Horizontal Transmission of the Symbiont in .

The recently discovered symbiont, , has a strong malaria transmission-blocking phenotype in , the predominant species complex member in many active transmission areas in eastern Africa. The ability of to block transmission together with vertical transmission and avirulence makes it a candidate for the development of a symbiont-based malaria transmission blocking strategy. We investigate the characteristics and efficiencies of transmission between mosquitoes. We show that is not transmitted between larvae but is effectively transmitted horizontally between adult mosquitoes. Notably, was only found to be transmitted between male and female , suggesting sexual horizontal transmission. In addition, cells were observed infecting the ejaculatory duct. Female that acquire horizontally are able to transmit the symbiont vertically to their offspring. We also investigate the possibility that can infect alternate hosts that live in the same habitats as their hosts, but find no other non-anopheline hosts. Notably, infections were found in another primary malaria African vector, . The finding that can be transmitted horizontally is relevant for the development of dissemination strategies to control malaria that are based on the targeted release of infected mosquitoes.