Extrachromosomal inheritance of susceptibility to trypanosome infection in tsetse flies

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Abstract
Differences in susceptibility to infection with Trypanosoma congolense between F1 families of Glossina morsitans morsitans indicated that susceptibility is maternally inherited in this species of tsetse fly. Twelve F1 families, six selected for susceptibility and six selected for refractoriness to infection, have been bred for up to 13 generations. The reciprocal differences demonstrated in the F1 generation persisted in these selected families over many generations, indicating that susceptibility/refractoriness to T. congolense infection is extrachromosomally inherited in G. m. morsitans. Repeated ‘backcrossing’ to males of the opposite strain showed that infection rates within families were independent of the contribution of the male parent. Susceptible families had a mean midgut infection rate of 76·9% and a mature (hypopharyngeal) infection rate of 47·9%. In the refractory families 88·9% of the flies failed to develop an infection, 11·1% had midgut infections and only 6·3% developed mature infections. Levels of midgut infection remained remarkably constant within families over generations, whether refractory or susceptible, while maturation rates varied between generations and between sexes. Males matured a significantly greater proportion of midgut infections than females in the susceptible families. It is suggested that the inheritance of susceptibility/refractoriness relates primarily to the establishment of midgut infections in G. m. morsitans, and that maturation of midgut infections is dependent upon environmental factors such as diet, and differences between sexes probably reflecting differences in rates of bloodmeal digestion. The fact that susceptibility is a heritable character in tsetse suggests that when assessing the ‘challenge’ presented by a fly population it is desirable to measure its susceptibility as well as its rate of infection with trypanosomes.