THE RATE OF ANTIGENIC VARIATION IN FLY-TRANSMITTED AND SYRINGE-PASSAGED INFECTIONS OF TRYPANOSOMA-BRUCEI

Rates of antigenic variation were measured in vivo in several populati ons of cloned lines of Trypanosoma brucei before and after cyclical tr ansmission through tsetse flies. Two cloned lines were adapted for use in laboratory conditions by extensive syringe passaging and rates of antigenic switching/cell/generation were less than 3 x 10(-6) and 1x10 (-4) in each line. Rates of switching were then determined after fly t ransmission of the first line and generated per capita rate values of greater than 2 x 10(-3) in three of four populations examined. In the fourth population the switch rate was lower: less than 7 x 10(-5) swit ches/cell/generation. These data show that rates of antigenic variatio n are several orders of magnitude lower in syringe-passaged lines, suc h as those routinely used in the majority of laboratory studies, compa red with most recently fly-transmitted lines. They also show that the reduction in switching rate associated with syringe passaging is rever sible and is thus unlikely to be caused by mutation.