The mechanisms that maintain adaptive genetic diversity in natural populations have long interested biologists. In vertebrates, the genes of the major histocompatibility complex (MHC) are highly diverse and functionally important, and these genes thereby provide an ideal opportunity to examine selection for diversity. The MHC codes for proteins that bind foreign peptides and then present them to T helper cells, which initiates an immune response (Klein 1990). Thus, the MHC acts as the interface between pathogens and the adaptive immune system. The MHC is known to be highly polymorphic in many taxa, specifically at the region of the protein that binds foreign peptides (referred to as the peptide binding region or PBR; reviewed in Bernatchez and Landry 2003). Three major nonexclusive hypotheses have been proposed to explain selection for diversity at the MHC: (1) overdominance, where heterozygotes are able to recognize a broader array of pathogens and therefore have increased fitness (Doherty and Zinkernagel 1975; Hughes and Nei 1988); (2) negative frequency-dependent selection, where pathogens and MHC alleles cycle through a coevolutionary arms-race (Clarke and Kirby 1966); and (3) spatial and temporal variation in selection, where pathogens vary spatially and temporally (Hedrick 2002). Other hypotheses include mate choice for MHC dissimilar alleles mediated by inbreeding avoidance (Penn 2002) and a nonadaptive mechanism proposed by van Oosterhout (2009) that selection occurs against mutations that accumulate near MHC loci.
Of the hypotheses that exist to explain the maintenance of diversity at the MHC, the most convincing evidence comes from overdominance where heterozygotes have a fitness advantage over homozygotes. For example, Oliver et al. (2009a) found that water voles (Arvicola terrestris) that were heterozygous at the MHC were infected by fewer parasites than homozygotes. Similarly, Kekäläinen et al. (2009) found that Arctic charr (Salvelinus alpinus) that were heterozygous at the MHC had a lower parasite load, higher condition, and higher survival rate than individuals that were homozygous at the MHC. Evidence for frequency-dependent selection is less common. Support for this hypothesis comes from studies that show a fitness disadvantage for the most common allele or a fitness advantage for rare alleles. For example, in humans the HLA-A11 allele confers resistance to infection with Epstein-Barr virus only in populations in which the allele is rare (de Campos-Lima et al. 1993); in populations with a high frequency of the allele, virus strains are monomorphic for a mutation that prevents presentation of immunodominant epitopes by HLA-A11 molecules. In several other systems, there is little support for either the overdominance or frequency-dependent selection hypotheses or the mechanism responsible for maintaining diversity remains to be determined.
Temporal variation in selection for MHC diversity is arguably the least well-studied mechanism of maintaining diversity, with only a handful of studies having examined the mechanism. Westerdahl et al. (2004) studied MHC allele frequency changes over time in great reed warblers (Acrocephalus arundinaceus) and found that MHC alleles tended to vary more among cohorts than microsatellite alleles. They concluded that selection differences among cohorts, and not neutral processes, were responsible for the differences in the MHC allele frequencies. Similarly, Charbonnel and Pemberton (2005) conducted a long-term survey of MHC variation in soay sheep (Ovis aries) and found that temporal variation was higher at MHC than neutral loci in one subdivision of sheep and that the strength of selection changed among years. Specifically, they showed that homogenizing selection was present in some years, where MHC population differentiation was more similar than expected by neutral diversity, but not present in other years (also see Jensen et al. 2008; Oliver et al. 2009b). If temporal variation in MHC diversity is common in wild populations, then selection studies on point estimates may miss some of the complexity of selection for MHC diversity. Clearly more studies of temporal variation at the MHC are needed.
Here, we study temporal variation at the MHC class IIB gene in nine wild populations of the guppy (Poecilia reticulata). The guppy is a tropical freshwater fish native to north-east South America and the neighboring islands of Trinidad and Tobago (Magurran 2005). In the mountainous regions of Northern Trinidad, populations are typically highly differentiated at neutral loci (Carvalho et al. 1991; Alexander et al. 2006; Crispo et al. 2006; Barson et al. 2009; Suk and Neff 2009). Despite this high population differentiation at neutral loci, in a previous study we uncovered homogenizing selection at the MHC across populations. Based on samples from 10 populations collected in 2006, we found that the MHC was less differentiated than estimates based on microsatellites for 73% of the population pairwise comparisons (Fraser et al. 2010). Similar homogenizing selection was found by van Oosterhout et al. (2006) for two populations within a single river in Northern Trinidad. We also showed that two common external parasites, Gyrodactylus turnbulli and G. bullatarudis (hereafter referred to as gyrodactylus), were the likely agents of the homogenizing selection. Gyrodactylus is a pervasive parasite that can be lethal to guppies (Scott and Anderson 1984). Specifically, we found that the Pore_a132 allele was common to all 10 populations and was associated with reduced gyrodactylus infection (Fraser and Neff 2010). The additive genetic effect of the Pore_a132 on parasite resistance was further supported by an experimental study in which guppies were challenged with gyrodactylus (Fraser and Neff 2009).
Here, we compare MHC class IIB genetic diversity in nine guppy populations that were surveyed in both 2006 and 2007 to determine whether there is temporal variation in selection on the MHC. Our sampling period corresponds to a minimum of two generations in guppies (Reznick et al. 1997). We compare the MHC class IIB genetic diversity within and between years to neutral diversity estimated from six microsatellite loci. We also examine the change in the putative selection pressure from gyrodactylus and relate it to the change in MHC variation across the years. In guppies, the MHC class IIB has undergone a single duplication event as up to four alleles have been found in individuals. Therefore, there are two MHC class II loci in the guppy that appear to be unlinked (McConnell et al. 1998; van Oosterhout et al. 2006; Fraser et al. 2010).