Infection-dependent phenotypes in MHC-congenic mice are not due to MHC: can we trust congenic animals?

Limited WKY chromosomal regions confer increases in anxiety and fear memory in a F344 congenic rat strain

This study investigated the interaction between genetic differences in stress reactivity/coping and environmental challenges, such as acute stress during adolescence on adult contextual fear memory and anxiety-like behaviors. Fischer 344 (F344) and the inbred F344;WKY-Stresp3/Eer congenic strain (congenic), in which chromosomal regions from the Wistar-Kyoto (WKY) strain were introgressed into the F344 background, were exposed to a modified forced swim test during adolescence, while controls were undisturbed. In adulthood, fear learning and memory, assessed by contextual fear conditioning, were significantly greater in congenic animals compared with F344 animals, and stress during adolescence increased them even further in males of both strains. Anxiety-like behavior, measured by the open field test, was also greater in congenic than F344 animals, and stress during adolescence increased it further in both strains of adult males. Whole genome sequencing of the F344;WKY-Stresp3/Eer strain revealed an enrichment of WKY genotypes in chromosomes 9, 14, and 15. An example of functional WKY sequence variations in the congenic strain, cannabinoid receptor interacting protein 1 (Cnrip1) had a Cnrip1 transcript isoform that lacked two exons. Although the original hypothesis that the genetic predisposition to increased anxiety of the WKY donor strain would exaggerate fear memory relative to the background strain was confirmed, the consequences of adolescent stress were strain independent but sex dependent in adulthood. Molecular genomic approaches combined with genetic mapping of WKY sequence variations in chromosomes 9, 14, and 15 could aid in finding quantitative trait genes contributing to the variation in fear memory.NEW & NOTEWORTHY This study found that 1) whole genome sequencing of congenic strains should be a criterion for their recognition; 2) sequence variations between Wistar-Kyoto and Fischer 344 strains at regions of chromosomes 9, 14, and 15 contribute to differences in contextual fear memory and anxiety-like behaviors; and 3) stress during adolescence affects these behaviors in males, but not females, and is independent of strain.

Bolder guppies do not have more mating partners, yet sire more offspring

BACKGROUND

Intra-individual stable but inter-individually variable behaviours, i.e. personalities, are commonly reported across diverse animal groups, yet the reasons for their maintenance remain controversial. Therefore, studying fitness consequences of personality traits is necessary to discriminate between alternative explanations.

RESULTS

Here, I measured boldness, a highly repeatable personality trait, and reproductive success in male guppies, Poecilia reticulata. I found that bolder males had higher reproductive success than their shyer conspecifics and they sired offspring with females who had larger clutches.

CONCLUSIONS

This result provides direct evidence for fitness consequences of boldness in the guppy. It suggests that the effect may be driven by bolder males mating with more fecund females.

Experimental viral evolution to specific host MHC genotypes reveals fitness and virulence trade-offs in alternative MHC types

The unprecedented genetic diversity found at vertebrate MHC (major histocompatibility complex) loci influences susceptibility to most infectious and autoimmune diseases. The evolutionary explanation for how these polymorphisms are maintained has been controversial. One leading explanation, antagonistic coevolution (also known as the Red Queen), postulates a never-ending molecular arms race where pathogens evolve to evade immune recognition by common MHC alleles, which in turn provides a selective advantage to hosts carrying rare MHC alleles. This cyclical process leads to negative frequency-dependent selection and promotes MHC diversity if two conditions are met: (i) pathogen adaptation must produce trade-offs that result in pathogen fitness being higher in familiar (i.e., host MHC genotype adapted to) vs. unfamiliar host MHC genotypes; and (ii) this adaptation must produce correlated patterns of virulence (i.e., disease severity). Here we test these fundamental assumptions using an experimental evolutionary approach (serial passage). We demonstrate rapid adaptation and virulence evolution of a mouse-specific retrovirus to its mammalian host across multiple MHC genotypes. Critically, this adaptive response results in trade-offs (i.e., antagonistic pleiotropy) between host MHC genotypes; both viral fitness and virulence is substantially higher in familiar versus unfamiliar MHC genotypes. These data are unique in experimentally confirming the requisite conditions of the antagonistic coevolution model of MHC evolution and providing quantification of fitness effects for pathogen and host. These data help explain the unprecedented diversity of MHC genes, including how disease-causing alleles are maintained.

Major histocompatibility complex-regulated odortypes: peptide-free urinary volatile signals

Major histocompatibility complex (MHC) genes influence urinary odors (odortypes) of mice. That volatile odorants are involved is supported by the observation that odortype identity can be detected from a distance. Furthermore, chemical analyses of urines have revealed numerous volatile odorants that differ in relative abundance between mice that differ only in MHC genotypes. In addition, urines from MHC-different mice evoke distinct odor-induced activity maps in the main olfactory bulbs. However, recent studies report that non-volatile MHC class I peptides may directly act as MHC-associated signals and may thereby be seen to call into question the evidence for a volatile MHC signal. To evaluate this question, we designed a procedure to collect peptide-free urinary volatiles and tested these volatiles for their ability to mediate chemosensory discrimination of MHC-congenic mice differing in their MHC genotype. The headspace volatiles from urines of C57BL/6 congenic mice (haplotypes H2(b) and H2(k)) were collected by solid phase microextraction (SPME). These volatiles were then desorbed into a gas chromatograph (GC) and the entire chromatographic eluate was collected into a buffer solution. Our results conclusively demonstrate that mice trained to discriminate between unadulterated urinary signals of the congenic mice generalize the discrimination, without reward or training, to the buffer solution containing the peptide-free urinary volatiles (p<0.001, binomial test). Thus volatile signals, perhaps along with non-volatile ones, are capable of mediating behavioral discriminations of mice of different MHC genotypes.

MHC haplotype involvement in avian resistance to an ectoparasite

Research on immune function in evolutionary ecology has frequently focused on avian ectoparasites (e.g., mites and lice). However, host immunogenetics involved with bird resistance to ectoparasites has not been determined. The critical role of the major histocompatibility complex (MHC) in adaptive immunity and high genetic variation found within the MHC make this gene complex useful for exploring the immunogenetic basis for bird resistance to ectoparasites. The objective of this study was to determine if the avian MHC influenced resistance to a blood-feeding ectoparasite. Four congenic lines of chickens, differing only at the MHC, were comparatively infested with a cosmopolitan ectoparasite of birds-northern fowl mite (NFM)-which is also a serious pest species of poultry. Mite infestations were monitored over time and mite densities (weekly and maximum) were compared among lines. Chickens with the MHC haplotype B21 were relatively resistant to NFM, compared with birds in the B15 congenic line (P < 0.02). To test for similar effects in an outbred genetic background, a separate experiment was performed with 107 commercial chickens (white leghorn, W-36 strain) infested with NFM. Hens were genotyped using a MHC microsatellite marker (LEI0258) and associations between MHC haplotype and NFM density were tested. The highest peak NFM populations occurred more often on hens with the B15 haplotype versus the B21 haplotype (P = 0.012), which supported the results of the congenic study. These data indicate the avian MHC influences ectoparasite resistance, which is relevant to disease ecology and avian-ectoparasite interaction.

The separate and combined effects of MHC genotype, parasite clone, and host gender on the course of malaria in mice

Background

The link between host MHC (major histocompatibility complex) genotype and malaria is largely based on correlative data with little or no experimental control of potential confounding factors. We used an experimental mouse model to test for main effects of MHC-haplotypes, MHC heterozygosity, and MHC × parasite clone interactions. We experimentally infected MHC-congenic mice (F2 segregants, homo- and heterozygotes, males and females) with one of two clones of Plasmodium chabaudi and recorded disease progression.

Results

We found that MHC haplotype and parasite clone each have a significant influence on the course of the disease, but there was no significant host genotype by parasite genotype interaction. We found no evidence for overdominance nor any other sort of heterozygote advantage or disadvantage.

Conclusion

When tested under experimental conditions, variation in the MHC can significantly influence the course of malaria. However, MHC heterozygote advantage through overdominance or dominance of resistance cannot be assumed in the case of single-strain infections. Future studies might focus on the interaction between MHC heterozygosity and multiple-clone infections.

Individual odortypes: interaction of MHC and background genes

Genes of the major histocompatibility complex (MHC) influence the urinary odors of mice. Behavioral studies have shown (1) that mice differing only at MHC have distinct urinary odors, suggesting an MHC odor phenotype or odortype; (2) that the MHC odortype can be recognized across different background strains; and (3) that the MHC odortype is not an additive trait. Very little is known about the odorants underlying this behavioral phenotype. We compared urinary volatile profiles of two MHC haplotypes (H2(b) and H2(k)) and their heterozygous cross (H2(b) x H2(k)) for two different background strains (C57BL/6J and BALB/c) using solid phase micro-extraction (SPME) headspace analysis and gas chromatography/mass spectrometry (GC/MS). Both MHC and background genes substantially influence the volatile profile. Of 148 compounds screened, 108 of them significantly differ between the six genotypes. Surprisingly, for numerous compounds, their MHC associations are moderated by background genes (i.e., there is a significant MHC x background interaction effect in the statistical model relating genotype to relative compound concentration). These interactions account for nearly 30% of the total genetic effect on the volatile profile. MHC heterozygosity further extends the odortype diversity. For many compounds, the volatile expression for the heterozygote is more extreme than the expression for either homozygote, suggesting a heterozygous-specific odortype. The remarkable breadth of effects of MHC variation on concentrations of metabolites and the interaction between MHC and other genetic variation implies the existence of as yet unknown processes by which variation in MHC genes gives rise to variation in volatile molecules in body fluids.

Detection of a new cerebral malaria susceptibility locus, using CBA mice

Human cerebral malaria (CM) during acute Plasmodium falciparum infection is a serious neurological complication that leads to coma and death. P. berghei ANKA infection of CBA mice is a useful experimental model of CM. To identify host susceptibility loci, we performed chromosomal mapping in crossbred populations of both CM-susceptible CBA and CM-resistant DBA/2 mice. One significant region for a CM-susceptible locus in CBA mice was mapped to H2 region on Chromosome 17, tentatively designated cmsc. cmsc was mapped to a different chromosomal region from that previously reported in the C57BL/6 mouse model of CM. It is possible that different loci contribute to CM in CBA and C57BL/6 mouse strains. Comparison of the function of CM susceptibility loci between CBA and C57BL/6 mice could have important implications for the study of the complex pathogenesis of CM in humans.