Africanization of a feral honey bee (Apis mellifera) population in South Texas: Does a decade make a difference?

The arrival to the United States of the Africanized honey bee (AHB), a hybrid between European honey bees and the African honey bee Apis mellifera scutellata, is a remarkable model for the study of biological invasions. This immigration has created an opportunity to study the dynamics of secondary contact of honey bee subspecies from African and European lineages in a feral population in South Texas. An 11-year survey (1991-2001) of this population showed that mitochondrial haplotype frequencies changed drastically over time from a resident population of eastern and western European maternal ancestry, to a population dominated by African haplotypes.  A subsequent study of the nuclear genome showed that the Africanization process included bidirectional gene flow between European and Africanized honey bees, giving rise to a new panmictic mixture of A. m. scutellata- and European-derived genes. In this study, we examined gene flow patterns in the same population 23 years after the first hybridization event occurred.  Of 89 tree cavities known to shelter colonies from earlier surveys, 28 were inhabited in 2013.  Of these, 25 colonies were of A. m. scutellata maternal ancestry, and 3 colonies were of western European maternal ancestry. No colonies of eastern European maternal ancestry were detected, although they were present in the earlier samples. Nuclear DNA revealed an increase in the introgression of A. m. scutellata-derived genes into the population. Our results suggest this feral population is an admixed swarm with a now greater presence of African-derived nuclear and mitochondrial genetic composition.