CONSEQUENCES OF POPULATION STRUCTURE ON GENES UNDER BALANCING SELECTION

Population Differences at MHC Do Not Explain Enhanced Resistance of Song Sparrows to Local Parasites

Infectious disease represents an emerging threat to natural populations, particularly when hosts are more susceptible to novel parasites (allopatric) than to parasites from the local area (sympatric). This pattern could arise through evolutionary processes (host populations become adapted to their local parasites and genetically differentiated from other populations at immune-related loci) and/or through ecological interactions (host individuals develop resistance to local parasites through previous exposure). The relative importance of these candidate mechanisms remains unclear. In jawed vertebrates, genes of the major histocompatibility complex (MHC) play a fundamental role in immunity and are compelling candidates for spatially varying selection. We recently showed that song sparrows (Melospiza melodia) are more susceptible to allopatric than to sympatric strains of malaria (Plasmodium). In the current study, to determine whether population differences at MHC explain this pattern, we characterized the peptide-binding regions of MHC (classes I and II) of birds that did or did not become infected in the previous experiment. We recovered up to 4 alleles per individual at class I, implying at least 2 loci, and up to 26 alleles per individual at class II, implying at least 13 loci. Individuals with more class I alleles were less likely to become infected by Plasmodium, consistent with parasite-mediated balancing selection. However, we found no evidence for population genetic differentiation at either class of MHC, based on 36 individuals sequenced. Resistance to sympatric parasites previously described for this system likely stems from individuals' prior immune experience, not from population differentiation and locally protective alleles at MHC.

Low Major Histocompatibility Complex Class II Variation in the Endangered Indo-Pacific Humpback Dolphin (Sousa chinensis): Inferences About the Role of Balancing Selection

It has been widely reported that the major histocompatibility complex (MHC) is under balancing selection due to its immune function across terrestrial and aquatic mammals. The comprehensive studies at MHC and other neutral loci could give us a synthetic evaluation about the major force determining genetic diversity of species. Previously, a low level of genetic diversity has been reported among the Indo-Pacific humpback dolphin (Sousa chinensis) in the Pearl River Estuary (PRE) using both mitochondrial marker and microsatellite loci. Here, the expression and sequence polymorphism of 2 MHC class II genes (DQB and DRB) in 32 S. chinensis from PRE collected between 2003 and 2011 were investigated. High ratios of non-synonymous to synonymous substitution rates, codon-based selection analysis, and trans-species polymorphism (TSP) support the hypothesis that balancing selection acted on S. chinensis MHC sequences. However, only 2 haplotypes were detected at either DQB or DRB loci. Moreover, the lack of deviation from the Hardy-Weinberg expectation at DRB locus combined with the relatively low heterozygosity at both DQB locus and microsatellite loci suggested that balancing selection might not be sufficient, which further suggested that genetic drift associated with historical bottlenecks was not mitigated by balancing selection in terms of the loss of MHC and neutral variation in S. chinensis. The combined results highlighted the importance of maintaining the genetic diversity of the endangered S. chinensis.