Variation and evolution of major histocompatibility complex DQB and DRB gene in bottlenose dolphins (Tursiops truncatus and T. aduncus)
Date Issued
2008
Date
2008
Author(s)
Yang, Wei-Cheng
Abstract
Variability of the major histocompatibility complex is very suitable for investigating aide range of open questions in evolutionary ecology and conservation because it reflectsvolutionarily relevant and adaptive processes within and between populations. First, thest full-length expressed DQB and DRB gene products in the order Cetacea from 2 speciesf bottlenose dolphins (genus Tursiops, family Delphinidae) of the western Pacific, whichiffer in their diet, microflora, and habitat, was presented. These species might represent aatural model through which we can examine the immunogenetic consequences of differentathogenic and environmental influences on the MHC of mammals. Blood samples from 2. aduncus (from Taiwan and Indonesia) and 2 T. truncatus (from Taiwan and Japan) weresed for RACE-PCR. The presence of 1 DQB locus and 2 DRB loci in this genus wasevealed. The presence of Tutr-DQB, Tuad-DRB, and Tutr-DRB suggests a functional roleor these molecules in pathogen-specific immune responses. However, several features doot support a traditional role for Tuad-DQB molecules in peptide binding, and further studys needed. The phylogenetic analysis revealed that the sequence divergence of these 2pecies might reflect different selective pressures between pathogens in oceanic (T.runcatus) and coastal (T. aduncus) waters. This study provides an essential foundation fornalyzing variations in MHC genes and studying infectious disease ecology in bottlenoseolphins.or a better understanding of the immunologic diversity in bottlenose dolphins, theequence variation of the exon 2 region of MHC DQB locus was analyzed in 62 bottlenoseolphins (42 T. truncatus and 20 T. aduncus), collected from strandings and fisheryycatch in Taiwanese waters. The 172 bp sequences amplified showed no more than twolleles in each individual. The high proportion of nonsynonymous nucleotide substitutionsnd the moderate amount of variation suggest positive selection pressure on this locus,rguing against a reduction in the marine environment selection pressure. The phylogeneticelationship among DQB exon 2 sequences of bottlenose dolphins and other delphinids didot coincide with taxonomic relationship, indicating a trans-species evolutionary pattern.oreover, there is no mixture but a clear division between T. truncatus and T. aduncus,ndicating that different selective pressures from pathogens exist in oceanic and coastalaters because shallow waters along the coast may influence by the terrestrial runoff andhe pathogen diversity and abundance in the coastal waters may be different from that inhe oceanic area. The habitat-specific alleles may have adaptive value for certain speciesnd can be discriminately selected.astly, the evolutionary relationship and estimates of divergence time of cetartiodactylsy using MHC class II genes was reported. The results suggest that cetaceans (T. truncatusnd T. aduncus) diverged from artiodactyls (pig, hippo, and ruminants) about 60 Mya orlightly earlier and the pathogen-driven directional selection is very likely the driving forcef the sequence divergence of MHC genes of the cetaceans and the terrestrial relatives.urthermore, the results show that T. truncatus and T. aduncus diverged about 24 Mya,hich predates the emergence of oldest dolphin (approximately 11 Mya) quite much. Itmplies that these allelic lineages were maintained for a long evolutionary period throughpeciation events by balancing selection and the long persistence time could allow theccumulation of multiple substitutions between allelic lineages. This research provides newnsights into the dynamics of MHC-pathogen coevolution and is the first study on thevolutionary history of cetartiodactyls using the MHC class II genes.
Subjects
bottlenose dolphin
MHC
variaton
evolution
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