Abstract
Genes of the major histocompatibility complex (MHC) play a key role in the initiation of immune response in jawed vertebrates. Variation at MHC loci can be used as an indicator of the genetic ‘health’ of natural populations and offer insight into potential susceptibility to epizootics. Here we present the first characterization of the sequence variation at two MHC class II loci (DQA and DQB) in the neotropical coastal (Sotalia guianensis) and riverine (Sotalia fluviatilis) dolphins, using cloning and direct sequencing of amplified genomic DNA. Four DQA and four DQB alleles were identified in 33 and 32 Sotalia samples, respectively, and high nucleotide diversity among these alleles was detected, similar to the findings described for other cetacean species. Positive selection was evidenced by an excess of dn/ds at the Peptide-Binding-Region of the DQB of Sotalia. The presence of common alleles at both loci in S. fluviatilis and S. guianensis are consistent with trans-species mode of evolution in the MHC. In contrast to observed low levels of mtDNA diversity at the population level, there was an apparent lack of reduction of DQA and DQB allelic variation in the Brazilian Coast population unit. This suggests either present or past balancing selection acting to maintain MHC variation in this population unit.
Highlights
Cell surface glycoproteins encoded by the Major Histocompatibility Complex (MHC) play a key role in the initiation of immune response in vertebrates, by binding foreign peptides and presenting them to T-cells (Hedrick, 1994)
A distinctive feature of the MHC is the high level of polymorphism exhibited in the Protein Binding Region (PBR) of class I and class II loci in most mammals
A total of 37 skin samples were obtained from S. fluviatilis and S. guianensis in twelve locations grouped into seven geographic regions throughout their range (Table 1, Figure 1)
Summary
Cell surface glycoproteins encoded by the Major Histocompatibility Complex (MHC) play a key role in the initiation of immune response in vertebrates, by binding foreign peptides and presenting them to T-cells (Hedrick, 1994). A distinctive feature of the MHC is the high level of polymorphism exhibited in the Protein Binding Region (PBR) of class I and class II loci in most mammals. This has been suggested as an adaptation to the large number of pathogens encountered by natural populations (Klein and Takahata, 1990). Overdominant selection through heterozygous advantage allows higher than expected levels of polymorphism to be maintained for longer periods of time at loci such as the MHC (Hughes and Nei, 1989), potentially increasing antigen recognition and presentation ability, providing, in theory, increased disease resistance (Potts and Slev, 1995; Penn et al, 2002). Balancing selection would operate in favor of increased allelic variation, making it possible for a species to have higher number of MHC alleles even if it has low or lacks variation at other neutral loci (Aguilar et al, 2004)
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