Snježana Ćurković: Polymorphism and selection in the major histocompatibility complex class II in bottlenose dolphin (Tursiops truncatus) from the Adriatic Sea. Faculty of Veterinary Medicine University of Zagreb. Zagreb. 2014.
The bottlenose dolphin (Tursiops truncatus) is the only marine mammal species resident in the Adriatic Sea. It is considered endangered and its small population is protected under Croatian law. Genetic diversity is essential for adaptability and long-term persistence of small and isolated populations, so adaptively important genes relevant to conservation were analyzed. Major histocompatibility complex (MHC) is thought to be crucial for pathogen presentation to immune cells and initiation of the immune response. The extreme polymorphism of these genes is believed to be maintained by balancing selection. The aims of this study were to examine the genetic diversity and evolutionary history at MHC class II DRA, DQA and DQB loci from the bottlenose dolphin population from the Adriatic Sea.
MATERIALS AND METHODS
Tissue samples from 50 bottlenose dolphins from the Adriatic Sea were collected from 1997 until 2011and 12 specimens from other Mediterranean locations. Total genomic DNA was extracted from muscle tissue and skin. Regions of exon 2 in the MHC II genes DRA, DQA and DQB were amplified using appropriate primers. PCR products were sequenced. New alleles were confirmed by sequencing in both directions and DNA cloning. Sequence processing, allele and haplotype identification, balancing selection testing and phylogenetic analysis were performed with appropriate computer programs.
A total of 25 alleles were found at exon 2 in the MHC II genes DRA, DQA and DQB. Thirteen identified alleles were new, while 12 alleles were previously described. DRA locus showed negligible variability (3 alleles) compared to considerable sequence variation of the DQA (10 alleles) and DQB locus (12 alleles). The alleles combined to form 12 DRA/DQA/DQB haplotypes in the bottlenose dolphin population from the Adriatic Sea and 11 haplotypes in bottlenose dolphins from other areas of the Mediterranean Sea. Five haplotypes, two DQA alleles and four DQB alleles appeared to be specific for Adriatic Sea samples, as they were absent in individuals from other locations. Three DQA alleles and three DQB alleles found in other Mediterranean locations and not present in the Adriatic Sea gave rise to three unique haplotypes. Significantly higher non-synonymous substitution rates than synonymous was found in DQA and DQB sequences indicating balancing selection at both loci. Phylogenetic analysis of DQB sequences confirmed trans-species polymorphism among the ordo Cetacea.
The low variability at the DRA bottlenose dolphin locus is comparable to the low variability of the DRA locus in other vertebrate species. In contrast, DQA and DQB loci displayed considerable sequence variation. The highest diversity determined in the DQB locus is consistent with the finding that most class II MHC polymorphism is concentrated in the beta genes. Unique haplotypes suggest population structuring among different locations of the Mediterranean Sea. Positive selection acts at both highly variable loci. Trans-species polymorphism at the DQB locus indicates the influence of balancing selection in the long term. The balancing selection observed in the bottlenose dolphins is higher than that reported from most other cetaceans and comparable to that seen in terrestrial mammals.