Paternal transmission of a secondary symbiont during mating in the viviparous tsetse fly

Abstract

Sodalis glossinidius, a maternally-inherited secondary symbiont of the tsetse fly, is a bacterium in the early/intermediate state of the transition towards symbiosis, representing an important model for investigating establishment and evolution of insect-bacteria symbiosis. The absence of phylogenetic congruence in tsetse-Sodalis co-evolution and the existence of Sodalis genotypic diversity in field flies is suggestive for a horizontal transmission route. However, to date no natural mechanism for the horizontal transfer of this symbiont has been identified. Using novel methodologies for the stable fluorescent-labeling and introduction of modified Sodalis in tsetse flies we unambiguously show that male-borne Sodalis is i) horizontally transferred to females during mating and ii) subsequently vertically transmitted to the progeny i.e. paternal transmission. This mixed mode of transmission has major consequences regarding Sodalis' genome evolution since it can lead to co-infections creating opportunities for lateral gene transfer which in turn could affect the interaction with the tsetse host.