Molecular evolution in immune genes across the avian tree of life.

All organisms encounter pathogens, and birds are especially susceptible to infection by malaria parasites and other haemosporidians. It is important to understand how immune genes, primarily innate immune genes which are the first line of host defense, have evolved across birds, a highly diverse group of tetrapods. Here, we find that innate immune genes are highly conserved across the avian tree of life and that although most show evidence of positive or diversifying selection within specific lineages or clades, the number of sites is often proportionally low in this broader context of putative constraint. Rather, the evidence shows a much higher level of negative or purifying selection in these innate immune genes - rather than adaptive immune genes - which is consistent with birds' long coevolutionary history with pathogens and the need to maintain a rapid response to infection. We further explored avian responses to haemosporidians by comparing differential gene expression in wild birds (1) uninfected with haemosporidians, (2) infected with Plasmodium, and (3) infected with Haemoproteus (Parahaemoproteus). We found patterns of significant differential expression with some genes unique to infection with each genus and a few shared between "treatment" groups, but none that overlapped with the genes included in the phylogenetic study.

Distribution and Prevalence of Haemosporidian Parasites in the Northern Cardinal (Cardinalis cardinalis).

Avian haemosporidian parasites provide a model system for understanding ecological and evolutionary host-parasite interactions. The diversity and distribution of these parasites remains incomplete, and, here, we provide the first range-wide assessment of avian haemosporidians in a continentally distributed host, the Northern Cardinal ( Cardinalis cardinalis ). Based on molecular techniques, we show geographical differences in prevalence and lineage diversity between host subspecies and identify several novel lineages. We use phylogenetic reconstruction to show where these lineages fit into the expanding evolutionary tree of avian haemosporidian lineages. All except 1 subspecies of Northern Cardinal are highly parasitized by a wide diversity of Plasmodium and Parahaemoproteus. Compared to published studies that used microscopy to determine prevalence in this host, we find a much higher number of infected individuals (67.4% vs. 45% or less). Consistent with previous studies, Parahaemoproteus from the Northern Cardinal was found to be highly host specific and geographically structured, whereas Plasmodium was less host specific and geographically unstructured.

Haemosporidian Prevalence and Parasitemia In the Tufted Titmouse (Baeolophus bicolor).

Hemosporidians are a diverse group of blood parasites that infect terrestrial vertebrates worldwide, but there is variability in parasite prevalence and parasitemia with infections ranging from virtually inconsequential to lethal. In this study, we determined prevalence and parasitemia of avian hemosporidians in the Tufted Titmouse ( Baeolophus bicolor ; n = 81). Plasmodium and Parahaemoproteus were detected and quantified from blood samples using microscopy, polymerase chain reaction (PCR), and quantitative PCR. Thirteen mitochondrial cytochrome b lineages of hemosporidian parasites were found, including generalist and specialist lineages, and the data indicate that prevalence is 69.1% (Plasmodium 89.3%; Parahaemoproteus 7.1%; double infection 3.6%). However, parasitemia was low in all infected birds. Seasonally, parasite prevalence varied significantly, although prevalence and parasitemia were not associated with host sex, age, or health. Observations of infection in this naturally infected bird provide details on host susceptibility that are applicable to the understanding of hemosporidian parasites in other avian hosts.