Leishmania mexicana: promastigotes migrate through osmotic gradients

During the insect phase of the parasite lifecycle, Leishmania promastigotes move from the midgut to the anterior regions of the alimentary tract of their sandfly vector. Chemotaxis of Leishmania promastigotes towards sugars has been reported, and the putative presence of sugar gradient in the insect foregut has been suggested to play a role in promastigote development in the insect. We have further investigated the potential of Leishmania mexicana promastigotes to respond to chemical stimulii. We find that promastigotes move towards concentrations of all substances tested and that this taxis requires the presence of an osmotic gradient. Our results indicate that behaviour that has previously been interpreted as chemotaxis is better understood as osmotaxis. The implications of this observation are discussed in the context of promastigote development.