Contrasting mode of evolution between the MHC class I genomic region and class II region in the three-spined stickleback (Gasterosteus aculeatus L.; Gasterosteidae: Teleostei)

Major Histocompatibility Complex (MHC) Genes and Disease Resistance in Fish

Fascinating about classical major histocompatibility complex (MHC) molecules is their polymorphism. The present study is a review and discussion of the fish MHC situation. The basic pattern of MHC variation in fish is similar to mammals, with MHC class I versus class II, and polymorphic classical versus nonpolymorphic nonclassical. However, in many or all teleost fishes, important differences with mammalian or human MHC were observed: (1) The allelic/haplotype diversification levels of classical MHC class I tend to be much higher than in mammals and involve structural positions within but also outside the peptide binding groove; (2) Teleost fish classical MHC class I and class II loci are not linked. The present article summarizes previous studies that performed quantitative trait loci (QTL) analysis for mapping differences in teleost fish disease resistance, and discusses them from MHC point of view. Overall, those QTL studies suggest the possible importance of genomic regions including classical MHC class II and nonclassical MHC class I genes, whereas similar observations were not made for the genomic regions with the highly diversified classical MHC class I alleles. It must be concluded that despite decades of knowing MHC polymorphism in jawed vertebrate species including fish, firm conclusions (as opposed to appealing hypotheses) on the reasons for MHC polymorphism cannot be made, and that the types of polymorphism observed in fish may not be explained by disease-resistance models alone.

Contrasting evolutionary histories of MHC class I and class II loci in grouse--effects of selection and gene conversion

Genes of the major histocompatibility complex (MHC) encode receptor molecules that are responsible for recognition of intracellular and extracellular pathogens (class I and class II genes, respectively) in vertebrates. Given the different roles of class I and II MHC genes, one might expect the strength of selection to differ between these two classes. Different selective pressures may also promote different rates of gene conversion at each class. Despite these predictions, surprisingly few studies have looked at differences between class I and II genes in terms of both selection and gene conversion. Here, we investigated the molecular evolution of MHC class I and II genes in five closely related species of prairie grouse (Centrocercus and Tympanuchus) that possess one class I and two class II loci. We found striking differences in the strength of balancing selection acting on MHC class I versus class II genes. More than half of the putative antigen-binding sites (ABS) of class II were under positive or episodic diversifying selection, compared with only 10% at class I. We also found that gene conversion had a stronger role in shaping the evolution of MHC class II than class I. Overall, the combination of strong positive (balancing) selection and frequent gene conversion has maintained higher diversity of MHC class II than class I in prairie grouse. This is one of the first studies clearly demonstrating that macroevolutionary mechanisms can act differently on genes involved in the immune response against intracellular and extracellular pathogens.

Molecular cloning and characterization of sea bass (Dicentrarchus labrax, L.) MHC class I heavy chain and β2-microglobulin

In this work, the gene and cDNA of sea bass (Dicentrarchus labrax) β2-microglobulin (Dila-β2m) and several cDNAs of MHC class I heavy chain (Dila-UA) were characterized. While Dila-β2m is single-copy, numerous Dila-UA transcripts were identified per individual with variability at the peptide-binding domain (PBD), but also with unexpected diversity from the connective peptide (CP) through the 3' untranslated region (UTR). Phylogenetic analysis segregates Dila-β2m and Dila-UA into each subfamily cluster, placing them in the fish class and branching Dila-MHC-I with lineage U. The α1 domains resemble those of the recently proposed L1 trans-species lineage. Although no Dila-specific α1, α2 or α3 sub-lineages could be observed, two highly distinct sub-lineages were identified at the CP/TM/CYT regions. The three-dimensional homology model of sea bass MHC-I complex is consistent with other characterized vertebrate structures. Furthermore, basal tissue-specific expression profiles were determined for both molecules, and expression of β2m was evaluated after poly I:C stimulus. Results suggest these molecules are orthologues of other β2m and teleost classical MHC-I and their basic structure is evolutionarily conserved, providing relevant information for further studies on antigen presentation in this fish species.

The evolution of MHC diversity: evidence of intralocus gene conversion and recombination in a single-locus system

Gene conversion, the unidirectional exchange of genetic material between homologous sequences, is thought to strongly influence patterns of genetic diversity. The high diversity of major histocompatibility complex (MHC) genes in many species is thought to reflect a long history of gene conversion events both within and among loci. Theoretical work suggests that intra- and interlocus gene conversion leave characteristic signatures of nucleotide diversity, but empirical studies of MHC variation have rarely been able to analyze the effects of conversion events in isolation, due to the presence of multiple gene copies in most species. The potbellied seahorse (Hippocampus abdominalis), a species with a single copy of the MH class II beta-chain gene (MHIIb), provides an ideal system in which to explore predictions on the effects of intralocus gene conversion on patterns of genetic diversity. The genetic diversity of the MHIIb peptide binding region (PBR) is high in the seahorse, similar to other vertebrate species. In contrast, the remainder of the gene shows a total absence of synonymous variation and low levels of intronic sequence diversity, concentrated in 3 short repetitive regions and 1-12 SNPs per intron. The distribution of substitutions across the gene results in a patchwork pattern of shared polymorphism between otherwise divergent sequences. The pattern of nucleotide diversity observed in the seahorse MHIIb gene is congruent with theoretical expectations for intralocus gene conversion, indicating that this evolutionary mechanism has played an important role in MHC gene evolution, contributing to both the high diversity in the PBR and the low diversity outside this region. Neutral variation at this locus may be further reduced due to biases in nucleotide composition and functional constraints.

Sympatric and allopatric divergence of MHC genes in threespine stickleback

Parasites can strongly affect the evolution of their hosts, but their effects on host diversification are less clear. In theory, contrasting parasite communities in different foraging habitats could generate divergent selection on hosts and promote ecological speciation. Immune systems are costly to maintain, adaptable, and an important component of individual fitness. As a result, immune system genes, such as those of the Major Histocompatibility Complex (MHC), can change rapidly in response to parasite-mediated selection. In threespine stickleback (Gasterosteus aculeatus), as well as in other vertebrates, MHC genes have been linked with female mating preference, suggesting that divergent selection acting on MHC genes might influence speciation. Here, we examined genetic variation at MHC Class II loci of sticklebacks from two lakes with a limnetic and benthic species pair, and two lakes with a single species. In both lakes with species pairs, limnetics and benthics differed in their composition of MHC alleles, and limnetics had fewer MHC alleles per individual than benthics. Similar to the limnetics, the allopatric population with a pelagic phenotype had few MHC alleles per individual, suggesting a correlation between MHC genotype and foraging habitat. Using a simulation model we show that the diversity and composition of MHC alleles in a sympatric species pair depends on the amount of assortative mating and on the strength of parasite-mediated selection in adjacent foraging habitats. Our results indicate parallel divergence in the number of MHC alleles between sympatric stickleback species, possibly resulting from the contrasting parasite communities in littoral and pelagic habitats of lakes.

Characterization of antisera raised against stickleback (Gasterosteus aculeatus) MHC class I and class II molecules

The three-spined stickleback (Gasterosteus aculeatus) is an important model organism for investigations on the maintenance of polymorphism of the major histocompatibility complex (MHC) of vertebrates. Analysis of functional aspects of MHC diversity in stickleback would benefit from the availability of MHC specific reagents. Here we characterize antisera raised against recombinant fusion proteins of stickleback MHC class I alpha and class II alpha and beta. Western blot analysis using recombinant proteins confirmed the specificity of the antisera. In brain and muscle preparations, neither of the MHC types was detectable. High levels of each MHC receptor type were observed in gills and spleen and lower levels in head kidneys. In histological sections of gills, epithelial cells of primary and secondary lamellae stained positive with MHC class I antiserum, while single, scattered cells stained positive for MHC class II. In sections of spleen and head kidney, considerable numbers of cells positive for either MHC type were detected. Molecular weight shift in SDS-PAGE after deglycosylation of MHC class I alpha and class II beta confirmed the predicted glyco-protein character of the molecules. The majority of MHC II alpha was not glycosylated; only a small fraction of MHC II alpha was susceptible to deglycosylation. This suggests differential expression of the two stickleback MHC II alpha genes (Gaac-DAA, Gaac-DBA) only one of which (Gaac-DBA) has a site for N-linked glycosylation.

Individual MHC class I and MHC class IIBdiversities are associated with male and female reproductive traits in the three-spined stickleback

Genes of the major histocompatibility complex (MHC) are indispensable for pathogen defence in vertebrates. With wild-caught three-spined sticklebacks (Gasterosteus aculeatus) we conducted the first study to relate individual reproductive parameters to both MHC class I and II diversities. An optimal MHC class IIB diversity was found for male nest quality. However, male breeding colouration was most intense at a maximal MHC class I diversity. One MHC class I allele was associated with male redness. Similarly, one MHC class IIB allele was associated with continuous rather than early female reproduction, possibly extending the reproductive period. Both alleles occurred more frequently with increasing individual allele diversity. We suggest that if an allele is currently not part of the optimum, it had not been propagated by choosy females. The parasite against which this allele provides resistance is therefore unlikely to have been predominant the previous year - a step to negative frequency-dependent selection.