Distribution and phenotypic population structure of the tsetse flies Glossina morsitans morsitans Westwood and Glossina morsitans centralis Machado in Zambia

Abstract

Glossina morsitans s.l. is an efficient vector of African trypanosomiasis, a debilitating and fatal disease of humans and livestock. This study aimed at investigating the current distribution and phenotypic population structure of G. m. centralis Machado and G. m. morsitans Westwood to inform effective vector management strategies in Zambia. To achieve these objectives, the study also evaluating a vehicle-mounted sticky trap (VST) for effective and rapid sampling of G. morsitans over large geographic areas. Randomised block design experiments were used to establish the optimal design of VST. An extensive VST based tsetse survey was then conducted in all tsetse belts in Zambia. The occurrence records obtained from this survey were used to model the distribution of G. m. centralis and G. m. morsitans using a Maxent species distribution model. Landmark-based wing geometric morphometrics was undertaken to investigate the population-level phenotypic variation of the two subspecies. There were no significant differences in catch indices of VST constructed using an all-blue, all-black and 1:1 blue-black panel. Overall, the VST oriented in-line and baited with butanone and 1-octen-3-ol, caught 2.42 and 2.60 times more G. m. centralis and G. m. morsitans respectively, than the standard mobile trapping device, the black-screen fly round. The VST survey captured a total of 15,602 flies with G. m. morsitans (58%) and G. m. centralis (39%) being the most abundant. The predicted potential distribution for G. m. centralis was 80,863 km2 while that of G. m. morsitans was 70,490 km2 representing a 47 and 29% reduction compared to their historical distributions, respectively. Significant differences in wing centroid size and shape were observed between G. morsitans sexes, subspecies and sample locations within each subspecies range. The populations of G. morsitans were found to exhibit significant population-level variation in fly size and wing shape which suggests high levels of population structuring. The main drivers of this structuring could be random genetic drift in G. m. centralis demes and local adaptation to environmental conditions in G. m. morsitans populations.