Parasite Selection for Immunogenetic Optimality

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Science  05 Sep 2003:
Vol. 301, Issue 5638, pp. 1343
DOI: 10.1126/science.1088293

In vertebrates, genes of the major histocompatibility complex (MHC), with their pronounced polymorphism, potentially represent outstanding examples for the selective advantages of genetic diversity (1). Theoretical models predicted that, within an individual, MHC alleles can be subjected to two opposing selective forces, resulting in an optimal number of genes at intermediate individual MHC diversity (2, 3). Diversifying selection increases heterozygosity and enables wider recognition of pathogens (4). This process is opposed by the need to delete T cells that react with self peptide–MHC combinations (5) from the repertoire, which has been proposed as a possible mechanism constraining expansion of MHC genes. Because too high MHC diversity might delimit T cell diversity, it might also impose limitations on the efficiency of pathogen recognition. However, empirical evidence demonstrating fitness benefits in terms of parasite resistance caused by this type of optimal MHC diversity has been lacking. Therefore, we tested whether three-spined sticklebacks (Gasterosteus aculeatus L.) carrying an intermediate level of individual MHC diversity also displayed the strongest level of resistance against parasite infection. Sticklebacks are particularly suited to test MHC optimality, because MHC class II genotypes can differ markedly in the number of MHC class IIB alleles (6). We caught fish from an outbred population and used these to breed six sibships of immunologically naïve fish (i.e., they had no previous contact to parasites). Immunogenetic diversity ranged from three to nine MHC class IIB alleles found in reverse-transcribed messenger RNA (mRNA) [see (6) for details on genotyping]. The MHC genotypes within these sibships segregated above and below the hypothesized optimal number of ∼5 MHC class IIB alleles, which had previously been estimated in an epidemiological field survey (7). In individual infection treatments, fish from all sibships were simultaneously exposed to three of the most abundant parasite species identified in the field (Fig. 1A) (8). After two rounds of infection, separated by an interval of 8 weeks, we found a significant minimal mean infection rate at an intermediate number of individual MHC class IIB variants [i.e., 5.82 expressed alleles (Fig. 1B)]. This result was also confirmed when sibships were considered separately [i.e., 4.96 alleles (Fig. 1C)] (9). The strong pattern only appeared when infection with all three parasites was accounted for simultaneously. This may not be surprising, because single alleles are expected to correlate with single diseases and multiple alleles can contribute to resistance against several infectious agents (2).

Fig. 1.

(A) Parasite species used to infect three-spined stickleback (G. aculeatus L.). (B) Relation between number of expressed MHC class IIB molecules and mean parasite load [expressed as summed residuals from General Linear Model analysis, models included exposure dose as covariate and sibship as random factor (9)] for double-exposed fish. The function matches a quadratic polynomial [r2 = 0.79, ANOVA F(2,4) = 7.38, P = 0.045] with a minimum of 5.82 alleles only when considering the combined effect from all three species [fit of polynomials for residuals from single parasite analysis: A. crassus, r2 = 0.06, F(2,4) = 0.12, P = 0.886; C. lacustris, r2 = 0.48, F(2,4)= 1.86, P = 0.268; D. spathaceum, r2 = 0.01, F(2,4) = 0.80, P = 0.991]. The dotted line shows the mean from all residuals. Error bars show 1 ±SE, controlled for sibship effects and exposure dose. (C) Relation of mean number of expressed MHC class IIB molecules per sibship and the mean of the slopes between parasite loads (controlled for exposure dose) associated with the four MHC class IIB genotypes of the corresponding sibship (9). The linear relation [f(x) = –0.265 + 0.053x, r 2 = 0.79, F(1,4) = 15.04, P = 0.013] has a zero intercept at 4.96 alleles, which can be equated with the immunogenetic optimum. Dashed lines show 95% confidence interval.

Infection decreased host body condition, which is a fitness-relevant trait in sticklebacks (10). Thus, intensity of infection correlated negatively to the change in body condition of the fish during the experiment [repeated measures analysis of variance (ANOVA), including control and singly infected fish: total number of parasites * time, F(1,141) = 4.351, P = 0.039]. Therefore, we have provided experimental evidence that multiple parasites can select for optimal rather than maximal MHC diversity and that intermediate rather than maximal genetic diversity confers the highest level of fitness.

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