1. Insects are commonly infected with bacterial endosymbionts. In addition to the costs and benefits associated with harbouring these symbionts, their rates of vertical and horizontal transmission are important determinants of symbiont prevalence. 2. Aphids are cyclical parthenogens and show virtually perfect maternal transmission of endosymbionts during asexual reproduction. Less clear is the role of the annual sexual generation, during which overwintering eggs are produced. Data from pea aphids ( Acyrthosiphon pisum Harris) suggest that maternal transmission failures and horizontal transmission via males may occur under sexual reproduction at least occasionally. No such data exist for other aphid species. 3. In the present study, the rates of maternal and paternal transmission of facultative endosymbionts during sexual reproduction in the black bean aphid, Aphis fabae ( Scopoli) were examined. Crosses were performed between clones infected with Hamiltonella defensa, clones infected with Regiella insecticola and clones without facultative endosymbionts, and eggs were overwintered under three different conditions. 4. Only one of 205 offspring from crosses testing for maternal transmission failed to inherit the symbiont present in the maternal clone, and in crosses testing for horizontal transmission, only one of 412 offspring acquired a facultative symbiont from the father. 5. These results show that in A. fabae, maternal transmission of H. defensa and R. insecticola is extremely reliable also during sexual reproduction, indicating that maternal transmission failures are unlikely to exert a significant influence on frequencies of infection in the field. Paternal transmission of endosymbionts was exceedingly rare, suggesting that this route of horizontal transmission may be less important than hitherto assumed.
