Identification of a TNF-TNFR-like system in malaria vectors (Anopheles stephensi) likely to influence Plasmodium resistance

Abstract

Identification of Plasmodium-resistance genes in malaria vectors remains an elusive goal despite the recent availability of high-quality genomes of several mosquito vectors. An. stephensi, with its three distinctly-identifiable forms at the egg stage, correlating with varying vector competence, offers an ideal species to discover functional mosquito genes implicated in Plasmodium resistance. Recently, the genomes of several strains of An. stephensi of the type-form, known to display high vectorial capacity, have been reported. Here, we report a chromosomal-level assembly of an intermediate-form of An. stephensi strain (IndInt), shown to have reduced vectorial capacity relative to a strain of type-form (IndCh). The contig level assembly with a L50 of 4 was scaffolded into chromosomes by using the genome of IndCh as the reference. The final assembly shows a heterozygous paracentric inversion, 3Li, involving 8 Mbp, which is syntenic to the extensively-studied 2La inversion implicated in Plasmodium resistance in An. gambiae involving 21 Mbp. Deep annotation of genes within the 3Li region in IndInt assembly using the state-of-the-art protein-fold prediction and other annotation tools reveals the presence of a TNF-like gene, which is the homolog of the eiger gene in Drosophila. Subsequent chromosome-wide searches revealed homologs of wengen (wgn) and grindelwald (grnd) genes in IndInt, which are known to be the receptors for eiger in Drosophila. We have identified all the genes in IndInt required for eiger-mediated signaling by analogy to TNF-TNFR system, suggesting the presence of a functionally active eiger signaling pathway present in IndInt. Comparative genomics of high-quality genome assemblies of the three type-forms with that of IndInt, reveals structurally disruptive mutations in eiger gene in all three strains of the type-form, alluding to compromised innate immunity in the type-form as the cause of high vectorial capacity in these strains. This is the first report of the presence of an intact evolutionarily-conserved TNF-TNFR signaling system in malaria vectors, with a potential role in Plasmodium resistance.