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.     

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     

Nonlinkage of major histocompatibility complex class I and class II loci in bony fishes

 In tetrapods, the functional (classical) class I and class II B loci of the major histocompatibility complex (Mhc) are tightly linked in a single chromosomal region. In an earlier study, we demonstrated that in the zebrafish, Danio rerio, order Cypriniformes, the two classes are present on different chromosomes. Here, we show that the situation is similar in the stickleback, Gasterosteus aculeatus, order Gasterosteiformes, the common guppy, Poecilia reticulata, order Cyprinodontiformes, and the cichlid fish Oreochromis niloticus, order Perciformes. These data, together with unpublished results from other laboratories suggest that in all Euteleostei, the classical class I and class II B loci are in separate linkage groups, and that in at least some of these taxa, the class II loci are in two different groups. Since Euteleostei are at least as numerous as tetrapods, in approximately one-half of jawed vertebrates, the class I and class II regions are not linked. Received: 30 August 1999 / Revised: 20 October 1999     

Analysis of zebrafish Mhc using BAC clones

 Although major histocompatibility complex (Mhc) genes have been identified in a number of species, little is yet known about their organization in species other than human and mouse. The zebrafish, Danio rerio, is a good candidate for full elucidation of the organization of its Mhc. As a step toward achieving this goal, a commercially available zebrafish BAC library was screened with probes specific for previously identified zebrafish class I and class II genes, as well as for genes controlling the proteasome subunits LMP7 and LMP2. Restriction maps of the individual positive clones were prepared and the Mhc (LMP7) genes localized to specific fragments. The total length of genomic DNA fragments with Mhc genes was approximately 1700 kilobases (kb) (200 kb of fragments bearing class I loci and 1500 kb of fragments bearing class II loci). One of the two class I loci (Dare-UCA) is closely associated with the LMP7 locus; the second class I locus (Dare-UAA) is more than 50 kb distant from the UCA locus and has no LMP genes associated with it. None of the class II genes are linked to the class I or the LMP genes. All six of the previously identified class II B genes and one of the three class II A genes were found to be present in the BAC clones; no new Mhc loci could be identified in the library. Each of the six previously identified class II B loci was found to be borne by a separate group of BAC clones. The Dare-DAB and -DAA loci were found on the same clone, approximately 15 kb apart from each other. An expansion of DCB and DDB loci was detected: the zebrafish genome may contain at least five closely related DCB and two closely related DDB loci which are presumably the products of relatively recent tandem duplication. These results are consistent with linkage studies and indicate that in the zebrafish, the class I and class II loci are on different chromosomes, and the class II loci are in three different regions, at least two of which are on different chromosomes. Received: 14 August 1997 / Revised: 16 September 1997     

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     

Genetic drift vs. natural selection in a long-term small isolated population: major histocompatibility complex class II variation in the Gulf of California endemic porpoise (Phocoena sinus)

Although many studies confirm long-term small isolated populations (e.g. island endemics) commonly sustain low neutral genetic variation as a result of genetic drift, it is less clear how selection on adaptive or detrimental genes interplay with random forces. We investigated sequence variation at two major histocompatibility complex (Mhc) class II loci on a porpoise endemic to the upper Gulf of California, México (Phocoena sinus, or vaquita). Its unique declining population is estimated around 500 individuals. Single-strand conformation polymorphism analysis revealed one putative functional allele fixed at the locus DQB (n = 25). At the DRB locus, we found two presumed functional alleles (n = 29), differing by a single nonsynonymous nucleotide substitution that could increase the stability at the dimer interface of alphabeta-heterodimers on heterozygous individuals. Identical trans-specific DQB1 and DRB1 alleles were identified between P. sinus and its closest relative, the Burmeister's porpoise (Phocoena spinipinnis). Comparison with studies on four island endemic mammals suggests fixation of one allele, due to genetic drift, commonly occurs at the DQA or DQB loci (effectively neutral). Similarly, deleterious alleles of small effect are also effectively neutral and can become fixed; a high frequency of anatomical malformations on vaquita gave empirical support to this prediction. In contrast, retention of low but functional polymorphism at the DRB locus was consistent with higher selection intensity. These observations indicated natural selection could maintain (and likely also purge) some crucial alleles even in the face of strong and prolonged genetic drift and inbreeding, suggesting long-term small populations should display low inbreeding depression. Low levels of Mhc variation warn about a high susceptibility to novel pathogens and diseases in vaquita.     

The function of mate choice in sticklebacks: optimizing Mhc genetics*

Sexual reproduction is an evolutionary ‘puzzle’. A sexual female ‘throws away’ half of her genes (during meiosis), and ‘fills up’ what she lost with genes from a male. Thus, sexual reproduction can only be successful if the offspring with the new mixture of genes should be more than twice as fit as if she had just made a copy of herself. A challenging hypothesis assumes that infectious diseases select for females that reshuffle the immune genes for their offspring in each generation. The required increase in quality could be achieved by females selectively ‘smelling out’ suitable immune‐genes (i.e. Mhc alleles) in potential partners, which, in combination with the female's genes, offer optimal resistance against quickly changing infectious diseases. It was found that most three spined sticklebacks Gasterosteus aculeatus in natural populations around Plön, Germany, had intermediate instead of maximal numbers of different Mhc class IIB alleles. Furthermore, fish with an intermediate number of different Mhc alleles were infected with the lowest number of both parasite species and parasites per species. This suggests that Mhc heterozygosity was optimized instead of maximized. Can this immunogenetic optimum be achieved through female choice? In a flow channel design that allowed the detection of olfactory signals only, it was found that female three‐spined sticklebacks that were ready to spawn preferred males as mates that in combination with their Mhc alleles would allow the production of offspring with the optimal number of Mhc alleles. Thus, mate choice in three‐spined sticklebacks could have the two‐fold advantage over asexual reproduction that is required to maintain sexual reproduction. The interaction of olfactory with visual signals in three‐spined stickleback mate choice is discussed. The three‐spined stickleback is a suitable model organism for studying the evolution of sexual reproduction in relation to optimizing offspring immune genetics although other fishes may be as suitable.