Cryptic preference for MHC-dissimilar females in male red junglefowl, Gallus gallus

Effects of different mating scenarios on embryo viability in brown trout

Mating with attractive or dominant males is often predicted to offer indirect genetic benefits to females, but it is still largely unclear how important such non-random mating can be with regard to embryo viability. We sampled a natural population of adult migratory brown trout (Salmo trutta), bred them in vitro in a half-sib breeding design to separate genetic from maternal environmental effects, raised 2098 embryos singly until hatching, and exposed them experimentally to different levels of pathogen stress at a late embryonic stage. We found that the embryos' tolerance to the induced pathogen stress was linked to the major histocompatibility complex (MHC) of their parents, i.e. certain MHC genotypes appeared to provide better protection against infection than others. We also found significant additive genetic variance for stress tolerance. Melanin-based dark skin patterns revealed males with 'good genes', i.e. embryos fathered by dark coloured males had a high tolerance to infection. Mating with large and dominant males would, however, not improve embryo viability when compared to random mating. We used simulations to provide estimates of how mate choice based on MHC or melanin-based skin patterns would influence embryos' tolerance to the experimentally induced pathogen stress.

MHC heterozygosity and survival in red junglefowl

Genes of the major histocompatibility complex (MHC) form a vital part of the vertebrate immune system and play a major role in pathogen resistance. The extremely high levels of polymorphism observed at the MHC are hypothesised to be driven by pathogen-mediated selection. Although the exact nature of selection remains unclear, three main hypotheses have been put forward; heterozygote advantage, negative frequency-dependence and fluctuating selection. Here, we report the effects of MHC genotype on survival in a cohort of semi-natural red junglefowl (Gallus gallus) that suffered severe mortality as a result of an outbreak of the disease coccidiosis. The cohort was followed from hatching until 250 days of age, approximately the age of sexual maturity in this species, during which time over 80% of the birds died. We show that on average birds with MHC heterozygote genotypes survived infection longer than homozygotes and that this effect was independent of genome-wide heterozygosity, estimated across microsatellite loci. This MHC effect appeared to be caused by a single susceptible haplotype (CD_c) the effect of which was masked in all heterozygote genotypes by other dominant haplotypes. The CD_c homozygous genotype had lower survival than all other genotypes, but CD_c heterozygous genotypes had survival probabilities equal to the most resistant homozygote genotype. Importantly, no heterozygotes conferred greater resistance than the most resistant homozygote genotype, indicating that the observed survival advantage of MHC heterozygotes was the product of dominant, rather than overdominant processes. This pattern and effect of MHC diversity in our population could reflect the processes ongoing in similarly small, fragmented natural populations.

Smelling wrong: hormonal contraception in lemurs alters critical female odour cues

Animals, including humans, use olfaction to assess potential social and sexual partners. Although hormones modulate olfactory cues, we know little about whether contraception affects semiochemical signals and, ultimately, mate choice. We examined the effects of a common contraceptive, medroxyprogesterone acetate (MPA), on the olfactory cues of female ring-tailed lemurs (Lemur catta), and the behavioural response these cues generated in male conspecifics. The genital odorants of contracepted females were dramatically altered, falling well outside the range of normal female variation: MPA decreased the richness and modified the relative abundances of volatile chemicals expressed in labial secretions. Comparisons between treatment groups revealed several indicator compounds that could reliably signal female reproductive status to conspecifics. MPA also changed a female's individual chemical 'signature', while minimizing her chemical distinctiveness relative to other contracepted females. Most remarkably, MPA degraded the chemical patterns that encode honest information about genetic constitution, including individual diversity (heterozygosity) and pairwise relatedness to conspecifics. Lastly, males preferentially investigated the odorants of intact over contracepted females, clearly distinguishing those with immediate reproductive potential. By altering the olfactory cues that signal fertility, individuality, genetic quality and relatedness, contraceptives may disrupt intraspecific interactions in primates, including those relevant to kin recognition and mate choice.

Self/nonself perception, reproduction and the extended MHC

Self/nonself perception governs mate selection in most eukaryotic species. It relies on a number of natural barriers that act before, during and after copulation. These hurdles prevent a costly investment into an embryo with potentially suboptimal genetic and immunological properties and aim at discouraging fertilization when male and female gametes exhibit extensive sharing of alleles. Due to the fact that several genes belonging to the extended major histocompatibility complex (xMHC) carry out crucial immune functions and are the most polymorphic within vertebrate genomes, it is likely that securing heterozygosity and the selection of rare alleles within this gene complex contributes to endowing the offspring with an advantage in fighting infections. Apart from MHC class I and II antigens, the products of several other genes within the xMHC are candidates for participating in mate choice, especially since the respective loci are subject to long-range linkage disequilibrium which may aid to preserve functionally connected alleles within a given haplotype. Among these loci are polymorphic odorant receptor genes that are expressed not only in the olfactory epithelium, but also within male reproductive tissues. They may thus not only be of importance in olfaction-influenced mate choice, by recognizing MHC-dependent individual-specific olfactory signals, but could also guide spermatozoa along chemical gradients to their target, the oocyte. By focusing on the human HLA complex and genes within its vicinity, we show here that the products of several xMHC-specified molecules might be involved in self/nonself perception during reproduction. Although the molecular details are often unknown, the existence of highly diverse, yet intertwined pre- and post-copulatory barriers suggests that xMHC-encoded proteins may be important for various stages of mate choice, germ cell development, as well as embryonic and foetal life in mammals and other vertebrates. Many of these genes should thus be regarded as crucial not only within the immune system, but also in reproduction.

MHC, mate choice and heterozygote advantage in a wild social primate

Preferences for mates carrying dissimilar genes at the major histocompatibility complex (MHC) may help animals increase offspring pathogen resistance or avoid inbreeding. Such preferences have been reported across a range of vertebrates, but have rarely been investigated in social species other than humans. We investigated mate choice and MHC dynamics in wild baboons (Papio ursinus). MHC Class II DRB genes and 16 microsatellite loci were genotyped across six groups (199 individuals). Based on the survey of a key segment of the gene-rich MHC, we found no evidence of mate choice for MHC dissimilarity, diversity or rare MHC genotypes. First, MHC dissimilarity did not differ from random expectation either between parents of the same offspring or between immigrant males and females from the same troop. Second, female reproductive success was not influenced by MHC diversity or genotype frequency. Third, population genetic structure analysis revealed equally high genotypic differentiation among troops, and comparable excess heterozygosity within troops for juveniles, at both Mhc-DRB and neutral loci. Nevertheless, the age structure of Mhc-DRB heterozygosity suggested higher longevity for heterozygotes, which should favour preferences for MHC dissimilarity. We propose that high levels of within-group outbreeding, resulting from group-living and sex-biased dispersal, might weaken selection for MHC-disassortative mate choice.

Mate choice and sexual selection: what have we learned since Darwin?

Charles Darwin laid the foundation for all modern work on sexual selection in his seminal book The Descent of Man, and Selection in Relation to Sex. In this work, Darwin fleshed out the mechanism of sexual selection, a hypothesis that he had proposed in The Origin of Species. He went well beyond a simple description of the phenomenon by providing extensive evidence and considering the far-reaching implications of the idea. Here we consider the contributions of Darwin to sexual selection with a particular eye on how far we have progressed in the last 150 years. We focus on 2 key questions in sexual selection. First, why does mate choice evolve at all? And second, what factors determine the strength of mate choice (or intensity of sexual selection) in each sex? Darwin provided partial answers to these questions, and the progress that has been made on both of these topics since his time should be seen as one of the great triumphs of modern evolutionary biology. However, a review of the literature shows that key aspects of sexual selection are still plagued by confusion and disagreement. Many of these areas are complex and will require new theory and empirical data for complete resolution. Overall, Darwin's contributions are still surprisingly relevant to the modern study of sexual selection, so students of evolutionary biology would be well advised to revisit his works. Although we have made significant progress in some areas of sexual selection research, we still have much to accomplish.