution (mean 25-37%). Therefore, the resulting feral honeybee population of south Texas was best viewed as a hybrid swarm.T HE evolutionary significance of natural hybridizaOur study of hybridization, which deals with one of tion has been debated for decades (Mayr 1942

Summary — New discriminant analysis procedures have been presented to identify Africanized and European honey bees. These procedures are founded on data from 2 103 samples of honey bees collected from colonies at several locations in the western hemisphere and Kangaroo Island, Australia. Various univariate and multivariate analyses have been used to select the morphological characteristics and the groups of characteristics to be used in the analysis. The multivariate discriminant analysis correctly identified 565 (95.6%) of the 591 Africanized colonies and correctly identified all of the 1 512 European colonies.

A population genetic analysis of honey bees of the Mexican neotropical Yucatan peninsula shows that the range expansion of Africanized bees there has involved extensive introgressive hybridization with European bees. Yucatan honey bee populations now include many colonies with intermediate morphologies. Genotypes of mitochondria have disassociated from historically correlated Africanized or European morphology, producing diverse phenotypic associations. This suggests that the size of resident European populations may be important in explaining previously reported asymmetrical hybridization. Evidence of natural hybridization is encouraging for the use of genetic management to mitigate the effects of Africanized bees in the United States.

Polymerase chain reaction (PCR)-ampliÞed mitochondrial DNA (mtDNA) assays have been used in studies of the Africanization process in neotropical feral and managed honey bee populations. The approach has been adopted, in conjunction with morphometric analysis, to identify Africanized bees for regulatory purposes in the United States such as in California. In this study, 211 Old World colonies, representing all known introduced subspecies in the United States, and 451 colonies from non-Africanized areas of the southern United States were screened to validate a rapid PCR-based assay for identiÞcation of Africanized honey bee mtDNA. This PCR-based assay requires a single enzyme digestion (BglII) of a single PCR-ampliÞed segment of the cytochrome b gene. The BglII polymorphism discriminates the mitochondrial haplotype (mitotype) of Apis mellifera scutellata L. (ancestor of Africanized bees) from that of A. m. mellifera, A. m. caucasia, A. m. ligustica, A. m. carnica, A. m. lamarcki, A. m. cypria, A. m. syriaca, and some A. m. iberiensis, but not from that of A. m. intermissa and some A. m. iberiensis. Nonetheless, given the very low frequency (Ͻ1%) of African non-A. m. scutellata mitotype present before arrival of Africanized bees in the United States, cytochrome b/BglII assay can be used to identify maternally Africanized bees with a high degree of reliability.

Abstract. The invasion of Africanized honeybees (Apis mellifera L.) in the Americas provides a window of opportunity to study the dynamics of secondary contact of subspecies of bees that evolved in allopatry in ecologically distinctive habitats of the Old World. We report here the results of an 11-year mitochondrial DNA survey of a feral honeybee population from southern United States (Texas). The mitochondrial haplotype (mitotype) frequencies changed radically during the 11-year study period. Prior to immigration of Africanized honeybees, the resident population was essentially of eastern and western European maternal ancestry. Three years after detection of the first Africanized swarm there was a mitotype turnover in the population from predominantly eastern European to predominantly A. m. scutellata (ancestor of Africanized honeybees). This remarkable change in the mitotype composition coincided with arrival of the parasitic mite Varroa destructor, which was likely responsible for severe losses experienced by colonies of European ancestry. From 1997 onward the population stabilized with most colonies of A. m. scutellata maternal origin.