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

Abstract

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.

A glance over the fence: Using phylogeny and species comparison for a better understanding of antigen recognition by human γδ T‐cells

“…29,33 Interestingly, some fish species have lost MHC class II genes but the functional consequences are unclear. 37 Apart from "classic" peptide-binding MHC molecules, a large number of class Ib exist. Their genes are found in the MHC locus on the human chromosome (Chr:)6 but also in MHC paralogous regions on other chromosomes.…”

The MHC Class Ia Genes in Chenfu’s Treefrog (Zhangixalus chenfui) Evolved via Gene Duplication, Recombination, and Selection

“…A great number of vertebrates have been observed to have high variation in MHC loci numbers, including fish [9,36], reptiles [37], birds [2,10,26], and mammals [15], which usually have more than one locus. For instance, three loci of MHC class I are expressed in Atlantic salmon [38], at least six loci are expressed in Blue tits [10], and 17 loci in a Cichlid species [39] but only two loci in Chinese sturgeon and paddlefish [9], as well as chickens [40].…”

Combining Multiple Approaches and Models to Dissect the Genetic Architecture of Resistance to Infections in Fish

“…The large variation of mhc genes and the specificity of the effects of such variations make them targets of choice for pathogen-specific resistance mechanisms. Table 1 summarizes the relevant QTL studies pointing toward the implication of adaptive mechanisms in disease resistance (also, reviewed in Yamaguchi and Dijkstra, 2019).…”