Onset of expression of the variant surface glycoproteins of Trypanosoma brucei in the tsetse fly studied using immunoelectron microscopy.

The acquisition of the variant surface glycoprotein (variable antigen) coat by metacyclic stage Trypanosoma brucei in the salivary glands of the tsetse fly, Glossina morsitans, has been studied in situ by transmission and scanning electron microscopy using monoclonal antibodies raised against metacyclic variable antigen types and complexed with horseradish peroxidase or colloidal gold. The coat is acquired after binary fission has ceased but while the parasite is still attached to the gland epithelium, i.e. before the mature metacyclic is released into the gland lumen. The variable antigen type heterogeneity previously observed in discharged mature metacyclics is here demonstrated in the nascent (attached) metacyclic population. The variant surface glycoprotein genes are thus not expressed in a fixed sequence since different metacyclic variable antigen types are present ab initio. The distribution of immunogold-marked nascent metacyclics of a particular variable antigen type, as shown by quadrat analysis of a scanning electron micrograph montage of the infected salivary gland epithelium, conforms to a Poisson series. This provides evidence that individual variant surface glycoprotein genes are stochastically activated and suggests that selective activation occurs after trypanosome division has ceased.