Immunogenetics

, Volume 65, Issue 1, pp 47–61

Evolution of the MHC-DQB exon 2 in marine and terrestrial mammals

Authors

  • María José Villanueva-Noriega
    • Departamento de Biología Evolutiva, Facultad de CienciasUniversidad Nacional Autónoma de México
    • Posgrado en Ciencias del Mar y LimnologíaUniversidad Nacional Autónoma de México
  • Charles Scott Baker
    • School of Biological SciencesUniversity of Auckland
    • Marine Mammal InstituteOregon State University
    • Departamento de Biología Evolutiva, Facultad de CienciasUniversidad Nacional Autónoma de México
    • School of Biological SciencesUniversity of Auckland
Original Paper

DOI: 10.1007/s00251-012-0647-8

Cite this article as:
Villanueva-Noriega, M.J., Baker, C.S. & Medrano-González, L. Immunogenetics (2013) 65: 47. doi:10.1007/s00251-012-0647-8

Abstract

On the basis of a general low polymorphism, several studies suggest that balancing selection in the class II major histocompatibility complex (MHC) is weaker in marine mammals as compared with terrestrial mammals. We investigated such differential selection among Cetacea, Artiodactyla, and Primates at exon 2 of MHC-DQB gene by contrasting indicators of molecular evolution such as occurrence of transpecific polymorphisms, patterns of phylogenetic branch lengths by codon position, rates of nonsynonymous and synonymous substitutions as well as accumulation of variable sites on the sampling of alleles. These indicators were compared between the DQB and the mitochondrial cytochrome b gene (cytb) as a reference of neutral expectations and differences between molecular clocks resulting from life history and historical demography. All indicators showed that the influence of balancing selection on the DQB is more variable and overall weaker for cetaceans. In our sampling, ziphiids, the sperm whale, monodontids and the finless porpoise formed a group with lower DQB polymorphism, while mysticetes exhibited a higher DQB variation similar to that of terrestrial mammals as well as higher occurrence of transpecific polymorphisms. Different dolphins appeared in the two groups. Larger variation of selection on the cetacean DQB could be related to greater stochasticity in their historical demography and thus, to a greater complexity of the general ecology and disease processes of these animals.

Keywords

Major histocompatibility complexDQBCytochrome bNonsynonymous nucleotide substitutionTranspecific polymorphismMarine mammals

Introduction

Mammals possess a cluster of genes, the major histocompatibility complex (MHC), which codes for proteins that participate in humoral and cellular immune responses. MHC molecules are subdivided into three classes, each having different functions. Class I molecules are expressed in virtually all cells of the organism and serve as recognition elements for natural killer cells. Class II molecules are expressed primarily in antigen-presenting cells which present antigen peptides to CD4+ T cells. Class III genes encode a heterogeneous set of proteins which includes complement components, steroid 21-hydroxylases, heat shock proteins, and cytokines (Goldsby et al. 2003). As far as is known, in placental mammals, the MHC class II genes DP, DN, DM, DO, DQ, and DR code for peptide-binding glycoproteins. Genes DP, DM, DQ, and DR usually code for one alpha chain and one beta chain, while gene DN code for an alpha chain and gene DO code for a beta chain. In some artiodactyls, DP genes are substituted by DI and DY genes. The general arrangement of MHC class II genes is conserved among mammals although some genes are missing in some species and the number of alpha and beta genes is variable (Beck and Trowsdale 1999; Takahashi et al. 2000; Goldsby et al. 2003; Kumánovics et al. 2003). Genes of the MHC are highly polymorphic in most mammals (Klein and Figueroa 1986; Edwards and Hedrick 1998; Gu and Nei 1999) presumably because of balancing selection acting over functional regions of the MHC proteins such as the peptide-binding regions (PBR) (Hughes and Nei 1988; Gu and Nei 1999). This balancing selection is thought to be driven by the load of pathogens in the environment and by inbreeding avoidance (Apanius et al. 1997).

Until recent years, research on MHC variation was conducted mainly on humans (Homo sapiens, HLA Complex), mice (Mus musculus, H-2 Complex), and a few wild and domesticated ungulates (Sigurdardóttir et al. 1992; Schwaiger et al. 1994; Swarbrick and Crawford 1997; Amills et al. 1998; Smith et al. 2005) with the view that MHC variation may be relevant for the viability of populations especially of those severely depleted (Bernatchez and Landry 2003; O'Brien et al. 1985; Potts and Wakeland 1990; Ellegren et al. 1993; Gutierrez-Espeleta et al. 2001). However, information linking MHC variability with disease susceptibility is still poor because the whole immunogenetic complex is not yet examined in relation with disease mechanisms (Acevedo-Whitehouse and Cunningham 2006). Also, the presence of particular alleles within the MHC genotype, rather than levels of heterozygosity and other population attributes, appears to be related with morbidity and mortality in animals (Paterson et al. 1998; Hutchings 2009). A similar situation has been found for the association between the human gene DRD4 and mental disorders (Tovo-Rodrigues et al. 2011) and could explain why some populations are highly susceptible to disease despite exhibiting a high degree of polymorphism at target MHC loci (Gutierrez-Espeleta et al. 2001) and also why populations with low levels of variation at MHC appear viable (Ellegren et al. 1993). Recent investigations using high-resolution methods as well as extensive samplings in species such as cheetahs (Acionyx jubatus) are also showing that MHC variation and population health status are actually higher than the traditional view of genetic depletion associated with high disease susceptibility (Castro-Prieto et al. 2011).

Investigation on MHC genes of marine mammals, mainly on DR and DQ genes, shows low levels of polymorphism for several species; this includes cetaceans (Trowsdale et al. 1989; Murray et al. 1995; Hayashi et al. 2003; Munguía-Vega et al. 2007; Nigenda-Morales et al. 2008) and pinnipeds (Slade 1992; Hoelzel et al. 1999). In other marine mammal species, MHC shows a high degree of variation (Baker et al. 2006; Bowen et al. 2006; Xu et al. 2007). Founder effect, population bottlenecks, and a low immunologic selective pressure in the marine environment have been invoked to explain the low levels of MHC polymorphism found in some marine mammals (Trowsdale et al. 1989; Slade 1992). Resistance to disease has been proposed to be a major factor driving the evolution of marine mammal MHC judging from the observation of mass mortalities caused by epizooties and naturally occurring toxins (Harwood and Hall 1990). However, it is not clear whether the apparent low diversity in MHC of some marine mammals is derived from a different regime of selection in the aquatic environment or from genetic drift associated with demographic fluctuations, or even low rates of mutational substitutions (e.g., Jackson et al. 2009). Although mortality in the marine environment is associated with a pattern of pathogenic load different to that of the terrestrial environment (McCallum et al. 2004), the selective regime over the MHC on both realms is not necessarily different, since the low diversity of MHC in some marine mammals might result from a different pattern of mortality.

In order to distinguish a differential regime of selection in the MHC between marine and terrestrial mammals from the effects of different life history and historical demography on the MHC molecular variation, here we examined indicators of molecular evolution at exon 2 of the DQB gene which codes for the PBR of class II MHC (Beck and Trowsdale 1999; Takahashi et al. 2000; Goldsby et al. 2003). Such indicators were examined in the set of related marine and terrestrial mammals Cetacea and Artiodactyla as well as in Primates as outgroup. We chose the DQB gene because it is present in at least three of the four main placental clades, i.e., Afrotheria, Euarchontoglires, and Laurasiatheria, available data cover a broader spectrum of terrestrial and marine mammal species to compare, and also the DQB evolutionary phenomenology relies mainly on nucleotide substitutions among orthologous genes of large phylogenetic longevity (Takahashi et al. 2000).

Balancing selection acting over the evolutionary history of the DQB gene can result in the retention of alleles among species for longer than expected under neutral evolution (Cutrera and Lacey 2007). Balancing selection may, therefore, be characterized by the occurrence of transpecific polymorphisms along the phylogeny of MHC genes (Klein and Figueroa 1986; Takahata 1990; Takahata et al. 1992). Maintenance of high variation in the MHC driven by balancing selection can also be shown through the analysis of substitution rates by codon position. This can be evaluated by phylogenetic branch lengths as well as by the rates of synonymous (dS) and nonsynonymous (dN) substitutions where dN has proved to be greater than dS in the PBR in comparison with non-PBR sites (Hughes and Nei 1988). We also explored the pattern of occurrence of variable sites at the PBR sites throughout the accumulation of alleles sampled. These molecular analyses represent the complementary phylogenetic and distance approaches.

An analysis of absolute molecular rates is restricted by differences among taxa in molecular clocks resulting from life history and demography (e.g., Jackson et al. 2009) by the lack of complete time calibration data and also because such analysis excludes examination of intraspecific variation. Since the mitochondrial cytochrome b gene (cytb) is a relatively conserved molecular marker which is under neutral evolution, its variation should reflect approximate phylogenetic distances among species in relation with their particular life histories and demographies (Irwin and Árnason 1994), giving, thus, a reference for contrast with variation at DQB (Takahata 1990). We have, thus, compared the patterns of molecular evolution listed above at the DQB with the cytb as a reference of molecular changes assumed to reflect neutral molecular variation as well as changes in life history and demography along history independent of balancing selection on the MHC.

Methods

DNA sequence data

We compiled from the GenBank (Benson et al. 2005) a total of 260 nucleotide sequences of the second exon in the DQB gene (171 bp), which codes for the PBR of the class II MHC antigen receptors. We compiled as well 248 sequences of the cytb gene (1,140 bp) for a total of 33 marine and terrestrial mammal species of the orders Cetacea, Artiodactyla, and Primates. We generated 20 of the analyzed DQB sequences from ten cetacean species and one artiodactyl (Table 1). These DQB data were obtained from tissue samples in the stocks of authors CSB and LMG by standard PCR amplification and cloning (Baker et al. 2006).
Table 1

Families and species of the three mammal orders examined including GenBank accessions as well as the number of sequences (n) for DQB and cytb genes

Order/family

Species

DQB GenBank accessiona

DQB n

cytb GenBank accession

Cytbn

Cetacea

Delphinidae

Cephalorhynchus hectori

DQ354629, DQ354628, Own (1)

3

EF093033, AF084071

2

Globicephala macrorhynchus

AB164228, AB164227, AB164226

3

AF084055, AF084054

2

Sagmatias obliquidens

AB164225, AB164224, Own (1)

3

EF093041, EF093040, EF093039, EF093038, EF093037, EF093036, AF084067

7

Tursiops truncatus

AB164221, Own (1)

2

DQ466029, DQ466028, DQ466027, DQ466026, DQ466025, EF093029, X92526, AF084095, AF084094, AF084093

10

Monodontidae

Delphinapterus leucas

U16990, U16989, U16988, U16987, U16986, Own (1)

6

DLU72037, X92531

2

Monodon monoceros

U16991, Own (1)

2

AJ554062, NC_005279, MMU72038, X92532

4

Phocoenidae

Neophocaena phocaenoides

AB164212, AB164214, AB164215, AB164216, AB164217, AB164218, AB164219

7

EF203442, DQ364692, DQ364691, AF334489, NPU09680

5

Balaenopteridae

Balaenoptera bonaerensis

AB164202, AB164203, AB164204, AB164205, AB164206, AB164207

6

X75581, NC_006926, AP006466

3

Ziphiidae

Mesoplodon europaeus

DQ354641, DQ354640

2

X92537

1

Mesoplodon grayi

DQ354639, Own (1)

2

AY579546, AY579545

2

Mesoplodon stejnegeri

AB164209, DQ354638

2

AY579554, AY579553

2

Physeteridae

Physeter macrocephalus

AB164208, Own (1)

2

X75589, AF304073, AJ277029, NC_002503

4

Balaenoptera physalus

AB164199, DQ300263, DQ300262, DQ300261, DQ354627, DQ354626, Own (1)

7

X61145, NC_001321

2

Megaptera novaeangliae

DQ354664, DQ354663, DQ354662, DQ354661, DQ354660, DQ354659, DQ354658, DQ354657, DQ354656, DQ354655, DQ354654, DQ354653, DQ354652, DQ354651, DQ354650, DQ354649, DQ354648, DQ354647, DQ354646, DQ354645, DQ354644, DQ354643, DQ354642, Own (10)

33

X75584, NC_006927, AP006467

3

Eschrichtiidae

Eschrichtius robustus

DQ354636, DQ354635

2

AJ554053, AP006471, NC_005270

3

Balaenidae

Balaena mysticetus

DQ354625, DQ354624, DQ354623

3

AJ554051, AP006472, NC_005268, X75588

4

Eubalaena australis

AB164198, DQ354634, DQ354633, DQ354632, DQ354631, DQ354630, Own (1)

7

DQ095153, DQ095152, NC_006930, AP006473

4

Subtotal

17

 

92

 

60

Artiodactyla

Hippopotamidae

Hippopotamus amphibius

Own (1)

1

Y08813, U07565, AJ010957, NC_000889

4

Bovidae

Bos indicus

AJ421636, AJ249896, AJ249897, AJ249898, AJ249718, AJ249717, AJ249716, X79348, X79352

9

AF531473, NC_005971, AF492350, AY126697

4

Bos taurus

U77795, U77796, U77798, AJ421635, AJ421634, AJ421633, AJ421632, AJ421631, AF376814, AF376808, Z48206, Z48205, NM_001034668, XM_585873, D37954, D37952, D37953, S43263, AY730728, AY730727, U77800, U77799, U77794, U77793, U77792, U77791, U77790, U77789, U77788

28

AY526085, AY521038, AY521037, AY521036, AY521035, AY521034, AY521033, AY521032, AY521031, AY521030, AY521063, AY521062, AY521061, AY521060, AY521059, AY521058, AY521057, AY521056, AY521055, AY521054, AY521053, AY521052, AY521051, AY521050, AY521049, AY521048, AY521047, AY521046, AY521045, AY521044, AY521043, AY521042, AY521041, AY521040, AY521039

35

Bubalus bubalis

AY699887, AY699886, AY699885, AY699884, AY699883, AY699882, AY699881, AY699880, AY699879, AY699878, AY699877, AY699876

12

EF409942, EF409941, EF409940, EF409939, AY702618 , AY488491 , NC_006295

7

Ovibos moschatus

AY077796, AY077795, AY077794, AY077793, AY077792, AY077791, AY077790, AY077789, AY077788

9

AY669322, OMU90303, OMU90302, OMU90301, OMU90300, OMU17862

6

Cervidae

Cervus elaphus

U39052, U39058, U39057, U39056, U39055, U39054, U39053, U39051, AF119786, AF119785, AF119784, AF119783, AF119782, AF119780, AF119779, AF119778, AF119777, AF119776, AF119775, AF119774

20

AJ000021, AY244490, AY044859, AY035871, AY347753, AY347752, AB021099, AB021098, AB021097, AB021096, AY148966, AY142327, AY142326, AY118199, AY118198, AY118197, AY070226, AY070225, AY070224, AY070223, AY070222, AY070221, AY044861, AY044860, AY044858, AY044856

26

Cervus nippon

AY679483, AY679482, AY679481, AY679480, AY679479, AY679478, AY679477, AY679476, AY679475, AY679474, AY679473, AY679472, AY679471, AY679470, AY679468, AY679467, AY679466, AY679465, AY679464, AY679463, AY679462, AY679461

22

AB160860, AY035876, AB021095, AB021094, AB021093, AB021092, AB021091, AB021090, NC_008462, EF058308, DQ985076, NC_007179, AB218689, NC_006993, NC_006973, AB211429, AB210267

17

Alces alces

AY077784, AY077785, AY077786, AY077787

4

AJ000026, AY090104, AY090103 , AY090102, AY090101, AY090100, AY090099

7

Suidae

Sus scrofa

AY135572, AY135571, AY135570, AY135569, AY135568

5

EF061505, EF061504, EF061503, EF061502, EF061501, EF061500, EF061499, EF061498, EF061497, EF061496, EF061495, EF061494, EF061493, EF061492, EF061491, EF061490, EF061489, EF061488, EF061487, EF061486, EF061485, EF061484, EF061483

23

Subtotal

9

 

110

 

129

Primates

Hominidae

Homo sapiens

NM_002123, M24364

2

EF459669, EF458167, EF459670, EF452294, EF452293, EF452295, EF449507, EF449506, EF061150, EF061159, EF061158, EF061157, EF061156, EF061155, EF061154, EF061153, EF061152, EF061151, EF061149, EF061148, EF061147, EF061146, EF061145, EF061144, AY495156, AY495324, AY495325, AY495326, AY495327, AY495328

30

Gorilla gorilla

M81265, M81266, M81267

3

X93347, NC_001645, GORMTCD38114

3

Pan troglodytes

AJ308040, AJ308039, AJ308038, M81260, M81262, M81263, M81264, M81259

8

NC_001643, AY585843, AY585842, AY585841, AY585840, AY585839, AY585838, AY585836, AY585835, AY585834, AY585833, X93338, X93339

13

Pongo pygmaeus

AJ308043, AJ308042, AJ308041, M81270, M81273, M81271, M81272

7

NC_001646, NC_002083

2

Cercopithecidae

Macaca arctoides

AJ308053, AJ308052, AJ308051

3

AY738634, AY685835, AY685834, AY685833, AY685832, AY685831, AY685830

7

Macaca mulatta

AJ308050, AJ308049, AJ308048, AJ308047, AJ308046, AJ308045, AJ308044, M81269, M81301, M81299, M81292, M81294, M81286, M81287, M81289, M81290, M81291, M81293, M81296, M81297, M81300, M81305

22

NC_005943, AY612638

2

Aotidae

Aotus nancymae

AF213642, AF213641, AF213640, AF213639, AF213638, AF213637, AF213636, AF213635, AF213634, AF213633, AF213632, AF213631, AF213630

13

AJ489745, AJ489746

2

Subtotal

7

 

58

 

59

Total

33

 

260

 

248

Bold numbers indicate subtotals of the number of species as well as DQB and cytb sequences examined for Cetacea, Artiodactyla and Primates

aOwn sequences were deposited in the Dryad Repository: http://dx.doi.org/10.5061/dryad.b33m7

Phylogeny and occurrence of ancestral polymorphisms

Separate phylogenies of the DQB and cytb sequences were reconstructed using the program MrBayes3.1.2 (Ronquist and Huelsenbeck 2003) partitioning sequence data in the three codon positions for which their own GTR + G models of nucleotide substitution were determined during the search as the ones selected by the program Modeltest 3.7 of Posada and Crandall (1998). Trees were built by four MC3 chains with temperature 0.5 in 10 million generations sampling trees every 10,000. Given the convergence of the likelihood values, the first 300 trees were discarded in the DQB analysis and the first 150 trees were discarded for the cytb. Consensus trees were finally determined using the majority rule of the program PAUP* 4.0 (Swofford 2001).

Each DQB sequence was identified with a vector of characters describing its classification at the levels of order, family, genus, and species. This character series was then mapped into the DQB phylogeny to identify unambiguous synapomorphies (i.e., shared derived characters) using the program MacClade 4 (Maddison and Maddison 2000). Synapomorphies of the classification vectors were interpreted as transpecific polymorphisms which result from the long-term persistence of MHC alleles beyond speciation events and which may even survive beyond divergence between genera and families. Since transpecific changes identified in the DQB tree may imply different taxonomical ranks above the species level (trans-order, trans-familial, and trans-genus), we refer to them in general as trans-taxon changes. A rate of trans-taxon polymorphism in DQB for the taxonomic levels (Rl) species within genera, genera within families and families within orders was then defined as follows:
$$ {R_l} = \frac{{{T_l}}}{{\left( {{L_{\text{cytbl}}}} \right)\left( {{b_{\text{cytbl}}}} \right)}} $$
where Tl stands for all the trans-taxon changes at DQB accumulated at taxonomic level l, Lcytbl is the mean length of branches at cytb phylogeny separating taxa at level l and bcytbl is the number of accumulated branches at level l on the cytb phylogeny. Rl is, thus, the number of trans-taxon changes per unitary branch length sampled at the cytb phylogeny. Rl was profiled for Cetacea, Artiodactyla, and Primates at the different levels l measured with the mean length of branches at cytb phylogeny separating taxa at level l (Lcytb). The number of accumulated trans-taxon changes at each taxonomic level was examined between paired taxa with the χ2 test after showing that cytb branch lengths are not significantly different among the taxa by the Student's t test. This, in turn, was made after examining the normality of data with the Kolmogorov–Smirnov d test.

Patterns of phylogenetic branch lengths

For the DQB and cytb phylogenies, Bayesian tree lengths between all sequences among all species pairs within Cetacea, Artiodactyla, and Primates were averaged. Therefore, for each species pair mean, the proportion DQB tree length/cytb tree length (LDQB/Lcytb), as a reference of DQB selection signal, was plotted against the cytb tree length (Lcytb) as a measure of neutral phylogenetic divergence. The plot obtained is a decaying curve asymptotic to the proportion LDQB/Lcytb = 1 and appearing as a power law in log–log representation; this is, LDQB/Lcytb = ALcytbb where A (>1) is the antilogarithm of the intercept (a) and b (<0) is the slope of the linear regression of the log (LDQB/Lcytb) vs. log (Lcytb) plot. In order to discriminate variation due to specific neutral rates among taxa, this comparison was partitioned by codon position, since LDQB/Lcytb values are expected to be higher in the first and second codon positions for balancing selection and higher at the third codon position for general neutrality. Parameters a, b, and the regression index (r2) were compared in the three main taxa by codon position (i) with reference to the whole sequence as aiaall, bi/ball and ri2/rall2. The regression parameters were also compared for each taxon j with reference to Primates as ajaPrimates, bj/bPrimates, and rj2/rPrimates2. Statistical significance for the differences of LDQB/Lcytb among taxa in the whole sequences as well as in the three codon positions was tested with a general linear model (GLM) dependent upon taxa and Lcytb using the program Statistica 7. GLM included the Levene's test for homogeneity of variances.

Patterns of nonsynonymous and synonymous substitutions

The nonsynonymous (dN) and synonymous (dS) substitution distances of Nei and Gojobori (1986) were calculated for all DQB sequence pairs using the program MEGA (Kumar et al. 2008). We obtained average dN and dS distances from pairwise comparisons between sequences belonging to different species within the three taxa groups, since the interspecific dN/dS ratio adequately measures selection (Kryazhimskiy and Plotkin 2008). The parameter for the selection coefficient 4Ns was then estimated from Eq. 3 of Nielsen and Yang (2003) by means of interpolation. The interspecific dN/dS ratio is related with the selection coefficient (s) as follows:
$$ \frac{{{d_{\text{N}}}}}{{{d_{\text{S}}}}} = \frac{{4Ns}}{{1 - {e^{{ - 4Ns}}}}} $$
where N is the long-term effective population size. Notice that dN/dS is nearly equal to 4Ns at large positive values of both (e.g., >3). When dN = 1 and dS = 0, values where replaced with the highest determined positive dN/dS ratio found in the comparison set. The values of 4Ns for each interspecies comparison of DQB within the three studied mammal orders were plotted against their respective average DQB Jukes–Cantor distance. Statistical significance for the differences of 4Ns among taxa was tested with a GLM dependent upon the DQB Jukes–Cantor distance and included the Levene's test for variance homogeneity using the program Statistica 7. Because the Jukes–Cantor distance of DQB is distributed differently among the three main taxa and also because the dN/dS ratio mismeasures selection within species (Kryazhimskiy and Plotkin 2008), the distribution of intraspecific dN values in the tree main taxa was examined with a Student's t test after demonstrating the normality of data with the Kolmogorov–Smirnov d test.

Accumulation of variable sites

The number of variable sites in sequence alignments, as additional evidence of selection-driven polymorphism, was determined for the DQB and cytb sequences for each species of Cetacea, Artiodactyla, and Primates. Thereafter, the proportion of variable sites per allele and nucleotide was determined and partitioned by codon position. After testing for normality of these data within the main taxa with the Kolmogorov–Smirnov d test, statistical significance for the differences between taxa were determined with the Student's t test. A profile for the accumulation of variable sites (vs) in dependence of the alleles sampled (s) for each group of species was built and fitted to the following ad hoc asymptotic–hyperbolic function:
$$ {v_s} = {v_{{\max }}}\frac{{s - 1}}{{k + s - 1}} $$
where s − 1 stands for the accumulation start of vs = 0 at s = 1, vmax is the asymptotic number of variable sites at an infinite number of alleles sampled and k is the number of alleles minus one at which half of the variable sites are observed. Parameters vmax and k and their standard errors were estimated for the three mammal orders examined with the least squares procedure available in the program SigmaPlot 8. Fitting was not practicable for the cytb sequences.

Results

Phylogeny and occurrence of ancestral polymorphisms

Separate phylogenies of the DQB and cytb were built for 260 and 248 sequences, respectively, belonging to Cetacea, Artiodactyla, and Primates (Table 1). The DQB phylogeny revealed several trans-taxon changes for both marine and terrestrial mammals (Fig. 1) suggesting the retention of allele diversity among divergent species in the three mammal orders. The cytb provided a phylogeny consistent with the known organismal phylogeny on which cetaceans and artiodactyls are linked in the clade Cetartiodactyla (Irwin and Árnason 1994; Montgelard et al. 1997; Murphy et al. 2001; Fig. 1a).
https://static-content.springer.com/image/art%3A10.1007%2Fs00251-012-0647-8/MediaObjects/251_2012_647_Fig1_HTML.gif
Fig. 1

a Bayesian majority rule consensus tree for the mitochondrial cytb gene (900 trees; LnL = −23,323.304) and the MHC-DQB gene (600 trees; LnL = −8634.446). DQB unambiguous trans-taxon events are indicated with arrows. b Detail of the cetacean trees. Sequences are abbreviated with one character indicating the order, one character indicating genus, three characters indicating specific name and a serial number

A mapping of species classification into our Bayesian phylogeny of the DQB, revealed a total of 25 unambiguous trans-taxon changes for marine and terrestrial mammal species of which 22 were transpecific or transgenus, two were transfamilial, and one was transorder linking the hippopotamus(Hippopotamus amphibius) DQB sequence with the Ziphiidae clade at a Jukes–Cantor distance as low as 0.06 for the comparison between the hippopotamus and the Gervais' beaked whale (Mesoplodon europaeus; Fig. 1b). Mysticetes showed DQB transpecific polymorphisms particularly among the humpback whale (Megaptera novaeangliae) and the fin whale (Balaenoptera physalus) and a transfamilial polymorphism was observed among the southern right whale (Eubalaena australis) and the gray whale (Eschrichtius robustus) (Fig. 1b). We found no transfamilial polymorphisms within Delphinoidea but found two transgenus polymorphisms between the bottlenose dolphin (Tursiops truncatus) and the short-finned pilot whale (Globicephala macrorhynchus) (Fig. 1b). Sequences from Bovidae and Cervidae families from the Artiodactyla were found much intermingled in the DQB tree as reported previously (Hassanin and Douzery 2003). There was also a consistent relationship between DQB sequences from moose (Alces alces) and musk ox (Ovibos moschatus). As for Primates, we found persistence of DQB alleles among the Catarrhini such as the stump-tailed macaque (Macaca arctoides) and the chimpanzee (Pan troglodytes). DQB sequences from night monkeys (Aotus nancymae: Platyrrhini) remained apart from the rest of the Primates examined (Fig. 1a).

Accumulated occurrence of trans-taxon polymorphisms profiled along the accumulated branch length of the cytb gene was lower for cetaceans despite the large number of species sampled (Table 1). Among the three analyzed mammal orders, artiodactyls showed the largest rate of accumulated trans-taxon variations (Fig. 2). No statistically significant differences were observed between the Lcytb distances of the three main taxa in the three taxonomic levels examined (interval of p 0.14–0.48). According to the χ2 test, no significant differences in the occurrence of trans-taxon polymorphisms for the three taxonomic levels were observed between cetaceans and primates (p > 0.31), but significant or marginally significant higher occurrence of trans-taxon variations were observed for artiodactyls compared with cetaceans (interval of p 0.02–0.11) and primates (interval of p 0.06–0.10).
https://static-content.springer.com/image/art%3A10.1007%2Fs00251-012-0647-8/MediaObjects/251_2012_647_Fig2_HTML.gif
Fig. 2

Rate of trans-taxon changes at the taxonomic levels of individuals within species (I), species within genera (S), genera within families (G), and families within orders (F). Black circles stand for Artiodactyla, open circles for Cetacea and gray triangles for Primates. Taxonomic levels are measured with the mean branch length at cytb phylogeny separating taxa (Lcytb). Horizontal bars indicate standard deviation of the average cytb lengths

Patterns of phylogenetic branch lengths

In all taxa and codon positions, the LDQB/Lcytb ratio showed a decaying relationship with the branch length of the cytb gene (Lcytb) which seems asymptotic to one (Fig. 3). Intercept values (a) in artiodactyls and primates (−0.59 and −0.67, respectively) were higher as compared with cetaceans (−0.80) (Table 2). This can be interpreted as a higher selection signal over the DQB sequences of terrestrial mammals. Higher values of the regression parameter a are also observed in the first and second codon positions as compared with the third position for all mammal orders examined. Lower r2 values indicate more variation in selection signal in cetaceans (0.72) as compared with artiodactyls and primates (0.96 and 0.97, respectively). For the whole sequence as well as for the three codon positions, the GLM identified a significant dependence of the LDQB/Lcytb ratio on Lcytb (p < 0.01). The estimated LDQB/Lcytb means were lower for cetaceans in the whole sequence and in the first codon position, while estimated means and standard errors of Lcytb were almost equal among the three main taxa in the entire sequence and at each codon position. For the whole sequence, the LDQB/Lcytb ratio was significantly different in the pairwise comparisons of cetaceans vs. artiodactyls and primates (p < 0.01) but no significant difference was found between artiodactyls and primates (p = 0.56). In the whole sequence, the Levene's test showed significant heterogeneity of variances for the LDQB/Lcytb ratio and for Lcytb between cetaceans and the other two main taxa (interval of p 0.00–0.09) but not between artiodactyls and primates (\( {p_{{L{\text{cyt}}b}}} = 0.77 \) and \( {p_{{L{\text{DQB}}/L{\text{cyt}}b}}} = 0.76 \)).
https://static-content.springer.com/image/art%3A10.1007%2Fs00251-012-0647-8/MediaObjects/251_2012_647_Fig3_HTML.gif
Fig. 3

Comparisons of branch length distances between the cytb gene (Lcytb) and the ratio between DQB and cytb lengths (LDQB/Lcytb) for Artiodactyla, Cetacea, and Primates in the whole sequence and the three codon positions. Intraspecific comparisons are indicated with the abbreviated species name using the first character of the genus name and the first three characters of the species name. Interspecific distances are shown in dark gray circles. The light gray area in the plots ground is the envelope of all points in all graphs

Table 2

Log–log regression values as well as GLM means and standard errors (SE) for branch length comparisons among cytb and DQB sequences shown in Fig. 3

 

Artiodactyla

Cetacea

Primates

All

A

−0.59

−0.80

−0.67

B

−0.90

−0.84

−0.93

r2

0.96

0.72

0.97

LDQB/Lcytb mean [SE]

3.81 [0.81]

1.87 [0.24]

3.86 [0.96]

Lcytb mean [SE]

0.11 [0.01]

0.13 [0.01]

0.11 [0.01]

1st

a

−0.07

−0.18

−0.11

b

−0.94

−0.88

−0.94

r2

0.98

0.88

0.96

LDQB/Lcytb mean [SE]

7.24 [2.55]

4.61 [1.12]

9.01 [3.02]

Lcytb mean [SE]

0.49 [0.04]

0.45 [0.02]

0.29 [0.03]

2nd

a

0.10

−0.137

−0.029

b

−0.85

−0.915

−0.892

r2

0.94

0.819

0.906

LDQB/Lcytb mean [SE]

23.2 [3.88]

21.2 [2.40]

19.6 [3.72]

Lcytb mean [SE]

0.05 [0.00]

0.06 [0.00]

0.06 [0.01]

3rd

A

−0.07

−0.20

−0.16

B

−0.84

−0.89

−0.99

r2

0.88

0.73

0.84

LDQB/Lcytb mean [SE]

1.16 [0.10]

0.97 [0.07]

1.88 [0.34]

Lcytb mean [SE]

0.87 [0.05]

0.95 [0.04]

0.51 [0.04]

Values of the LDQB/Lcytb ratio in the third codon position were the lowest and the most similar among the three major taxa examined. Comparisons of parameters a, b, and r2 with reference to primates show that variation in cetaceans is higher (rj2/rPrimates2 = 0.75) and that selective signal is lower (aj − aPrimates = −0.13). Artiodactyls showed a pattern similar to primates (r2/rPrimates2 > 0.99) with a slightly higher selective signal (aj − aPrimates = 0.09) (Table 2; Fig. 3). For all codon positions, the slope parameter (b) is lower in cetaceans than that of artiodactyls and primates with noticeable exception of the second codon position for which the cetacean slope is higher. A slower decrease of the LDQB/Lcytb ratio with reference to Lcytb, indicated by the slope parameter in cetaceans, may reflect the lower selection signal on the DQB as inferred from the values of the intercept parameter (a). The GLM mean value of Lcytb was higher for the third codon position and lower for the second codon position, while the GLM mean LDQB/Lcytb ratio was higher for the second codon position and lower for the third codon position (Table 2). The LDQB/Lcytb ratio was similar and not significantly different between artiodactyls, cetaceans, and primates in the second codon position. For the first and third codon positions, primates exhibited the highest LDQB/Lcytb ratio and cetaceans, the lowest. Differences were significant or marginally significant in the third codon position for all comparisons (interval of p 0.00–0.10) and significant only between cetaceans and primates at the first codon position (p = 0.06).

Artiodactyl species from the Bovidae family showed the highest intraspecific selective signal in this group, whereas species from the Cervidae family showed the lowest. All mysticetes showed consistent high selective signal, while ziphiids, monodontids, the sperm whale (Physeter macrocephalus), and the finless porpoise (Neophocaena phocaenoides) revealed the lowest selective pressure in the cetacean group. No evident pattern among species was observed in the Delphinidae and primates (Fig. 3).

Patterns of nonsynonymous and synonymous substitutions

In the three mammal orders examined, the dN/dS ratio expressed as 4Ns decreases in parallel to the DQB sequence distance between species (Fig. 4). For the paired taxa comparisons and for the global comparison among the three taxa, the GLM showed a significant dependence of 4Ns on the DQB sequence distance (p < 0.01). However, this distance was not equally distributed among taxa; cetaceans showed the highest values of 4Ns and the lower DQB sequence distance, while primates showed the lower 4Ns values and the higher DQB sequence distance. Mean 4Ns value was 4.62 for cetaceans, 0.37 for artiodactyls, and −0.45 for primates, while the mean DQB distances were 0.07, 0.14, and 0.14, respectively (Fig. 4). After testing the normality of the DQB sequence distances with the Kolmogorov–Smirnov d test (p > 0.10), the Student's t test showed a significant difference between cetaceans and the two terrestrial orders of mammals (p < 0.02) as well as a marginally significant difference between artiodactyls and primates (p = 0.10). This pattern was similar in the cytb gene and precluded application of the GLM, since the three main taxa cannot be compared on the same levels of variation of DQB or cytb sequences. Notice, however, that linear regressions for the plot 4Ns vs. DQB distance indicate an expected 4Ns value for cetaceans of −2.2 at the mean DQB distance of 0.14 in artiodactyls and primates which corresponds to 4Ns values of 0.40 and −0.40, respectively (Fig. 4).
https://static-content.springer.com/image/art%3A10.1007%2Fs00251-012-0647-8/MediaObjects/251_2012_647_Fig4_HTML.gif
Fig. 4

Selection parameter 4Ns on the MHC-DQB gene in reference of the DQB Jukes–Cantor distance. Black circles and line stand for values and linear regression of Artiodactyla, open circles and dashed line for the corresponding Cetacea results and gray triangles and line for the Primates data

Therefore, we compared the distribution of intraspecific dN values in the three taxa. Within species, synonymous and nonsynonymous sequence distances of the DQB are higher and more variable than the distances of the cytb. For the DQB also, nonsynonymous distance is larger than the synonymous distance in the three mammal orders examined. Mean intraspecific distance of the cytb gene is lower for cetaceans (0.004) and higher for primates (0.017). Nonsynonymous distance within species of the DQB is similar for artiodactyls and primates (0.11 and 0.10, respectively) and lower for cetaceans (0.05) (Table 3). After testing the normality of data, the Student's t test showed significant differences comparing the DQB distances between cetaceans and the two terrestrial groups (p < 0.01) and significant or marginally significant differences for the cytb distance (interval of p 0.01–0.07). The difference between artiodactyls and primates was not significant either for the DQB distance or for the cytb distance (interval of p 0.63–065).
Table 3

Average Jukes–Cantor distance (d), nonsynonymous (dN) and synonymous (dS) distances and their standard errors (SE) of cytb and DQB sequences of the examined species. Abbreviation nc stands for not calculated values due to the lack of data

 

cytb alleles

DQB alleles

cytb d [SE]

DQBd [SE]

DQBdN [SE]

DQBdS [SE]

Artiodactyla

A. alces

7

4

0.009 [0.003]

0.104 [0.019]

0.117 [0.031]

0.069 [0.027]

B. bubalis

7

12

0.014 [0.003]

0.168 [0.020]

0.174 [0.033]

0.152 [0.038]

B. indicus

4

9

0.001 [0.001]

0.114 [0.016]

0.122 [0.024]

0.094 [0.031]

B. taurus

35

28

0.006 [0.003]

0.108 [0.013]

0.115 [0.023]

0.091 [0.020]

C. elaphus

26

20

0.037 [0.003]

0.102 [0.015]

0.101 [0.023]

0.109 [0.028]

C. nippon

17

22

0.025 [0.003]

0.100 [0.015]

0.104 [0.024]

0.089 [0.028]

H. amphibius

4

1

0.009 [0.002]

nc

nc

nc

O. moschatus

6

9

0.002 [0.001]

0.075 [0.013]

0.085 [0.020]

0.049 [0.020]

S. scrofa

23

5

0.009 [0.001]

0.068 [0.015]

0.079 [0.023]

0.038 [0.024]

Mean

14

12

0.012 [0.004]

0.105 [0.011]

0.112 [0.010]

0.086 [0.013]

Cetacea

B. bonaerensis

3

6

0.004 [0.001]

0.071 [0.014]

0.084 [0.025]

0.035 [0.020]

B. mysticetus

4

3

0.001 [0.001]

0.049 [0.014]

0.062 [0.028]

0.012 [0.012]

B. physalus

2

7

0.000 [0.000]

0.071 [0.013]

0.081 [0.021]

0.030 [0.015]

C. hectori

2

3

0.002 [0.001]

0.012 [0.007]

0.016 [0.009]

0.000 [0.000]

D. leucas

2

6

0.004 [0.002]

0.031 [0.009]

0.039 [0.016]

0.008 [0.008]

E. australis

4

7

0.000 [0.000]

0.072 [0.015]

0.094 [0.025]

0.014 [0.015]

E. robustus

3

2

0.001 [0.001]

0.055 [0.017]

0.066 [0.033]

0.021 [0.018]

G. macrorhynchus

2

3

0.003 [0.002]

0.085 [0.019]

0.106 [0.035]

0.022 [0.017]

L. obliquidens

7

3

0.003 [0.001]

0.071 [0.017]

0.086 [0.026]

0.023 [0.020]

M. europaeus

1

2

nc

0.019 [0.011]

0.026 [0.015]

0.000 [0.000]

M. grayi

2

2

0.010 [0.005]

0.013 [0.009]

0.000 [0.000]

0.000 [0.000]

M. monoceros

4

2

0.002 [0.001]

0.000 [0.000]

0.000 [0.000]

0.000 [0.000]

M. novaeangliae

3

33

0.002 [0.001]

0.078 [0.012]

0.082 [0.021]

0.056 [0.022]

M. stejnegeri

2

2

0.003 [0.003]

0.000 [0.000]

0.000 [0.000]

0.000 [0.000]

N. phocaenoides

5

7

0.013 [0.004]

0.026 [0.008]

0.035 [0.013]

0.000 [0.000]

P. macrocephalus

4

2

0.002 [0.001]

0.007 [0.007]

0.010 [0.010]

0.000 [0.000]

T. truncatus

10

2

0.011 [0.002]

0.039 [0.017]

0.034 [0.026]

0.036 [0.039]

Mean

4

5

0.004 [0.001]

0.041 [0.007]

0.048 [0.009]

0.015 [0.004]

Primates

A. nancymae

2

13

0.003 [0.001]

0.093 [0.014]

0.099 [0.023]

0.075 [0.024]

G. gorilla

3

3

0.008 [0.002]

0.124 [0.023]

0.132 [0.032]

0.102 [0.040]

H. sapiens

30

2

0.003 [0.001]

0.141 [0.031]

0.152 [0.040]

0.111 [0.053]

M. arctoides

7

3

0.009 [0.003]

0.098 [0.020]

0.094 [0.026]

0.109 [0.046]

M. mulatta

2

22

0.000 [0.000]

0.114 [0.015]

0.110 [0.024]

0.126 [0.025]

P. pygmaeus

2

7

0.079 [0.009]

0.103 [0.018]

0.085 [0.023]

0.162 [0.048]

P. troglodytes

13

8

0.018 [0.003]

0.069 [0.014]

0.060 [0.018]

0.098 [0.033]

Mean

8

8

0.017 [0.011]

0.106 [0.009]

0.105 [0.011]

0.112 [0.010]

Bold numbers highlights mean values of Cetacea, Artiodactyla and Primates

The results above indicated that statistical comparisons of the rate of nonsynonymous substitution at the DQB between cetaceans and terrestrial mammals depend on differences between taxa affecting molecular clocks which are slower in cetaceans for the two genes examined. However, plotting of the cytb and DQBdN distances showed a pattern of variation noticeably different for cetaceans in which at least two groups of dN are observed. A group of high values of DQBdN (>0.06) is formed by all mysticetes examined together with the short-finned pilot whale and the North Pacific white-sided dolphin (Sagmatias obliquidens), while a group of low dN values (<0.04) is formed by ziphiids, the sperm whale, monodontids, the Hector's dolphin (Cephalorhynchus hectori), the bottlenose dolphin, and the finless porpoise (Table 3; Fig. 5). This pattern is the same as the one observed in the analysis of phylogenetic branch lengths (Fig. 3). The plot of the cytb and DQBdN distances also suggests that in spite of the differential variation of the cytb distance, the DQBdN of cetaceans is, in general, lower than its values for artiodactyls and primates (Fig. 5).
https://static-content.springer.com/image/art%3A10.1007%2Fs00251-012-0647-8/MediaObjects/251_2012_647_Fig5_HTML.gif
Fig. 5

Jukes–Cantor distance of the cytb gene and the nonsynonymous DQB Jukes–Cantor distance (dN) within species of Artiodactyla (black rectangles), Cetacea (white rectangles) and Primates (gray rectangles) in Table 3. The dashed line is the diagonal. Species names are indicated with abbreviations as in Fig. 3. The inner graph is the same plot showing only cetaceans with distinction of two groups having different levels of DQBdN which correspond to different marine habitats

Accumulation of variable sites

DQB sequences of terrestrial mammals showed a higher proportion of variable sites compared to cetacean sequences. Cetaceans, which had the largest number of species analyzed in this investigation (17 species, 92 alleles), had on average less than 8 % of variable sites (13/171), while artiodactyls, which had fewer species and a similar amount of sequences analyzed (nine species, 110 alleles), had 25 % (43/171) of variable sites (Table 4). Both taxa had a comparable sampling of divergence degree as measured by the cytb (Fig. 2). Mean proportion of variable sites on cytb sequences was also lower in cetaceans (0.5 %) as compared with artiodactyls and primates (3 % and 2 %, respectively) and thus, the general proportion of variable sites at the cytb gene was lower than it is in the DQB. The Student's t test showed significant differences in the proportion of variable sites per allele and nucleotide between cetaceans and the two groups of terrestrial mammals. These differences were found for the DQB and the cytb sets (p < 0.02). The proportion of variable sites per allele and nucleotide was not statistically different between artiodactyls and primates for the DQB (p = 0.50) or for the cytb (p = 0.66).
Table 4

Sampling of species, alleles, and variable sites for the cytb and DQB of the mammal orders examined. Parameters vmax and k, their standard errors (SE), and the fit regression index for the estimation of total variable sites at DQB are also shown

 

Artiodactyla

Cetacea

Primates

Species

9

17

7

DQB alleles

110

92

58

cytb alleles

129

60

59

DQB variable sites (171 bp)

43

13

43

cytb variable sites (1,140 bp)

37

6

27

DQBvmax [SE]

92.7 [19.9]

55.9 [11.5]

65.8 [15.4]

DQBk [SE]

8.96 [4.84]

9.42 [3.58]

2.95 [2.22]

r2

0.86

0.79

0.72

Estimation of the asymptotic number of variable sites at the DQB sequence yielded 56 ± 11 (SE; r2 = 0.79) for cetaceans, 93 ± 20 (SE; r2 = 0.79) for artiodactyls, and 66 ± 15 (SE; r2 = 0.72) for primates (Fig. 6). Distribution of variable sites among codon positions in the cytb gene was similar in the three mammal orders occurring more in the third position and less in the second position. Variable sites in the DQB gene in both terrestrial mammal orders were almost equally distributed among the three codon positions, while cetaceans showed more occurrences of variable sites in the first position and less in the third position.
https://static-content.springer.com/image/art%3A10.1007%2Fs00251-012-0647-8/MediaObjects/251_2012_647_Fig6_HTML.gif
Fig. 6

Accumulation of variable sites on DQB and cytb alleles sampled. Each point represent a species among the examined Artiodactyla (black circles, A), Cetacea (open circles, C), and Primates (gray triangles, P). The maximum and minimum accumulation profiles of a fitted hyperbolic function are plotted for the DQB using the function parameters vmax and k and their standard errors, SE (Table 4; \( {v_{{\max }}} + {\text{S}}{{\text{E}}_{{v\max }}} \) and k − SEk for upper bound as well as \( {v_{{\max }}} - {\text{S}}{{\text{E}}_{{v\max }}} \) and k + SEk for lower bound). Fitted function intervals are dark gray for Artiodactyla, open space between black lines for Cetacea, and light gray for Primates

Discussion

All indicators of molecular evolution examined pointed to weaker balancing selection in MHC-DQB of cetaceans in comparison with artiodactyls and primates. The analyses on nonsynonymous substitutions and the accumulation of variable sites impeded statistical examination of selection on the DQB given the general lower rates of substitution in cetaceans. However, the general weaker balancing selection observed in this taxon seemed not related to differences in sampling of species and alleles among taxa. Analyses of the LDQB/Lcytb and dN/dS ratios also showed a higher variation in the strength of balancing selection for the DQB in cetaceans which was unrelated to sampling differences. Because molecular clocks differ between taxa, phylogenetic approaches showed more clearly a different selection mode on the cetacean DQB than distance approaches.

We found apparent transorder sharing of an allele between the hippopotamus and a beaked whale with a sequence similarity of 0.94. Although the phylogenetic relationship between Cetacea and Hippopotamidae is not surprising given other molecular evidence (Gatesy 1997; Nikaido et al. 1999; Boisserie et al. 2005), the preservation of this allelic similarity for over 50 Mya (Thewissen et al. 2007) seems remarkable.

Variable degrees of polymorphism have been described for the different MHC genes in several mammal species (Amills et al. 1998; Baker et al. 2006), each having a particular role in the immune response to specific pathogens (Acevedo-Whitehouse and Cunningham 2006). Also, duplication of MHC genes allows the expression of alternative MHC proteins in one individual. In the California sea lion (Zalophus californianus), for example, the DRB is a family of eight loci, each of which exhibits limited variability but which may have different loci configurations among individuals (two to eight loci for a total of 40 configurations). This yields a highly variable set of possible DR proteins within and between individuals (Bowen et al. 2006). Duplication of DQB has been reported in some cetacean species (Baker et al. 2006; Xu et al. 2007) and is widely reported in artiodactyls showing, in some cases, high variation between duplicated loci (Sigurdardóttir et al. 1992; Amills et al. 1998). MHC duplication and all its multigenic features comprise a genotypic level of polymorphism different to the variation in nucleotide sequence that we have examined for the DQB gene. MHC function depends on sequence variation among individuals within loci, variation of loci configurations (genotypes) within and between individuals, and expression patterns of genes on cells. These three levels of genetic structure and function may be subjected to different modes and strengths of natural selection. Further research on polymorphism at different MHC loci as well as on the patterns of MHC expression on cells against different pathogens are needed to properly assess MHC evolution in marine and terrestrial environments (Acevedo-Whitehouse and Cunningham 2006).

The apparent low levels of polymorphism previously reported for some species of marine mammals does not occur for all species (Hoelzel et al. 1999; Baker et al. 2006; Xu et al. 2007). For example, at least 23 different DQB alleles have been reported in humpback whales (Baker et al. 2006). As is in our sampling, the ziphiids, the sperm whale, monodontids, and the finless porpoise formed a group with low DQB polymorphism, while mysticetes exhibited a relatively high DQB variation similar to that of terrestrial mammals which is concordant with a higher occurrence of transpecific polymorphisms. Different dolphin species appeared in the two groups; the bottlenose dolphin and the Hector's dolphin in the low DQB variation group and the short-finned pilot whale and the North Pacific white-sided dolphin in the high DQB variation group. The distinction of these two groups among cetaceans suggests that life differences between coastal and pelagic realms as well as between warm and cold environments might influence the selection regime on the DQB.

The attributes of epidemic processes in the marine environments, mostly high dispersal and openness (McCallum et al. 2004), seem involved in mass mortalities and other stochastic phenomena which are relevant for the life history and demography of marine mammals (Harwood and Hall 1990). The stronger and less variable balancing selection found in terrestrial mammals might, thus, result from a lower stochasticity in their demography part of which could be related with domestication in artiodactyls of our sampling such as bovines, pigs, and even deer. However, the pattern of stronger and less variable balancing selection of the DQB occurs also in primates for which life history and demography are different. This suggests that the evolutionary pattern of DQB in artiodactyls and primates might be general for terrestrial mammals. The differences of epidemiology among marine and terrestrial environments seem to have the same basis on dispersal, habitat isolation and population openness (McCallum et al. 2004) than the contrasting rates of chromosomal evolution and speciation between cetaceans, artiodactyls, and primates (Árnason 1972; Bush et al. 1977). Therefore, the particular DQB evolution of cetaceans might be part of a general stochastic phenomenology of high dispersal in habitats at once highly continuous and highly contrasting.

In conclusion, our results show that DQB evolution in cetaceans follows the general pattern of balancing selection observed in other mammals (Takahata 1990). However, we found a clear indication of a general weaker balancing selection for the DQB of cetaceans and a larger variation on its strength. Such evolutionary variation for cetaceans also seems, in our results, associated to differences between marine habitats. Further comparative studies on local selective pressures occurring in subpopulations or restricted populations (e.g., Vassilakos et al. 2009) are needed to disentangle environmental, demographic, and phylogenetic influences on the evolution of mammal MHC and their consequences for conservation.

Acknowledgments

We are grateful to all people who, at the labs and/or in the sea, have contributed to this work. In particular, we acknowledge the technical work of M. Dalebout, J. Murrell, M.R. Robles, and D. Steel as well as the review and advice from M.L. Fanjul, D. Heimeier, and M. Uribe, and the scholar orientation by G. Vilaclara. We appreciate the assistance of J. Zúñiga on statistical tests as well as the comments of two anonymous reviewers who greatly improved this article. Institutional, academic, legal, and funding supports were received from The Marsden Foundation, University of Auckland, Consejo Nacional de Ciencia y Tecnología, Facultad de Ciencias-Universidad Nacional Autónoma de México, Posgrado en Ciencias del Mar y Limnología-Universidad Nacional Autónoma de México, Secretaría del Medio Ambiente y Recursos Naturales, Instituto Nacional de Ecología, and Convention on International Trade in Endangered Species of Wild Fauna and Flora.

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© Springer-Verlag 2012