Phylogeny and evolution of the SERA multigene family in the genus Plasmodium

The serine repeat antigen gene family of Plasmodium falciparum (Pf-SERA) consists of nine gene members. By sequence similarity search, 45 genes were identified to be homologous to the Pf-SERA genes in the ongoing seven Plasmodium genome sequencing project databases for the species: P. reichenowi, P. vivax, P. knowlesi, P. yoelii, P. berghei, P. chabaudi, and P. gallinaceum. In combination with additional PCR-based sequencing, we found that almost all SERA genes in each species were aligned in a tandem cluster and sandwiched between two conserved hypothetical protein genes, except for P. reichenowi, which could not be confirmed. The minimum and maximum numbers of clustered genes were 2 and 12 for P. gallinaceum and P. vivax, respectively. The best tree of the maximum likelihood analysis demonstrated that all Plasmodium SERA homologues, except for SERA1 of P. gallinaceum (Pg-SERA1), can be classified into four groups, represented by Pf-SERA5, Pf-SERA6, Pf-SERA7, and Pf-SERA8. Genes in the Pf-SERA8 group, although highly divergent and distantly related to the sequences of other groups, were not pseudogenes. P. berghei SERA5, the counterpart of Pf-SERA8, was expressed in the mosquito stage. P. gallinaceum lacks the orthologues to Pf-SERA5, Pf-SERA6, and Pf-SERA7, suggesting that P. gallinaceum diverged from a common ancestor of all eight Plasmodium species examined before gene duplication(s) occurred to generate these paralogous groups. Here, we reveal an evolutionary trail of SERA gene cluster in the genus Plasmodium and discuss a phylogeny of Plasmodium species from the viewpoint of the evolution of a multigene family.