Discussion

Trypanosoma brucei undergoes a complex life cycle in the tsetse fly vector. The parasite´s development occurs during a journey through the different microenvironments of the fly´s interior organs. For the trypanosome it involves crossing various barriers and confined surroundings, concurrent with major morphological changes. The unicellular flagellate´s motility, which is directly dependent on morphology, is essential for its survival and successful development. This work details cell motility, morphology and collective behaviour of different developmental stages using high spatio-temporal resolution microscopic analysis. Swimming patterns of solitary swimmers were analysed in vivo and in vitro, as well as collective motion at the single cell level in vivo by using fluorescently labelled parasites. Light sheet fluorescence microscopy was established as a powerful tool for the 3D analysis of the infection process in the tsetse fly´s digestive tract. This provides information about tissue topology with unprecedented resolution. We are able to visualise fluorescent trypanosomes inside the surprisingly complex folding of the peritrophic matrix and can begin to correlate the microenvironments and trypanosome behaviour to high-speed motility data. We propose that the infection process is a succession of solitary and collective motion and represents an adaption to the varying and convoluted microenvironments trypanosomes have to face inside the tsetse fly.