Distinct roles for Pbs21 and Pbs25 in the in vitro ookinete to oocyst transformation of Plasmodium berghei

We have developed an in vitro culture system for early sporogonic stages of Plasmodium berghei, which can be used to study developmental events normal ly taking place in the midgut of an infected mosquito. These include penetr ation of insect cells by the mature ookinete, transformation into oocysts a nd the early development of the latter, sustained through several rounds of nuclear division, The system, based upon co-culture of enriched ookinetes with several established insect cell lines, was used to study the developme nt of mutant ookinetes lacking both the Pbs21 and Pbs25 surface proteins. M otility and entry of double knockout and Pb21 single knockout ookinetes int o the insect cells are normal, but the number of ookinetes successfully tra nsforming into oocysts expressing the CSP protein are substantially reduced , Finally, using the yeast two-hybrid system we also show that Pbs25 has th e capacity to homodimerise as well as to form heterodimers with Pbs21.