Experimental evidence for hybridization of closely related lineages in Plasmodium relictum

Over 50 avian Plasmodium species have been described. However, PCR-based information shows much broader diversity of genetic lineages in these parasites. This discrepancy indicates insufficient knowledge about taxonomic diversity and boundaries of a single species in avian Plasmodium species. In recent taxonomy, most of genetically closely related lineages that share the same morphology and development patterns are attributed to the same biological species, but there is no information if these lineages are able to cross. This information is crucial to understand if these lineages form single or multiple evolutionary units. Due to presence of sexual process and sporogonic development of Plasmodium parasites in mosquitoes, self and cross-fertilization can occur and be identified during the oocyst stage. We initiated in vivo hybridization experiments of two widespread Plasmodium relictum lineages (pSGS1 and pGRW11) in experimentally infected Culex pipiens pipiens form molestus mosquitoes. To study putative hybrid oocysts, we used a laser microdissection technique together with PCR-based analyses of mitochondrial and nuclear genes. We demonstrate that both pSGS1 and pGRW11 lineages develop in infected mosquitoes in parallel, but also form hybrid oocysts of these two lineages. Our results are in accord to a recent global phylogeographic study of P. relictum that suggested that cross-fertilization between pSGS1 and pGRW11 might occur. This information helps to understand population structure, gene flow and the evolutionary process of haemosporidian parasites.