1. ¹Behaviour and Genetics of Social Insects Lab, School of Biological Sciences A12, University of Sydney, Sydney, Australia
2. ²Honeybee Research Section, ARC-Plant Protection Research Institute, Stellenbosch, South Africa
3. ³Department of Botany and Zoology, DST-NRF Centre of Excellence for Invasion Biology, University of Stellenbosch, Matieland, South Africa

Correspondence: Dr M Beekman, Behaviour and Genetics of Social Insects Lab, School of Biological Sciences A12, University of Sydney, Macleay Building, Sydney, New South Wales 2006, Australia. E-mail: mbeekman@bio.usyd.edu.au

Received 21 February 2007; Revised 31 July 2007; Accepted 3 August 2007; Published online 12 September 2007.

Abstract

Hybrid zones are found wherever two populations distinguishable on the basis of heritable characters overlap spatially and temporally and hybridization occurs. If hybrids have lower fitness than the parental types a tension zone may emerge, in which there is a barrier to gene flow between the two parental populations. Here we discuss a hybrid zone between two honeybee subspecies, Apis mellifera capensis and A. m. scutellata and argue that this zone is an example of a tension zone. This tension zone is particularly interesting because A. m. capensis can be a lethal social parasite of A. m. scutellata. However, despite its parasitic potential, A. m. capensis appears to be unable to increase its natural range unassisted. We propose three interlinked mechanisms that could maintain the South African honeybee hybrid zone: (1) low fitness of intercrossed and genetically mixed colonies arising from inadequate regulation of worker reproduction; (2) higher reproductive success of A. m. scutellata via both high dispersal rates into the hybrid zone and increased competitiveness of males, countered by (3) the parasitic nature of A. m. capensis.