Immune defense genes are sometimes highly variable in host populations, reflecting selective pressure to combat diverse pathogens. In other instances, where there are only a few dominant pathogens, natural selection may favor only one or a few defense alleles. Here, we show that both adaptive strategies can occur in the same genes under different circumstances.

We examined diversity in the APL1 genes of the human malaria vector mosquito Anophleles gambiae, which play a role in defense against malaria parasites. We found that the APL1 genes are exceptionally polymorphic, being 10-fold more diverse than typical A. gambiae genes.
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