title: Low Major Histocompatibility Complex Class II Variation in the Endangered Indo-Pacific Humpback Dolphin (Sousa chinensis): Inferences About the Role of Balancing Selection.
abstract: 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.
authors.affiliation: *These authors contributed equally to the work.
authors.forename: Xiyang
authors.initials: X
authors.lastname: Zhang
authors.affiliation: *These authors contributed equally to the work.
authors.forename: Wenzhi
authors.initials: W
authors.lastname: Lin
authors.forename: Ruilian
authors.initials: R
authors.lastname: Zhou
authors.forename: Duan
authors.initials: D
authors.lastname: Gui
authors.forename: Xinjian
authors.initials: X
authors.lastname: Yu
authors.forename: Yuping
authors.initials: Y
authors.lastname: Wu
mesh_terms: D000483:Alleles; D000595:Amino Acid Sequence; D000818:Animals; D004289:Dolphins / Q000235:genetics*; D056727:Endangered Species; D005802:Genes, MHC Class II; D040961:Genetic Drift; D014644:Genetic Variation; D005828:Genetics, Population; D006239:Haplotypes; D000949:Histocompatibility Antigens Class II / Q000235:genetics*; D018895:Microsatellite Repeats; D008969:Molecular Sequence Data; D010802:Phylogeny; D012641:Selection, Genetic; D017422:Sequence Analysis, DNA
chemical_list: D000949:Histocompatibility Antigens Class II
keywords: DQB; DRB; MHC expression; balancing selection; trans-species polymorphism