Genetic characterization of Hawaiian isolates of Plasmodium relictum reveals mixed-genotype infections

Abstract

BackgroundThe relatively recent introduction of a highly efficient mosquito vector and an avian pathogen (Plasmodium relictum) to an isolated island ecosystem with naïve, highly susceptible avian hosts provides a unique opportunity to investigate evolution of virulence in a natural system. Mixed infections can significantly contribute to the uncertainty in host-pathogen dynamics with direct impacts on virulence. Toward further understanding of how host-parasite and parasite-parasite relationships may impact virulence, this study characterizes within-host diversity of malaria parasite populations based on genetic analysis of the trap (thrombospondin-related anonymous protein) gene in isolates originating from Hawaii, Maui and Kauai Islands.MethodsA total of 397 clones were produced by nested PCR amplification and cloning of a 1664 bp fragment of the trap gene from two malarial isolates, K1 (Kauai) and KV115 (Hawaii) that have been used for experimental studies, and from additional isolates from wild birds on Kauai, Maui and Hawaii Islands. Diversity of clones was evaluated initially by RFLP-based screening, followed by complete sequencing of 33 selected clones.ResultsRFLP analysis of trap revealed a minimum of 28 distinct RFLP haplotypes among the 397 clones from 18 birds. Multiple trap haplotypes were detected in every bird evaluated, with an average of 5.9 haplotypes per bird. Overall diversity did not differ between the experimental isolates, however, a greater number of unique haplotypes were detected in K1 than in KV115. We detected high levels of clonal diversity with clear delineation between isolates K1 and KV115 in a haplotype network. The patterns of within-host haplotype clustering are consistent with the possibility of a clonal genetic structure and rapid within-host mutation after infection.ConclusionAvian malaria (P. relictum) and Avipoxvirus are the significant infectious diseases currently affecting the native Hawaiian avifauna. This study shows that clonal diversity of Hawaiian isolates of P. relictum is much higher than previously recognized. Mixed infections can significantly contribute to the uncertainty in host-pathogen dynamics with direct implications for host demographics, disease management strategies, and evolution of virulence. The results of this study indicate a widespread presence of multiple-genotype malaria infections with high clonal diversity in native birds of Hawaii, which when coupled with concurrent infection with Avipoxvirus, may significantly influence evolution of virulence.ReviewersThis article was reviewed by Joseph Schall (nominated by Laura Landweber), Daniel Jeffares (nominated by Anthony Poole) and Susan Perkins (nominated by Eugene Koonin).

Transcriptome sequencing and analysis of Plasmodium gallinaceum reveals polymorphisms and selection on the apical membrane antigen-1

“…This prediction is supported by the high levels of diversity in nuclear genes from malaria parasites found in avian hosts of Hawaii, including the thrombospondin-related anonymous protein ( trap ) gene, which encodes a protein involved in immuno-evasion and erythrocyte invasion [ 77 ]. Although trap evolves under positive selection in human malaria parasites, no evidence for positive selection was found in trap of avian malaria parasites [ 78 ]. Here, for the first time, evidence of positive selection as a driving force in the evolution and diversification of an erythrocyte invasion gene ( ama - 1 ) in avian malaria parasites is provided.…”

Identification and characterization of the merozoite surface protein 1 (msp1) gene in a host-generalist avian malaria parasite, Plasmodium relictum (lineages SGS1 and GRW4) with the use of blood transcriptome

“…Within this system, msp1 -p19 would be a promising target for investigating cross-immunity between different species and changes of this cross-immunity depending on the phylogenetic distance of the parasite. Further, P. relictum has caused large population declines and mortality in endemic host communities, when introduced into formerly parasite-free areas (e.g., Hawaii) [32-34]. Here, msp1 -p19 could be used as a candidate gene to investigate whether individual birds that survive malaria infection carry antibodies against this peptide, in order to examine whether diverse host species target the same structures and pathways in the parasite when fighting the disease.…”

Immunity, resistance and tolerance in bird–parasite interactions

“…Traditionally, the study of avian malaria parasites has been carried out using natural populations of hosts . The advent of modern molecular techniques has promoted the discovery of an unsuspected diversity of parasite lineages and confirmed that, as for mammalian Plasmodia , individual hosts harbour mixed infections . Unravelling the cost of infection and the resistance/tolerance towards avian malaria has been a more challenging task, because as mentioned above this usually requires the use of experimental infections.…”