Polymorphism and transcription of Mhc class I genes in a passerine bird, the great reed warbler

 The class I genes of the major histocompatibility complex (Mhc) are here investigated for the first time in a passerine bird. The great reed warbler is a rare species in Sweden with a few semi-isolated populations. Yet, we found extensive Mhc class I variation in the study population. The variable exon 3, corresponding to the α₂ domain, was amplified from genomic DNA with degenerated primers. Seven different genomic class I sequences were detected in a single individual. One of the sequences had a deletion leading to a shift in the reading frame, indicating that it was not a functional gene. A randomly selected clone was used as a probe for restriction fragment length polymorphism (RFLP) studies in combination with the restriction enzyme Pvu II. The RFLP pattern was complex with 21–25 RFLP fragments per individual and extensive variation. Forty-nine RFLP genotypes were detected in 55 tested individuals. To study the number of transcribed genes, we isolated 14 Mhc class I clones from a cDNA library from a single individual. We found eight different sequences of four different lengths (1.3–2.2 kilobases), suggesting there are at least four transcribed loci. The number of nonsynonymous substitutions (d _N ) in the peptide binding region of exon 3 were higher than the number of synonymous substitutions (d _S ), indicating balancing selection in this region. The number of transcribed genes and the numerous RFLP fragments found so far suggest that the great reed warbler does not have a "minimal essential Mhc" as has been suggested for the chicken. Received: 13 May 1998 / Revised: 18 August 1998     

Sequence analysis of a polymorphic Mhc class II gene in Pacific salmon

Polymorphism of the nucleotide sequences encoding 149 amino acids of linked major histocompatibility complex (Mhc) class II 131 and 132 peptides, and of the intervening intron (548–773 base pairs), was examined within and among seven Pacific salmon (Oncorhynchus) species. Levels of nucleotide diversity were higher for theB1 sequence than forB2 or the intron in comparisons both within and between species. For the codons of the peptide binding region of the BI sequence, the level of nonsynonymous nucleotide substitution (d_N) exceeded the level of synonymous substitution (d_S) by a factor of ten for within-species comparisons, and by a factor of four for between-species comparisons. The excess of d_N indicates that balancing selection maintains diversity at this salmonidMhc class II locus, as is common forMhc loci in other vertebrates. Levels of nucleotide diversity for both the exon and intron sequences were greater among than within species, and there were numerous species-specific nucleotides present in both the coding and noncoding regions. Thus, neighbor-joining analysis of both the intron and exon regions provided phylogenies in which the sequences clustered strongly by species. There was little evidence of shared ancestral (trans-species) polymorphism in the exon phylogeny, and the intron phylogeny depicted standard relationships among the Pacific salmon species. The lack of shared allelicB1 lineages in these closely related species may result from severe bottlenecks that occurred during speciation or during the ice ages that glaciated the rim of the north Pacific Ocean approximately every 100 000 years in the Pleistocene.The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession numbers U34692-U34720     

The Mhc class II of the Black grouse (Tetrao tetrix) consists of low numbers of B and Y genes with variable diversity and expression

We found that the Black grouse (Tetrao tetrix) possess low numbers of Mhc class II B (BLB) and Y (YLB) genes with variable diversity and expression. We have therefore shown, for the first time, that another bird species (in this case, a wild lek-breeding galliform) shares several features of the simple Mhc of the domestic chicken (Gallus gallus). The Black grouse BLB genes showed the same level of polymorphism that has been reported in chicken, and we also found indications of balancing selection in the peptide-binding regions. The YLB genes were less variable than the BLB genes, also in accordance with earlier studies in chicken, although their functional significance still remains obscure. We hypothesize that the YLB genes could have been under purifying selection, just as the mammal Mhc-E gene cluster.     

Single locus typing of MHC class I and class II B loci in a population of red jungle fowl

In species with duplicated major histocompatibility complex (MHC) genes, estimates of genetic variation often rely on multilocus measures of diversity. It is possible that such measures might not always detect more detailed patterns of selection at individual loci. Here, we describe a method that allows us to investigate classical MHC diversity in red jungle fowl (Gallus gallus), the wild ancestor of the domestic chicken, using a single locus approach. This is possible due to the well-characterised gene organisation of the ‘minimal essential’ MHC (BF/BL region) of the domestic chicken, which comprises two differentially expressed duplicated class I (BF) and two class II B (BLB) genes. Using a combination of reference strand-mediated conformation analysis, cloning and sequencing, we identify nine BF and ten BLB alleles in a captive population of jungle fowl. We show that six BF and five BLB alleles are from the more highly expressed locus of each gene, BF2 and BLB2, respectively. An excess of non-synonymous substitutions across the jungle fowl BF/BL region suggests that diversifying selection has acted on this population. Importantly, single locus screening reveals that the strength of selection is greatest on the highly expressed BF2 locus. This is the first time that a population of red jungle fowl has been typed at the MHC region, laying the basis for further research into the underlying processes acting to maintain MHC diversity in this and other species.     

Trans-species polymorphism of class IIMhc loci in danio fishes

 A characteristic feature of the major histocompatibility complex (Mhc) polymorphism in mammals is the existence of allelic lineages shared by related species. This trans-species polymorphism has thus far been documented only in primates, rodents, and artiodactyls. In this communication we provide evidence that it also exists in cyprinid (bony) fishes at the class II A and B loci coding for the α and β polypeptide chains of the class II α : β heterodimers. The study has focused on three species of the family Cyprinidae, subfamily Rasborinae: the zebrafish (Danio rerio), the giant danio (D. malabaricus), and the pearl danio (D. albolineatus). The polymerase chain reaction was used to amplify and then sequence intron 1 and exon 2 of the class II B loci and exon 2 of the class II A loci in these species. Phylogenetic analysis of the sequences revealed the existence of allelic lineages whose divergence predates the divergence of the three species at both the A and B loci. The lineages at the B locus in particular are separated by large genetic distances. The polymorphism is concentrated in the peptide-binding region sites and is apparently maintained by balancing selection. Sharing of this unique Mhc feature by both bony fishes and mammals suggests that the main function of the Mhc (presentation of peptides to T lymphocytes) has not changed during the last 400 million years of its evolution. Received: 6 December 1995 / Revised: 6 February 1996     

Rapid Evolution by Positive Selection and Gene Gain and Loss: PLA<Subscript>2</Subscript> Venom Genes in Closely Related <Emphasis Type="Italic">Sistrurus</Emphasis> Rattlesnakes with Divergent Diets

Rapid evolution of snake venom genes by positive selection has been reported previously but key features of this process such as the targets of selection, rates of gene turnover, and functional diversity of toxins generated remain unclear. This is especially true for closely related species with divergent diets. We describe the evolution of PLA₂ gene sequences isolated from genomic DNA from four taxa of Sistrurus rattlesnakes which feed on different prey. We identified four to seven distinct PLA₂ sequences in each taxon and phylogenetic analyses suggest that these sequences represent a rapidly evolving gene family consisting of both paralogous and homologous loci with high rates of gene gain and loss. Strong positive selection was implicated as a driving force in the evolution of these protein coding sequences. Exons coding for amino acids that make up mature proteins have levels of variation two to three times greater than those of the surrounding noncoding intronic sequences. Maximum likelihood models of coding sequence evolution reveal that a high proportion (∼30%) of all codons in the mature protein fall into a class of codons with an estimated d _N /d _S (ω) ratio of at least 2.8. An analysis of selection on individual codons identified nine residues as being under strong (p < 0.01) positive selection, with a disproportionately high proportion of these residues found in two functional regions of the PLA₂ protein (surface residues and putative anticoagulant region). This is direct evidence that diversifying selection has led to high levels of functional diversity due to structural differences in proteins among these snakes. Overall, our results demonstrate that both gene gain and loss and protein sequence evolution via positive selection are important evolutionary forces driving adaptive divergence in venom proteins in closely related species of venomous snakes.     

Mhc-DRB Genes and the Origin of New World Monkeys

The major histocompatibility complex (Mhc) is a family of loci characterized by its relatively rapid evolutionary turnover, large genetic distances between genes, and long persistence of allelic lineages effected by balancing selection. These features render the Mhc highly suitable for answering questions concerning speciation and adaptive radiation. The aim of the present study was to use Mhc-DRB genes to make inferences about the founding population of the Platyrrhini. Three segments, each approximately 300 base pairs in length, of the platyrrhine DRB genes were amplified by the polymerase chain reaction and sequenced. The segments were derived from intron 2, exon 3, and exon 6 of DRB genes from different species of New World monkeys. The results of the study have revealed that on a phylogenetic tree, all of the tested platyrrhine genes appear to form a single cluster, while all catarrhine DRB genes form a distinct cluster, although the bootstrap values fail to provide statistically significant support for the separation of these two clades. This observation suggests that the multiple platyrrhine genes originated from a single ancestral gene after the divergence of the Platyrrhini and Catarrhini and thus contradicts the results of an earlier study in which some exon 2 DRB sequences appeared to predate the split of the two primate groups. The inconsistency in the DRB gene phylogeny can be explained by postulating convergent evolution for the peptide-binding region of the DRB exon 2 sequences. The phylogeny of the platyrrhine DRB genes (except for exon 2) is relatively "shallow"; the distances between genes are relatively short (in comparison to the catarrhine DRB genes), and there is a tendency for sequences of individual species to cluster together. The phylogeny of the platyrrhine DRB genes is consistent with the postulate that a small population founded the group and that there is an ongoing adaptive radiation from small, relatively isolated founding populations.     

Elimination of self-tolerogen turns nonresponder mice into responders

Two sets of genes control the immune response ofH-2 ^d mice to the synthetic antigen poly(Glu⁵⁰Tyr⁵⁰) (GT). One set involves class II major histocompatibility complex (Mhc) loci encoding an A^d product that serves as a recognition context to GT-reactive helper T cells (T_h). The other one is a background gene, the product of which, in association with the same Mhc-restricting element, mimics the GT/A^d complex. Mice expressing the GT-mimicking background-encoded structure (Im^gt), which is preferentially displayed on B lymphoblasts, do not respond to GT as a consequence of self-tolerance. On the other hand, elimination of cells bearing Im^gt renders these mice responsive to GT, demonstrating that tolerance to self can impoverish the immune system. Im^gt is probably not identical to GT, but resembles it in the way it forms complexes with A^d molecules ofMhc.     

Disentangling the effects of recombination,selection, and demography on the genetic variation at a major histocompatibility complex class II gene in the alpine chamois

The major histocompatibility complex (MHC) harbours some of the most polymorphic loci in vertebrate genomes. MHC genes are thought to be subject to some form of balancing selection, most likely pathogen‐mediated selection. Hence, MHC genes are excellent candidates for exploring adaptive processes. In this study, we investigated the genetic variation at exon 2 of the DRB class II MHC locus in 191 alpine chamois (Rupicapra rupicapra) from 10 populations in the eastern Alps of Italy. In particular, we were interested in distinguishing and estimating the relative impact of selective and demographic factors, while taking into account the confounding effect of recombination. The extremely high d_n/d_s ratio and the presence of trans‐species polymorphisms suggest that a strong long‐term balancing selection effect has been operating at this locus throughout the evolutionary history of this species. We analysed patterns of genetic variation within and between populations, and the mitochondrial D‐loop polymorphism patterns were analysed to provide a baseline indicator of the effects of demographic processes. These analyses showed that (i) the chamois experienced a demographic decline in the last 5000–30 000 years, most likely related to the postglacial elevation in temperature; (ii) this demographic process can explain the results of neutrality tests applied to MHC variation within populations, but cannot justify the much weaker divergence between populations implied by MHC as opposed to mitochondrial DNA; (iii) similar sets of divergent alleles are probably maintained with similar frequencies by balancing selection in different populations, and this mechanism is also operating in small isolated populations, which are strongly affected by drift.     

Patterns of nucleotide sequence variation in <Emphasis Type="Italic">ICAM1</Emphasis> and <Emphasis Type="Italic">TNF</Emphasis> genes in twelve ethnic groups of India: roles of demographic history and natural selection

We have studied DNA sequence variation in and around the genes ICAM1 and TNF, which play functional and correlated roles in inflammatory processes and immune cell responses, in 12 diverse ethnic groups of India, with a view to investigating the relative roles of demographic history and natural selection in shaping the observed patterns of variation. The total numbers of single nucleotide polymorphisms (SNPs) detected at the ICAM1 and TNF loci were 29 and 12, respectively. Haplotype and allele frequencies differed significantly across populations. The site frequency spectra at these loci were significantly different from those expected under neutrality, and showed an excess of intermediate-frequency variants consistent with balancing selection. However, as expected under balancing selection, there was no significant reduction of F _ST values compared to neutral autosomal loci. Mismatch distributions were consistent with population expansion for both loci. On the other hand, the phylogenetic network among haplotypes for the TNF locus was similar to expectations under population expansion, while that for the ICAM1 was as expected under balancing selection. Nucleotide diversity at the ICAM1 locus was an order of magnitude lower in the promoter region, compared to the introns or exons, but no such difference was noted for the TNF gene. Thus, we conclude that the pattern of nucleotide variation in these genes has been modulated by both demographic history and selection. This is not surprising in view of the known allelic associations of several polymorphisms in these genes with various diseases, both infectious and noninfectious.     

Evolution of Mhc–DRB Introns: Implications for the Origin of Primates

Introns are generally believed to evolve too rapidly and too erratically to be of much use in phylogenetic reconstructions. Few phylogenetically informative intron sequences are available, however, to ascertain the validity of this supposition. In the present study the supposition was tested on the example of the mammalian class II major histocompatibility complex (Mhc) genes of the DRB family. Since the Mhc genes evolve under balancing selection and are believed to recombine or rearrange frequently, the evolution of their introns could be expected to be particularly rapid and subject to scrambling. Sequences of intron 4 and 5 DRB genes were obtained from polymerase chain reaction-amplified fragments of genomic DNA from representatives of six eutherian orders—Primates, Scandentia, Chiroptera, Dermoptera, Lagomorpha, and Insectivora. Although short stretches of the introns have indeed proved to be unalignable, the bulk of the intron sequences from all six orders, spanning >85 million years (my) of evolution, could be aligned and used in a study of the tempo and mode of intron evolution. The analysis has revealed the Mhc introns to evolve at a rate similar to that of other genes and of synonymous sites of non-Mhc genes. No evidence of homogenization or large-scale scrambling of the intron sequences could be found. The Mhc introns apparently evolve largely by point mutations and insertions/deletions. The phylogenetic signals contained in the intron sequences could be used to identify Scandentia as the sister group of Primates, to support the existence of the Archonta superorder, and to confirm the monophyly of the Chiroptera. Received: 26 October 1998 / Accepted: 21 December 1998     

Evolutionary relationships among the primate Mhc-DQA1 and DQA2 alleles

The variation of the Mhc-DQA1 and DQA2 loci of ten different primate species (hominoids and Old World monkeys) was studied in order to obtain an insight in the processes that generate polymorphism of major histocompatibility complex (Mhc) class II genes and to establish the evolutionary relationships of their alleles. To that end nucleotide sequences of 36 Mhc class II DQA1 and seven DQA2 second exons were determined and phylogenetic trees that illustrate their evolutionary relationships were constructed. We demonstrate the existence of four primate Mhc-DQA1 allele lineages, two of which probably existed before the separation of the ancestors of the hominoids and Old World monkeys (approximately 22–28 million years ago). Mhc-DQA2 sequences were found only in the hominoid species and showed little diversity. We found no evidence for a major contribution of recombinational events to the generation of allelic diversity of the primate Mhc-DQA1 locus. Instead, our data suggest that the primate Mhc-DQA1 and DQA2 loci are relatively stable entities that mutated primarily as a result of point mutations.The nucleotide sequence data reported in this paper have been submitted to the GenBank nucleotide sequence database and have been assigned the accession numbers M76186-M76229.     

Genetic Variation of Microsatellite Loci in the Major Histocompatibility Complex (MHC) Region in the Southeast Asian House Mouse (<Emphasis Type="Italic">Mus musculus castaneus</Emphasis>)

Major histocompatibility complex (MHC) genes are the most polymorphic loci known for vertebrates. Here we employed five microsatellite loci closely linked to the MHC region in an attempt to study the amount of genetic variation in 19 populations of the southeast Asian house mouse (Mus musculus castaneus) in Taiwan. The overall polymorphism at the five loci was high (H_e = 0.713), and the level of polymorphism varied from locus to locus. Furthermore, in order to investigate if selection is operating on MHC genes in natural mouse populations, we compared the extent and pattern of genetic variation for the MHC-linked microsatellite loci (the MHC loci) with those for the microsatellite loci located outside the MHC region (the non-MHC loci). The number of alleles and the logarithm of variance in repeat number were significantly higher for the MHC loci than for the non-MHC loci, presumably reflecting linkage to a locus under balancing selection. Although three statistical tests used do not provide support for selection, their lack of support may be due to low statistical power of the tests, to weakness of selection, or to a profound effect of genetic drift reducing the signature of balancing selection. Our results also suggested that the populations in the central and the southwestern regions of Taiwan might be one part of a metapopulation structure.     

Adaptive and neutral genetic differentiation among Scottish and endangered Irish red grouse (<Emphasis Type="Italic">Lagopus lagopus scotica</Emphasis>)

Studying patterns of intra-specific genetic variation among populations allows for a better understanding of population structure and local adaptation. However, those patterns may differ according to the genetic markers applied, as neutral genetic markers reflect demographic processes and random genetic drift, whereas adaptive markers also carry the footprint of selection. In combination, neutral and adaptive genetic markers permit to assess the relative roles of drift and selection in shaping population structure. Among the best understood adaptive genetic loci are the genes of the major histocompatibility complex (MHC). We here study variation and differentiation at neutral SNP markers and MHC class II genes in red grouse (Lagopus lagopus scotica) from Ireland and Scotland. Irish red grouse populations are fragmented and drastically declining, but red grouse are abundant in Scotland. We find evidence for positive selection acting on the MHC genes and variation in MHC gene copy numbers among Irish individuals. Furthermore, there was significant population differentiation among red grouse from Ireland and Scotland at the neutral SNP markers (F_ST = 0.084) and the MHC-BLB genes (F_ST: BLB1 = 0.116, BLB2 = 0.090, BLB3 = 0.104). Differentiation at the MHC-BLB1 was significantly higher than at the neutral SNP markers, suggesting that selection plays an important role in shaping MHC variation, in addition to genetic drift. We speculate that the observed differentiation pattern might be due to local adaptation to different parasite regimes. These findings have strong conservation implications and we advise against the introduction of Scottish red grouse to supplement Irish populations.     

Mhc‐linked survival and lifetime reproductive success in a wild population of great tits

Major histocompatibility complex (Mhc) genes are frequently used as a model for adaptive genetic diversity. Although associations between Mhc and disease resistance are frequently documented, little is known about the fitness consequences of Mhc variation in wild populations. Further, most work to date has involved testing associations between Mhc genotypes and fitness components. However, the functional diversity of the Mhc, and hence the mechanism by which selection on Mhc acts, depends on how genotypes map to the functional properties of Mhc molecules. Here, we test three hypotheses that relate Mhc diversity to fitness: (i) the maximal diversity hypothesis, (ii) the optimal diversity hypothesis and (iii) effect of specific Mhc types. We combine mark–recapture methods with analysis of long‐term breeding data to investigate the effects of Mhc class I functional diversity (Mhc supertypes) on individual fitness in a wild great tit (Parus major) population. We found that the presence of three different Mhc supertypes was associated with three different components of individual fitness: survival, annual recruitment and lifetime reproductive success (LRS). Great tits possessing Mhc supertype 3 experienced higher survival rates than those that did not, whereas individuals with Mhc supertype 6 experienced higher LRS and were more likely to recruit offspring each year. Conversely, great tits that possessed Mhc supertype 5 had reduced LRS. We found no evidence for a selective advantage of Mhc diversity, in terms of either maximal or optimal supertype diversity. Our results support the suggestion that specific Mhc types are an important determinant of individual fitness.     

High allelic variation of MHC class II alpha antigen and the role of selection in wild and cultured Sparus aurata populations

Genetic polymorphism and differentiation in wild and cultured sea bream samples were studied after amplification, cloning, and partial sequence of the major histocompatibility complex (MHC) class II alpha antigen. Forty-one alleles were detected from 43 unrelated individuals and sequence alignment of the obtained alleles revealed 28 polymorphic sites. High heterozygosity values and allelic richness were unveiled for both wild and cultured populations. The substitution pattern (d_N /d_S = 0.7) is not consistent with the effect of diversifying selection, indicating lower selection pressure on the a2 domain, as well as that too few advantageous non-synonymous mutations have accumulated as substrate for the diversifying selection to act. Comparison with previously published results on microsatellite markers suggests that balancing selection in MHC genes reduces the genetic drift and bottleneck effects that are common in aquaculture and which are known to reduce genetic variation at neutral markers. The present study stresses that both coding and non-coding loci should be analyzed for designing proper management strategies.     

Increased positive selection pressure within the complementarity determining regions of the T‐cell receptor β gene in New World monkeys

Because of the long‐term co‐evolution of TCR and MHC molecules, numerous nucleotide substitutions have accumulated within the domains of TCRβ genes. We previously found that nonsynonymous nucleotide substitutions occurred more frequently in complementarity determining region (CDR)β than in CDRα, even though only a limited number of common marmoset (Callithrix jacchus) and human T‐cell receptor β variable (TRBV) sequences were compared. This interesting finding raised the question of whether the increased selective pressure within CDRβ was species‐specific. In this study, we identified 21 TRBV region sequences from the common marmoset and performed comparative sequence analyses of the T‐cell receptor α variable (TRAV) and TRBV regions from human, chimpanzee, rhesus monkey, cotton‐top tamarin, Ma's night monkey, and common marmoset. The ratios of the number of nonsynonymous nucleotide substitutions per site (d_N) to the d_S values (d_N/d_S) were less than 1 within the framework regions (FRs) of TRAV and TRBV region sequences, suggesting that purifying selection is largely dominant within the FRs. In contrast, the d_N values were statistically significantly greater for CDRβ than for CDRα only in New World monkeys. Also, increased d_N/d_S ratios (d_N/d_S>1) were observed within CDRβ between humans and New World monkeys and, interestingly, between New World monkeys, which share a relatively recent common ancestor. Moreover, phylogenetic analysis by maximum likelihood analysis provided firm evidence to support that positive selection occurred within CDRβ along New World monkey lineages. These results suggest that increased positive selection pressure within CDRβ is common in New World monkeys rather than being species‐specific. This study provides an intriguing insight into the co‐evolution of TCR and MHC molecules within primates. Am. J. Primatol. 73:1082–1092, 2011. © 2011 Wiley‐Liss, Inc.     

Linkage Relationships and Haplotype Variation of the Major Histocompatibility Complex Class I <Emphasis Type="BoldItalic">A</Emphasis> Genes in the Cichlid Fish <Emphasis Type="BoldItalic">Oreochromis niloticus</Emphasis>

: The haplochromine cichlid species flocks of the East African Great Lakes are one of the best examples of adaptive radiation. Analysis of genetic variation among these species provides valuable information on species relationships and timing of speciation events. Although the haplochromine cichlids generally display little genetic variation, the major histocompatibility complex (Mhc) genes have been found to be highly variable. A study of the linkage relationships of the Mhc class I A genes in the cichlid fish Oreochromis niloticus was therefore undertaken. Class I loci were identified, and their segregation in seven mothers and their haploid embryos was determined. In total, 56 class I A sequences were found among the seven families. A strong concordance of segregation was observed in five haplotypes among the embryos, indicating a close linkage of all loci. The number of loci per haplotype varied from 11 to 17, while the total number of distinct loci found among all families was 22. These findings show that all class I A loci are linked in a single genetic cluster in O. niloticus.     

Variation in MHC genotypes in two populations of house sparrow (Passer domesticus) with different population histories

Small populations are likely to have a low genetic ability for disease resistance due to loss of genetic variation through inbreeding and genetic drift. In vertebrates, the highest genetic diversity of the immune system is located at genes within the major histocompatibility complex (MHC). Interestingly, parasite‐mediated selection is thought to potentially maintain variation at MHC loci even in populations that are monomorphic at other loci. Therefore, general loss of genetic variation in the genome may not necessarily be associated with low variation at MHC loci. We evaluated inter‐ and intrapopulation variation in MHC genotypes between an inbred (Aldra) and a relatively outbred population (Hestmannøy) of house sparrows (Passer domesticus) in a metapopulation at Helgeland, Norway. Genomic (gDNA) and transcribed (cDNA) alleles of functional MHC class I and IIB loci, along with neutral noncoding microsatellite markers, were analyzed to obtain relevant estimates of genetic variation. We found lower allelic richness in microsatellites in the inbred population, but high genetic variation in MHC class I and IIB loci in both populations. This suggests that also the inbred population could be under balancing selection to maintain genetic variation for pathogen resistance.     

Sequence variation at the major histocompatibility complex DRB loci in beluga (Delphinapterus leucas) and narwhal (Monodon monoceros)

 The variation at loci with similarity to DRB class II major histocompatibility complex loci was assessed in 313 beluga collected from 13 sampling locations across North America, and 11 narwhal collected in the Canadian high Arctic. Variation was assessed by amplification of exon 2, which codes for the peptide binding region, via the polymerase chain reaction, followed by either cloning and DNA sequencing or single-stranded conformation polymorphism analysis. Two DRB loci were identified in beluga: DRB1, a polymorphic locus, and, DRB2, a monomorphic locus. Eight alleles representing five distinct lineages (based on sequence similarity) were found at the beluga DRB1 locus. Although the relative number of alleles is low when compared with terrestrial mammals, the amino acid variation found among the lineages is moderate. At the DRB1 locus, the average number of nonsynonymous substitutions per site is greater than the average number of synonymous substitutions per site (0.0806 : 0.0207, respectively;P<0.01). Most of the 31 amino acid substitutions do not conserve the physiochemical properties of the residue, and 21 of these are located at positions implicated as forming pockets responsible for the selective binding of foreign peptide side chains. Only DRB1 variation was examined in 11 narwhal, revealing a low amount of variation. These data are consistent with an important role for the DRB1 locus in the cellular immune response of beluga. In addition, the ratio of nonsynonymous to synonymous substitutions is similar to that among primate alleles, arguing against a reduction in the balancing selection pressure in the marine environment. Two hypotheses may explain the modest amount of Mhc variation when compared with terrestrial mammals: small population sizes at speciation or a reduced neutral substitution rate in cetaceans. Received: 15 July 1997 / Revised: 24 March 1998