Parasite infections impose high costs on both human and animal populations, increasing morbidity and mortality, particularly in hosts under ecological stress [1–3]. Understanding the genetic regulation of parasite resistance in natural population is of major importance for understanding host-parasite evolution and host sexual selection processes. In the past, great effort has been devoted to study major histocompatibility complex (MHC) diversity and compatibility as a key element of genetic regulation of parasite resistance and a potential driving force in sexual selection processes, respectively [4–12]: host genetic variation can be promoted by parasites through frequency-dependent selection on advantageous resistance alleles [10, 13], and individuals that are heterozygous at the MHC are expected to have a selection advantage and to be better capable of combating a variety of infectious agents than MHC homozygotes [5, 7, 14]. Additionally, some studies report correlations between parasite resistance and individual heterozygosity that are explained by reduced fitness values of homozygous individuals for traits that are controlled by directionally dominant loci [13, 15, 16]. Still, there is accumulating evidence that individual heterozygosity often appears to be a weak predictor of parasite infection and the importance of specific alleles of candidate genes in regulation of parasite infection has been suggested [15, 17]. In this respect, cytokine genes such as interleukins are natural candidates due to their major regulatory role in helminth parasite susceptibility , and recently Fumagalli and colleagues  highlighted their evolutionary significance as a target of balancing selective processes.
Immunity to helminth parasite infections is mainly mediated by CD4+ T-helper 2- (TH2) lymphocytes with promotion of TH2 immune responses (humoral immune responses) being dependent on the cytokine interleukin-4 (IL-4) . IL-4 not only induces and sustains TH2 responses and suppresses TH1 responses, but also initiates immunoglobulin (Ig) isotype switching to IgE, which plays an essential role in anti-parasite immunity . Evidence mainly from human diseases is accumulating that single nucleotide polymorphisms (SNPs) in the promoter region of the interleukin 4 gene (IL4) affect its transcription, resulting in altered IL-4 protein levels and, hence, in either higher or lower IgE titres [21–24]. In this way, IL4 SNPs can effectively influence the intensity of various infections [22, 25–30], including enteric pathogens [31–34]. Still, despite its key role in the regulation of parasite infections and resulting evolutionary significance of IL4 polymorphisms , empirical evidence of the importance of IL4 on the intensity of parasite infections in natural population is lacking.
This study integrates field parasitology and population genetics to investigate the functional significance of polymorphisms in the IL4 gene on gastro-intestinal infections in a wild non-human primate: the red-fronted lemur (Eulemur fulvus rufus).
Red-fronted lemurs live in small multi-male, multi-female groups of 5-12 individuals with an even or slightly male-biased adult sex ratio [35, 36]. Reproduction is highly seasonal with only one mating period per year. During this three-to four-week period, females are in oestrus for approximately one day and mate promiscuously with several males [37, 38], resulting in a very low male mating skew within a group . In contrast, reproductive skew is very high as male reproductive success is positively correlated with male dominance rank . Variation in parasite infection of the red-fronted lemur population on which this study is based on has been investigated in detail and is known to differ significantly among individuals but not between males of different rank .
The specific objectives of our study were: (1) to identify promoter SNPs in the IL4 gene of the red-fronted lemur, (2) to associate both the respective IL4 SNP genotypes and a measure of individual heterozygosity with intensities of nematode infections, (3) to identify a possible functional role of the IL4 alleles in selective processes by exploring long-term fitness consequences between males of different genotype constitutions. We expected frequency-dependent selection to result in a higher frequency of genotypes, which provide the best resistance to parasites . Further, if IL4 does obtain a functional role in selective processes, we predicted a fitness advantage of individuals with a beneficial IL4 genotype.