Is mate choice in humans MHC-dependent?

A comparative evaluation of the role of olfaction in attachment

Olfactory information plays an important role in the attachment and bonding processes for both humans and non-human animals. Odor cues obtained via individual body odor facilitate attachment and bonding processes across species with regard to both mate selection and mother-infant bonding. The purpose of the present paper is to summarize the role of odor as it pertains to bond formation and maintenance in the mother-infant bond for human infants and non-human animal infants, and for mate selection among human adults and non-human animals. We then synthesize this summary with literature on attachment and existing evidence for the relationships between olfaction and attachment processes. Finally, we suggest avenues for areas of future research.

Genetic characteristics of spouse selection based on short tandem repeats in DNA and lunula count on fingertip

OBJECTIVE

The aim of this study was to assess the correlation of spouse selection with short tandem repeats (STRs) in DNA and with the number of fingertip lunulae to investigate the role of heredity in spouse selection.

METHODS

We randomly selected a total of 286 couples (husband and wife) as a couple group while 200 paired subjects (a man randomly matched with a woman as a pair of subjects) were selected as a non-spouse group for DNA typing, and to investigate lunulae in spouse selection, a total of 554 couples were selected as a couple group and 500 pairs of subjects were selected as a control group.

RESULTS

A significant difference of STR matching number (a large value implies a higher genetic similarity) between spouse group and non-spouse group were observed (12.3 ± 2.7 vs. 11.8 ± 2.6; p < 0.05). A significant difference of the lunula matching number (difference of lunula counts between a paired subjects, a lower value implies a higher genetic similarity) between two groups were also observed for the lunula counts (1.55 ± 1.88 vs. 3.53 ± 2.40; p < 0.01).

CONCLUSION

Significant and unprecedented relationships were found between the couples and polymorphic STRs, and between spouse selection and lunula counts. Polymorphic STRs and fingertip lunulae counts provide an initial insight into the potentially important contributions that genetic characteristics may play a key role in spouse selection.

Olfatory HLA-associated mechanism of formation of married couples in the development of congenital heart diseases in children

Taking into account the significance of the HLA complex in the inflammatory and immune responses, we can assume that the potential for limiting or developing pathology in the next generation will be determined at the stage of selection of certain alleles in the spousal genotype.The aim. To study the role of HLA assortativity in couples with healthy children and couples with children with congenital heart diseases (CHD) through the prism of immunogenetic mechanisms of mutual olfactory choice.Materials and methods. We studied the distribution of HLA-DRB1 alleles in married couples with healthy children and with children having CHD. To identify the associations of HLA-DRB1 alleles with odor preferences, we also studied the group included young males and females. HLA-DRB1 gene typing was carried out in all participants.Results. The combination of HLA-DRB1 alleles in couples with healthy children was similar to the combination of these alleles in the mutual olfactory sympathy between unfamiliar young males and females. Allele combinations in the spouses from the experimental group differ from the group of random selection. The frequency of matches for HLA-DRB1 alleles in married couples with children having CHD without was significantly higher than in the control group.Conclusion. The first stage of selection, associated with olfactory selection, is aimed to the whole population, and it is significantly manifested in the control group (married couples with healthy children). At the same time, in the experimental group (couples with children having CHD), some deviations from the main selection were discovered. Generally, specific HLA-DRB1 allele combinations obtained on the basis of olfactory assessments, indicate the involvement of HLA molecules in pheromone reception.

Genetic slippage after sex maintains diversity for parasite resistance in a natural host population

Although parasite-mediated selection is a major driver of host evolution, its influence on genetic variation for parasite resistance is not yet well understood. We monitored resistance in a large population of the planktonic crustacean Daphnia magna over 8 years, as it underwent yearly epidemics of the bacterial pathogen Pasteuria ramosa. We observed cyclic dynamics of resistance: Resistance increased throughout the epidemics, but susceptibility was restored each spring when hosts hatched from sexual resting stages. Host resting stages collected across the year showed that largely resistant host populations can produce susceptible sexual offspring. A genetic model of resistance developed for this host-parasite system, based on multiple loci and strong epistasis, is in partial agreement with our findings. Our results reveal that, despite strong selection for resistance in a natural host population, genetic slippage after sexual reproduction can be a strong factor for the maintenance of genetic diversity of host resistance.

Genetic Association between Farrowing Rates and Swine Leukocyte Antigen Alleles or Haplotypes in Microminipigs

We have previously reported specific swine leukocyte antigen (SLA) haplotype associations with significant effects on several reproduction performance traits in a highly inbred miniature pig population of Microminipigs (MMPs). In this study, to clarify the effects on farrowing rates of SLA similarity between mating partners in the MMP population, we compared the farrowing rates as a measure of reproductive success after 1063-cumulative matings among the following three groups of mating partners: (1) completely sharing SLA class I or class II haplotypes or alleles between partners (CS), (2) only one sharing the haplotypes or alleles (OS), and (3) non-sharing the haplotypes or alleles (NS). Average farrowing rates in CS groups consisting of completely sharing SLA class II haplotypes or DRBI and DQB1 alleles were lowest in the three groups. Moreover, lower farrowing rates were indicated in mating pairs with smaller amino acid pairwise genetic distances of SLA-1, SLA-3, DRB1 and DQB1 alleles between the pairs. These results suggested that the dissimilarity of SLA class I and class II alleles between mating partners markedly improved reproductive performance; therefore, SLA alleles or haplotypes are potentially useful genetic markers for the selection of mating pairs in breeding programs and epistatic studies of reproductive traits of MMPs.

Inbreeding Coefficient and Distance in MHC Genes of Parents as Predictors of Reproductive Success in Domestic Cat

Simple Summary

Inbreeding and low diversity in MHC (major histocompatibility complex) genes can have a significant impact on the survival and quality of offspring in mammals. At the same time, a decrease in genetic diversity can be disastrous for animals at individual and species level. For felines, studies of the effects of inbreeding and low variety in MHC genes are conducted on populations with a low number of animals, where there is a high probability of a shortage of available partners, and, accordingly, their choice. The use of model species, especially domestic cats, allows us to identify the main consequences of inbreeding and the lack of a choice of partners for future offspring. The survival of offspring in a domestic cat is primarily affected by the degree of similarity/difference in the genes of the parents’ MHC. Parents with the maximum distance in MHC genes have a larger proportion of surviving kittens, and this effect is most pronounced immediately after birth. In parents with the minimum distance in MHC genes, a significant percentage of kittens are either stillborn or die on the first day after birth. However, inbreeding and the similarity of parents in MHC genes in domestic cats did not affect the body mass of kittens.

Abstract

Inbreeding and low diversity in MHC genes are considered to have a negative effect on reproductive success in animals. This study presents an analysis of the number and body mass of offspring in domestic cat, depending on the inbreeding coefficient and the degree of similarity in MHC genes of class I and II in parents. Inbred partners had a lower number of live kittens at birth than outbred ones. At the same time, the inbreeding coefficient did not affect the litter size and the number of offspring who survived until the period of transition to solid food. The most significant predictor for the number of surviving offspring was the degree of parental similarity in MHC genes: the parents with the maximum distance in MHC genes had more survived kittens. Moreover, this effect was most pronounced immediately after birth. A significant percentage of kittens from parents with a minimum distance in MHC genes were either stillborn or died on the first day after birth. By the age of transition to solid food, this effect is no longer so pronounced. Furthermore, neither the inbreeding coefficient nor the distance in MHC genes of parents had any effect on the body mass of kittens.

Assortative mating and within-spouse pair comparisons

Spousal comparisons have been proposed as a design that can both reduce confounding and estimate effects of the shared adulthood environment. However, assortative mating, the process by which individuals select phenotypically (dis)similar mates, could distort associations when comparing spouses. We evaluated the use of spousal comparisons, as in the within-spouse pair (WSP) model, for aetiological research such as genetic association studies. We demonstrated that the WSP model can reduce confounding but may be susceptible to collider bias arising from conditioning on assorted spouse pairs. Analyses using UK Biobank spouse pairs found that WSP genetic association estimates were smaller than estimates from random pairs for height, educational attainment, and BMI variants. Within-sibling pair estimates, robust to demographic and parental effects, were also smaller than random pair estimates for height and educational attainment, but not for BMI. WSP models, like other within-family models, may reduce confounding from demographic factors in genetic association estimates, and so could be useful for triangulating evidence across study designs to assess the robustness of findings. However, WSP estimates should be interpreted with caution due to potential collider bias.

Detangling the mechanisms and timing of MHC-dependent sexual selection using Soay sheep

Immune defence is a key component of fitness, and individuals are expected to have evolved preferences for mates that ensure immunocompetent offspring. Potential preferences include those for mates with specific heritable immune gene profiles ("good genes") or for immunogenetically dissimilar mates to increase offspring immune gene diversity. The major histocompatibility complex (MHC) is by far the most investigated immune gene in mate choice studies, but we still know very little about its role in sexual selection for genetic benefits. In this issue of Molecular Ecology, Huang et al. capitalize on the extraordinary wealth of behavioural, life history and genetic/genomic data from the free-living Soay sheep population on the Island of Hirta to address this problem. While the authors find evidence of both pre- and postcopulatory MHC-based sexual selection, postcopulatory MHC-dissimilar mate choice is indistinguishable from genome-wide effects, suggesting it is a byproduct of inbreeding avoidance in Soay sheep. The study's comprehensive sampling ensures that inferences are generalizable to the entire population and provides a gold standard for studies investigating immune gene-based sexual selection.

Hormonal Contraception and Violent Death: The Physiological and Psychological Links

In the past decade, two large prospective cohort studies of British and American women have been conducted which found a statistically significant increase in the risk of violent death in ever-users of hormonal contraceptives. Research on the effects of hormonal contraceptives upon the behaviors of intimate partners and on the physiology of women using hormonal contraceptives has provided insight into the possible basis for the resulting increase in violent death. This review examines the changes that are potential contributors to the reported increase.

HLA class II peptide-binding-region analysis reveals funneling of polymorphism in action

BACKGROUND

HLA-class II proteins hold important roles in key physiological processes. The purpose of this study was to compile all class II alleles reported in human population and investigate patterns in pocket variants and their combinations, focusing on the peptide-binding region (PBR).

METHODS

For this purpose, all protein sequences of DPA1, DQA1, DPB1, DQB1 and DRB1 were selected and filtered, in order to have full PBR sequences. Proportional representation was used for pocket variants while population data were also used.

RESULTS

All pocket variants and PBR sequences were retrieved and analyzed based on the preference of amino acids and their properties in all pocket positions. The observed number of pocket variants combinations was much lower than the possible inferred, suggesting that PBR formation is under strict funneling. Also, although class II proteins are very polymorphic, in the majority of the reported alleles in all populations, a significantly less polymorphic pocket core was found.

CONCLUSIONS

Pocket variability of five HLA class II proteins was studied revealing favorable properties of each protein. The actual PBR sequences of HLA class II proteins appear to be governed by restrictions that lead to the establishment of only a fraction of the possible combinations and the polymorphism recorded is the result of intense funneling based on function.

The scent of attraction and the smell of success: crossmodal influences on person perception

In recent decades, there has been an explosion of research into the crossmodal influence of olfactory cues on multisensory person perception. Numerous peer-reviewed studies have documented that a variety of olfactory stimuli, from ambient malodours through to fine fragrances, and even a range of chemosensory body odours can influence everything from a perceiver's judgments of another person's attractiveness, age, affect, health/disease status, and even elements of their personality. The crossmodal and multisensory contributions to such effects are reviewed and the limitations/peculiarities of the research that have been published to date are highlighted. At the same time, however, it is important to note that the presence of scent (and/or the absence of malodour) can also influence people's (i.e., a perceiver's) self-confidence which may, in turn, affect how attractive they appear to others. Several potential cognitive mechanisms have been put forward to try and explain such crossmodal/multisensory influences, and some of the neural substrates underpinning these effects have now been characterized. At the end of this narrative review, a number of the potential (and actual) applications for, and implications of, such crossmodal/multisensory phenomena involving olfaction are outlined briefly.

Structural dissimilarity of partners' immune genes increases sperm viability in women's reproductive tract

Human leucocyte antigen (HLA) genes appear to mediate pre- and post-copulatory mate choice towards HLA-dissimilar ('compatible') partners. However, since genetically distinct alleles often have similar immunogenic properties, genetic dissimilarity is not necessarily an accurate predictor of the functional compatibility of HLA alleles and, hence, may not reflect partners' true compatibility. Furthermore, it has remained unclear whether other genes of the immune system could also play a role in male-female compatibility. We studied whether the immunoglobulin binding regions (eplets) of HLA molecules and the immunoglobulin structural dissimilarity of the partners affect their gamete-level compatibility. We exposed sperm of multiple men to follicular fluid or cervical mucus of multiple women and tested whether sperm viability in these reproductive secretions was influenced by HLA eplet and immunoglobulin structural dissimilarity between partners. We found that eplet dissimilarity positively affects sperm viability in follicular fluid, whereas immunoglobulin dissimilarity enhanced sperm viability in cervical mucus. Together, these findings indicate that structural characteristics of both HLA alleles and immunoglobulins may facilitate cryptic female choice towards immunologically compatible partners. Our results, thus, indicate that partners' genetic compatibility may have wider immunological basis than traditionally has been assumed. Relative contribution of different immunogenetic factors to overall compatibility of the reproductive partners needs to be clarified in future studies.

Differential attraction in mosquito-human interactions and implications for disease control

Mosquito-borne diseases are a major burden on human health worldwide and their eradication through vector control methods remains challenging. In particular, the success of vector control interventions for targeting diseases such as malaria is under threat, in part due to the evolution of insecticide resistance, while for other diseases effective control solutions are still lacking. The rate at which mosquitoes encounter and bite humans is a key determinant of their capacity for disease transmission. Future progress is strongly reliant on improving our understanding of the mechanisms leading to a mosquito bite. Here, we review the biological factors known to influence the attractiveness of mosquitoes to humans, such as body odour, the skin microbiome, genetics and infection by parasites. We identify the knowledge gaps around the relative contribution of each factor, and the potential links between them, as well as the role of natural selection in shaping vector–host–parasite interactions. Finally, we argue that addressing these questions will contribute to improving current tools and the development of novel interventions for the future.

This article is part of the theme issue ‘Novel control strategies for mosquito-borne diseases'.

Marriage does not relate to major histocompatibility complex: a genetic analysis based on 3691 couples

Optimization of chances for healthy offspring is thought to be one of the factors driving mate choice and compatibility of the major histocompatibility complex (MHC) is assumed to determine the offspring's fitness. While humans have been claimed to be able to perceive information of MHC compatibility via the olfactory channel, it remains unknown whether humans use such information for mate choice. By investigation of 3691 married couples, we observed that the high polymorphism of MHC leads to a low chance for homozygous offspring. MHC similarity between couples did not differ from chance, we hence observed no MHC effect in married couples. Hormonal contraception at the time of relationship initiation had no significant effect towards enhanced similarity. A low variety of alleles within a postcode area led to a higher likelihood of homozygous offspring. Based on this data, we conclude that there is no pattern of MHC dis-assortative mating in a genetically diverse Western society. We discuss the question of olfactory mate preference, in-group mating bias and the high polymorphism as potential explanations.

Post-copulatory genetic matchmaking: HLA-dependent effects of cervical mucus on human sperm function

Several studies have demonstrated that women show pre-copulatory mating preferences for human leucocyte antigen (HLA)-dissimilar men. A fascinating, yet unexplored, possibility is that the ultimate mating bias towards HLA-dissimilar partners could occur after copulation, at the gamete level. Here, we explored this possibility by investigating whether the selection towards HLA-dissimilar partners occurs in the cervical mucus. After combining sperm and cervical mucus from multiple males and females (full factorial design), we found that sperm performance (swimming velocity, hyperactivation, and viability) was strongly influenced by the male–female combination. This indicates that sperm fertilization capability may be dependent on the compatibility between cervical mucus (female) and sperm (male). We also found that sperm viability was associated with partners' HLA dissimilarity, indicating that cervical mucus may selectively facilitate later gamete fusion between immunogenetically compatible partners. Together, these results provide novel insights into the female-mediated sperm selection (cryptic female choice) in humans and indicate that processes occurring after copulation may contribute to the mating bias towards HLA-dissimilar partners. Finally, by showing that sperm performance in cervical mucus is influenced by partners' genetic compatibility, the present findings may promote a deeper understanding of infertility.

No evidence for a relationship between MHC heterozygosity and life history strategy in a sample of North American undergraduates

Although allelic diversity at the major histocompatibility complex (MHC) has implications for adaptive immunity, mate choice, and social signalling, how diversity at the MHC influences the calibration of life history strategies remains largely uninvestigated. The current study investigated whether greater MHC heterozygosity was associated with markers of slower life history strategies in a sample of 789 North American undergraduates. Contrary to preregistered predictions and to previously published findings, MHC heterozygosity was not related to any of the psychological life history-relevant variables measured (including short- vs. long-term sexual strategy, temporal discounting, the Arizona life history battery, past and current health, disgust sensitivity, and Big Five personality traits). Further, no meaningful effects emerged when analysing women and men separately. Possible reasons for why the current results are inconsistent with previous work are discussed.

Chemical signals from eggs facilitate cryptic female choice in humans

Mate choice can continue after mating via chemical communication between the female reproductive system and sperm. While there is a growing appreciation that females can bias sperm use and paternity by exerting cryptic female choice for preferred males, we know surprisingly little about the mechanisms underlying these post-mating choices. In particular, whether chemical signals released from eggs (chemoattractants) allow females to exert cryptic female choice to favour sperm from specific males remains an open question, particularly in species (including humans) where adults exercise pre-mating mate choice. Here, we adapt a classic dichotomous mate choice assay to the microscopic scale to assess gamete-mediated mate choice in humans. We examined how sperm respond to follicular fluid, a source of human sperm chemoattractants, from either their partner or a non-partner female when experiencing a simultaneous or non-simultaneous choice between follicular fluids. We report robust evidence under these two distinct experimental conditions that follicular fluid from different females consistently and differentially attracts sperm from specific males. This chemoattractant-moderated choice of sperm offers eggs an avenue to exercise independent mate preference. Indeed, gamete-mediated mate choice did not reinforce pre-mating human mate choice decisions. Our results demonstrate that chemoattractants facilitate gamete-mediated mate choice in humans, which offers females the opportunity to exert cryptic female choice for sperm from specific males.

Interdisciplinary challenges for elucidating human olfactory attractiveness

Many species use chemicals to communicate. In humans, there is increasing evidence that chemicals conveyed by the body are extremely important in interpersonal relationships. However, many aspects of chemical communication remain to be explored to fully understand this function in humans. The aim of this article is to identify relevant challenges in this field, with a focus on human attractiveness in the context of reproduction, and to put forward roadmaps for future studies that will hopefully extend to a wider range of social interactions. The first challenge consists in not being limited to body (mal)odours from the axilla. Preliminary data on how the odour of the face and head is perceived are presented. Second, there is a crucial need to increase our knowledge of the chemical bases of human chemical communication. Third, cross-cultural approaches must not be overlooked, because they have a major input in understanding the universal and culture-specific aspects of chemical communication. Fourth, the influence of specific cultural practices such as contraceptive and fragrance use is likely to be prominent and, therefore, needs to be well described. The fifth and last challenge for research projects in this field is the integration of different disciplines such as behavioural sciences, social sciences, neurosciences and microbiology. This article is part of the Theo Murphy meeting issue 'Olfactory communication in humans'.

Genomic evidence for MHC disassortative mating in humans

Although pervasive in many animal species, the evidence for major histocompatibility complex (MHC) disassortative mating in humans remains inconsistent across studies. Here, to revisit this issue, we analyse dense genotype data for 883 European and Middle Eastern couples. To distinguish MHC-specific effects from socio-cultural confounders, the pattern of relatedness between spouses in the MHC region is compared to the rest of the genome. Couples from Israel exhibit no significant pattern of relatedness across the MHC region, whereas across the genome, they are more similar than random pairs of individuals, which may reflect social homogamy and/or cousin marriages. On the other hand, couples from The Netherlands and more generally from Northern Europe are significantly more MHC-dissimilar than random pairs of individuals, and this pattern of dissimilarity is extreme when compared with the rest of the genome. Our findings support the hypothesis that the MHC influences mate choice in humans in a context-dependent way: MHC-driven preferences may exist in all populations but, in some populations, social constraints over mate choice may reduce the ability of individuals to rely on such biological cues when choosing their mates.

Assortative mating at loci under recent natural selection in humans

Genetic correlation between mates at specific loci can greatly alter the evolutionary trajectory of a species. Genetic assortative mating has been documented in humans, but its existence beyond population stratification (shared ancestry) has been a matter of controversy. Here, we develop a method to measure assortative mating across the genome at 1,044,854 single-nucleotide polymorphisms (SNPs), controlling for population stratification and cohort-specific cryptic relatedness. Using data on 1683 human couples from two data sources, we find evidence for both assortative and disassortative mating at specific, discernible loci throughout the entire genome. Then, using the composite of multiple signals (CMS) score, we also show that the group of SNPs exhibiting the most assortativity has been under stronger recent positive selection. Simulations using realistic inputs confirm that assortative mating might indeed affect changes in allele frequency over time. These results suggest that genetic assortative mating may be speeding up evolution in humans.

Multiplicative fitness, rapid haplotype discovery, and fitness decay explain evolution of human MHC

The major histocompatibility complex (MHC) is a central component of the vertebrate immune system and hence evolves in the regime of a host-pathogen evolutionary race. The MHC is associated with quantitative traits which directly affect fitness and are subject to selection pressure. The evolution of haplotypes at the MHC HLA (HLA) locus is generally thought to be governed by selection for increased diversity that is manifested in overdominance and/or negative frequency-dependent selection (FDS). However, recently, a model combining purifying selection on haplotypes and balancing selection on alleles has been proposed. We compare the predictions of several population dynamics models of haplotype frequency evolution to the distributions derived from 6.59-million-donor HLA typings from the National Marrow Donor Program registry. We show that models that combine a multiplicative fitness function, extremely high haplotype discovery rates, and exponential fitness decay over time produce the best fit to the data for most of the analyzed populations. In contrast, overdominance is not supported, and population substructure does not explain the observed haplotype frequencies. Furthermore, there is no evidence of negative FDS. Thus, multiplicative fitness, rapid haplotype discovery, and rapid fitness decay appear to be the major factors shaping the HLA haplotype frequency distribution in the human population.

Assortative Mating on Ancestry-Variant Traits in Admixed Latin American Populations

Assortative mating is a universal feature of human societies, and individuals from ethnically diverse populations are known to mate assortatively based on similarities in genetic ancestry. However, little is currently known regarding the exact phenotypic cues, or their underlying genetic architecture, which inform ancestry-based assortative mating. We developed a novel approach, using genome-wide analysis of ancestry-specific haplotypes, to evaluate ancestry-based assortative mating on traits whose expression varies among the three continental population groups – African, European, and Native American – that admixed to form modern Latin American populations. Application of this method to genome sequences sampled from Colombia, Mexico, Peru, and Puerto Rico revealed widespread ancestry-based assortative mating. We discovered a number of anthropometric traits (body mass, height, and facial development) and neurological attributes (educational attainment and schizophrenia) that serve as phenotypic cues for ancestry-based assortative mating. Major histocompatibility complex (MHC) loci show population-specific patterns of both assortative and disassortative mating in Latin America. Ancestry-based assortative mating in the populations analyzed here appears to be driven primarily by African ancestry. This study serves as an example of how population genomic analyses can yield novel insights into human behavior.

Human Leukocyte Antigen similarity decreases partners' and strangers' body odor attractiveness for women not using hormonal contraception

The Human Leukocyte Antigen (HLA) is a gene complex that encodes important elements of the human immune system. HLA profile is communicated via olfaction and interindividual diversity is assumed to be advantageous for mate choice. Additionally, HLA diversity appears to enhance satisfaction and sexual attraction in existing romantic partnerships. However, whether this effect is transmitted via body odors and whether it results in an attraction towards HLA-dissimilar individuals and/or an avoidance of HLA-similar ones remains unclear. In the present study, we genotyped couples and asked each participant to rate a body odor sample from their partner and from three strangers of the opposite sex who expressed a similar or dissimilar HLA-B and HLA-C genotype. We found no statistically significant preference for HLA similarity or dissimilarity in men. Among women, the observed effects differed depending on hormonal contraception status. Like men, women on hormonal contraception did not exhibit significant HLA-related preferences. However, for women not using hormonal contraceptives, odors from HLA-B and HLA-C similar donors were significantly less attractive than those from HLA-dissimilar donors, regardless of whether the samples were from a partner or a stranger. Our findings support the hypothesis that HLA similarity is perceived via body odors and that such similarity affects human attraction. This mechanism may serve an evolutionarily adaptive function in preventing prospective offspring from having decreased immunocompetence, or decreasing the chance of kin mating.

Genomic analysis of MHC-based mate choice in the monogamous California mouse

Variation at Major Histocompatibility Complex (MHC) genes is thought to be an important mechanism underlying mate choice in vertebrates, with individuals typically predicted to prefer MHC-dissimilar reproductive partners. However, analyses based on individual MHC loci have generated contradictory results regarding the role of these genes in mate-choice decisions. To provide a more comprehensive assessment of relationships between MHC variation and mating behavior, we used an exome capture strategy to characterize variability at 13 MHC loci, 312 innate immune system genes, and 1044 nonimmune genes in 25 obligate monogamous pairs of California mice (Peromyscus californicus) from 2 free-living populations of this species in Monterey County, California. Pairwise genotypic comparisons and analyses of SNP-based allelic differences failed to detect disassortative mating based on MHC variability; reproductive partners were not more dissimilar than randomly generated male-female pairs at MHC, innate or nonimmune loci. Within populations, individuals tended to be more closely related at MHC genes than at innate or nonimmune genes. Consistent with the functional role of immunogenes, the 2 study populations were highly differentiated at MHC and innate genes but not at nonimmune loci. Collectively, our results suggest that MHC genetic variation in California mice reflects local differences in pathogen exposure rather than disassortative mating based on variability at MHC Class I and II genes.

Female Japanese quail visually differentiate testosterone-dependent male attractiveness for mating preferences

Biased mating due to female preferences towards certain traits in males is a major mechanism driving sexual selection, and may constitute an important evolutionary force in organisms with sexual reproduction. In birds, although the role of male ornamentation, plumage coloration, genetic dissimilarity, and body size have on mate selection by females have been examined extensively, few studies have clarified exactly how these characteristics affect female mate preferences. Here, we show that testosterone (T)-dependent male attractiveness enhances female preference for males of a polygamous species, the Japanese quail. A significant positive correlation between female mating preference and circulating T in the male was observed. The cheek feathers of attractive males contained higher levels of melanin and were more brightly colored. The ability of females to distinguish attractive males from other males was negated when the light source was covered with a sharp cut filter (cutoff; < 640 nm). When females were maintained under short-day conditions, the expression of retinal red-sensitive opsin decreased dramatically and they became insensitive to male attractiveness. Our results showed that female preference in quail is strongly stimulated by male feather coloration in a T-dependent manner and that female birds develop a keen sense for this coloration due to upregulation of retinal red-sensitive opsin under breeding conditions.

Negative-Assortative Mating in the White-Throated Sparrow

Nonrandom mating based on phenotype has been observed in a number of organisms, but a very high proportion of these examples are of assortative mating. The strongest example of negative-assortative mating is for white-striped versus tan-striped crown in the white-throated sparrow, where about 98% of the observed pairings (mated pairs or social pairs) are between mates with different phenotypes and the correlation between mating types is -0.964. Although nonrandom mating has been explored theoretically for decades, these models have generally not focused on specific well-documented examples. Here we have developed a model to investigate the dynamics and equilibrium of this iconic example. The observed pattern of mating appears to be the result of 96% negative-assortative mating and a 17% advantage of W (white) male × T (tan) female matings compared to the reciprocal T male × W female matings. The equilibrium heterozygosity given these values is 0.500, very close to the 0.501 observed in our large sample of pairings, and this heterozygosity has been maintained for the 29 years from 1988 to 2016. In addition, the equilibrium frequency of 2m inversion determining the white-striped phenotype has been maintained at a frequency very close to its equilibrium frequency of 0.25. Overall, this model demonstrates how evolutionary genetic models can be used to understand negative-assortative mating.

Soluble major histocompatibility complex molecules in immune regulation: highlighting class II antigens

The involvement of major histocompatibility complex (MHC) antigens in the development and regulation of immune response has been well defined over the years, starting from maturation, antigenic peptide loading, migration to the cell membrane for recognition by the T-cell receptor and recycling for immune response cessation. During this intracellular trafficking, MHC antigens find a way to be excreted by the cells, because they can be found as soluble MHC class I (sMHC-I) and class II (sMHC-II) molecules in all body fluids. Although secretion mechanisms have not been sufficiently studied, sMHC molecules have been shown to display important immunoregulatory properties. Their levels in the serum have been shown to be altered in a variety of diseases, including viral infections, inflammation, autoimmunities and cancer, etc. while they seem to be involved in a number of physiological reactions, including maintenance of tolerance, reproduction, as well as mate choice vis-à-vis species evolution. The present review aims to present the thus far existing literature on sMHC molecules and point out the importance of these molecules in the maintenance of immune homeostasis.

Individual biological sensitivity to environmental influences: testing the differential susceptibility properties of the 5HTTLPR polymorphism in relation to depressive symptoms and delinquency in two adolescent general samples

The gene–environment interaction research field in psychiatry has traditionally been dominated by the diathesis–stress framework, where certain genotypes are assumed to confer increased risk for adverse outcomes in a stressful environment. In later years, theories of differential susceptibility, or biological sensitivity, suggest that candidate genes that interact with environmental events do not exclusively confer a risk for behavioural or psychiatric disorders but rather seem to alter the sensitivity to both positive and negative environmental influences. The present study investigates the susceptibility properties of the serotonin transporter-linked polymorphic region (5HTTLPR) in relation to depressive symptoms and delinquency in two separate adolescent community samples: n = 1457, collected in 2006; and n = 191, collected in 2001. Two-, three-, and four-way interactions between the 5HTTLPR, positive and negative family environment, and sex were found in relation to both depressive symptoms and delinquency. However, the susceptibility properties of the 5HTTLPR were distinctly less pronounced in relation to depressive symptoms. If the assumption that the 5HTTLPR induces differential susceptibility to both positive and negative environmental influences is correct, the previous failures to measure and control for positive environmental factors might be a possible explanation for former inconsistent findings within the research field.

Genomewide association study of HLA alloimmunization in previously pregnant blood donors

BACKGROUND

Alloimmunization through blood transfusion, transplantation, or circulating fetal cells during pregnancy is a significant concern. Some exposed individuals make alloantibodies while others do not, implying variation in genetic risk factors.

STUDY DESIGN AND METHODS

We conducted a genomewide association study (GWAS) of 9,427,497 single-nucleotide polymorphisms (SNPs) to identify genetic variants for HLA alloimmunization in previously pregnant blood donors with (n = 752) and without (n = 753) HLA Class I or II alloantibodies.

RESULTS

A SNP in the neurexophilin 2 (NXPH2) gene surpassed genome-wide significance (p = 2.06 × 10-8 ), with multiple adjacent markers p < 10-6 , for women with anti-Class I alloantibodies only. Little is currently known about the function of NXPH2, although gene family members have been shown to impact immunity. SNPs in the E2F7 gene, a transcription factor related to cell cycle control and cellular proliferation, also approached genomewide significance (p = 2.5 × 10-7 ).

CONCLUSION

Further work to extend the GWAS approach and to characterize variants in NXPH2 and E2F7 in the context of alloantibody formation is warranted.

MHC-Dependent Mate Selection within 872 Spousal Pairs of European Ancestry from the Health and Retirement Study

Disassortative mating refers to the phenomenon in which individuals with dissimilar genotypes and/or phenotypes mate with one another more frequently than would be expected by chance. Although the existence of disassortative mating is well established in plant and animal species, the only documented example of negative assortment in humans involves dissimilarity at the major histocompatibility complex (MHC) locus. Previous studies investigating mating patterns at the MHC have been hampered by limited sample size and contradictory findings. Inspired by the sparse and conflicting evidence, we investigated the role that the MHC region played in human mate selection using genome-wide association data from 872 European American spouses from the Health and Retirement Study (HRS). First, we treated the MHC region as a whole, and investigated genomic similarity between spouses using three levels of genomic variation: single-nucleotide polymorphisms (SNPs), classical human leukocyte antigen (HLA) alleles (both four-digit and two-digit classifications), and amino acid polymorphisms. The extent of MHC dissimilarity between spouses was assessed using a permutation approach. Second, we investigated fine scale mating patterns by testing for deviations from random mating at individual SNPs, HLA genes, and amino acids in HLA molecules. Third, we assessed how extreme the spousal relatedness at the MHC region was compared to the rest of the genome, to distinguish the MHC-specific effects from genome-wide effects. We show that neither the MHC region, nor any single SNPs, classic HLA alleles, or amino acid polymorphisms within the MHC region, were significantly dissimilar between spouses relative to non-spouse pairs. However, dissimilarity in the MHC region was extreme relative to the rest of genome for both spousal and non-spouse pairs. Despite the long-standing controversy, our analyses did not support a significant role of MHC dissimilarity in human mate choice.

Mate choice in sticklebacks reveals that immunogenes can drive ecological speciation

Adaptation to ecologically contrasting niches can lead to the formation of new species. Theoretically, this process of ecological speciation can be driven by pleiotropic "magic traits" that genetically link natural and sexual selection. To qualify as a true magic trait, the pleiotropic function of a gene must be reflected in biologically relevant mechanisms underlying both local adaptation and mate choice. The immune genes of the major histocompatibility complex (MHC) contribute to parasite resistance and also play a major role in sexual selection. Hence, the MHC may encode a candidate magic trait. Using diverging 3-spined stickleback populations from a connected lake-river habitat, we show with mate choice experiments in a flow channel that polymorphic MHC genes probably underlie assortative mating with respect to particular habitat-adapted ecotypes, potentially resulting in reproductive isolation. By manipulating olfactory cues in controlled experiments, we show that female sticklebacks employ MHC-dependent male olfactory signals to select mates with which they can achieve a habitat-specific MHC gene structure that optimally protects their offspring against local parasites. By using MHC-based olfactory signals, females thus select individuals of their own population as mates. Our results demonstrate how mate choice and parasite resistance may be functionally linked. These findings suggest that MHC genes are pleiotropic and encode a true magic trait of biologically significant effect.

Odour-based discrimination of similarity at the major histocompatibility complex in birds

Many animals are known to preferentially mate with partners that are dissimilar at the major histocompatibility complex (MHC) in order to maximize the antigen binding repertoire (or disease resistance) in their offspring. Although several mammals, fish or lizards use odour cues to assess MHC similarity with potential partners, the ability of birds to assess MHC similarity using olfactory cues has not yet been explored. Here we used a behavioural binary choice test and high-throughput-sequencing of MHC class IIB to determine whether blue petrels can discriminate MHC similarity based on odour cues alone. Blue petrels are seabirds with particularly good sense of smell, they have a reciprocal mate choice and are known to preferentially mate with MHC-dissimilar partners. Incubating males preferentially approached the odour of the more MHC-dissimilar female, whereas incubating females showed opposite preferences. Given their mating pattern, females were, however, expected to show preference for the odour of the more MHC-dissimilar male. Further studies are needed to determine whether, as in women and female mice, the preference varies with the reproductive cycle in blue petrel females. Our results provide the first evidence that birds can use odour cues only to assess MHC dissimilarity.

Predicting Romantic Interest at Zero Acquaintance: Evidence of Sex Differences in Trait Perception but Not in Predictors of Interest

We evaluated five competing hypotheses about what predicts romantic interest. Through a half-block quasi-experimental design, a large sample of young adults (i.e., responders; n = 335) viewed videos of opposite-sex persons (i.e., targets) talking about themselves and responders rated the targets' traits and their romantic interest in the target. We tested whether similarity, dissimilarity, or overall trait levels on mate value, physical attractiveness, life history strategy, and the Big-Five personality factors predicted romantic interest at zero acquaintance, and whether sex acted as a moderator. We tested the responders' individual perception of the targets' traits, in addition to the targets' own self-reported trait levels and a consensus rating of the targets made by the responders. We used polynomial regression with response surface analysis within multilevel modeling to test support for each of the hypotheses. Results suggest a large sex difference in trait perception; when women rated men, they agreed in their perception more often than when men rated women. However, as a predictor of romantic interest, there were no sex differences. Only the responders' perception of the targets' physical attractiveness predicted romantic interest; specifically, responders' who rated the targets' physical attractiveness as higher than themselves reported more romantic interest.

Present Yourself! By MHC Class I and MHC Class II Molecules

Since the discovery of MHC molecules, it has taken 40 years to arrive at a coherent picture of how MHC class I and MHC class II molecules really work. This is a story of the proteases and MHC-like chaperones that support the MHC class I and II molecules in presenting peptides to the immune system. We now understand that the MHC system shapes both the repertoire of presented peptides and the subsequent T cell response, with important implications ranging from transplant rejection to tumor immunotherapies. Here we present an illustrated review of the ins and outs of MHC class I and MHC class II antigen presentation.

Influence of HLA on human partnership and sexual satisfaction

The major histocompatibility complex (MHC, called HLA in humans) is an important genetic component of the immune system. Fish, birds and mammals prefer mates with different genetic MHC code compared to their own, which they determine using olfactory cues. This preference increases the chances of high MHC variety in the offspring, leading to enhanced resilience against a variety of pathogens. Humans are also able to discriminate HLA related olfactory stimuli, however, it is debated whether this mechanism is of behavioural relevance. We show on a large sample (N = 508), with high-resolution typing of HLA class I/II, that HLA dissimilarity correlates with partnership, sexuality and enhances the desire to procreate. We conclude that HLA mediates mate behaviour in humans.

Negative-assortative mating for color in wolves

There is strong negative-assortative mating for gray and black pelage color in the iconic wolves in Yellowstone National Park. This is the first documented case of significant negative-assortative mating in mammals and one of only a very few cases in vertebrates. Of 261 matings documented from 1995 to 2015, 63.6% were between gray and black wolves and the correlation between mates for color was -0.266. There was a similar excess of matings of both gray males × black females and black males × gray females. Using the observed frequency of negative-assortative mating in a model with both random and negative-assortative mating, the estimated proportion of negative-assortative mating was 0.430. The estimated frequency of black wolves in the population from 1996 to 2014 was 0.452 and these frequencies appear stable over this 19-year period. Using the estimated level of negative-assortative mating, the predicted equilibrium frequency of the dominant allele was 0.278, very close to the mean value of 0.253 observed. In addition, the patterns of genotype frequencies, that is, the observed proportion of black homozygotes and the observed excess of black heterozygotes, are consistent with negative-assortative mating. Importantly these results demonstrate that negative-assortative mating could be entirely responsible for the maintenance of this well-known color polymorphism.

Tolerance to noninherited maternal antigens, reproductive microchimerism and regulatory T cell memory: 60 years after 'Evidence for actively acquired tolerance to Rh antigens'

Compulsory exposure to genetically foreign maternal tissue imprints in offspring sustained tolerance to noninherited maternal antigens (NIMA). Immunological tolerance to NIMA was first described by Dr. Ray D. Owen for women genetically negative for erythrocyte rhesus (Rh) antigen with reduced sensitization from developmental Rh exposure by their mothers. Extending this analysis to HLA haplotypes has uncovered the exciting potential for therapeutically exploiting NIMA-specific tolerance naturally engrained in mammalian reproduction for improved clinical outcomes after allogeneic transplantation. Herein, we summarize emerging scientific concepts stemming from tolerance to NIMA that includes postnatal maintenance of microchimeric maternal origin cells in offspring, expanded accumulation of immune suppressive regulatory T cells with NIMA-specificity, along with teleological benefits and immunological consequences of NIMA-specific tolerance conserved across mammalian species.

Genetic and socioeconomic study of mate choice in Latinos reveals novel assortment patterns

Nonrandom mating in human populations has important implications for genetics and medicine as well as for economics and sociology. In this study, we performed an integrative analysis of a large cohort of Mexican and Puerto Rican couples using detailed socioeconomic attributes and genotypes. We found that in ethnically homogeneous Latino communities, partners are significantly more similar in their genomic ancestries than expected by chance. Consistent with this, we also found that partners are more closely related--equivalent to between third and fourth cousins in Mexicans and Puerto Ricans--than matched random male-female pairs. Our analysis showed that this genomic ancestry similarity cannot be explained by the standard socioeconomic measurables alone. Strikingly, the assortment of genomic ancestry in couples was consistently stronger than even the assortment of education. We found enriched correlation of partners' genotypes at genes known to be involved in facial development. We replicated our results across multiple geographic locations. We discuss the implications of assortment and assortment-specific loci on disease dynamics and disease mapping methods in Latinos.

Cross-Generational Reproductive Fitness Enforced by Microchimeric Maternal Cells

Exposure to maternal tissue during in utero development imprints tolerance to immunologically foreign non-inherited maternal antigens (NIMA) that persists into adulthood. The biological advantage of this tolerance, conserved across mammalian species, remains unclear. Here, we show maternal cells that establish microchimerism in female offspring during development promote systemic accumulation of immune suppressive regulatory T cells (Tregs) with NIMA specificity. NIMA-specific Tregs expand during pregnancies sired by males expressing alloantigens with overlapping NIMA specificity, thereby averting fetal wastage triggered by prenatal infection and non-infectious disruptions of fetal tolerance. Therefore, exposure to NIMA selectively enhances reproductive success in second-generation females carrying embryos with overlapping paternally inherited antigens. These findings demonstrate that genetic fitness, canonically thought to be restricted to Mendelian inheritance, is enhanced in female placental mammals through vertically transferred maternal cells that promote conservation of NIMA and enforce cross-generational reproductive benefits.

Major histocompatibility complex and mate choice in the polygynous primate: the Sichuan snub-nosed monkey (Rhinopithecus roxellana)

The highly polymorphic genes within the major histocompatibility complex (MHC) not only play a major role in immunity resistance, but also seem to provide hints for mate choice in some animal populations. In the present study we investigated MHC-related mate choice in a small natural population (group size 40-55 individuals) of a polygynous primate, the Sichuan snub-nosed monkey (Rhinopithecus roxellana). We found that there was no evidence either for MHC-disassortative mating, or for females to mate with males based on MHC heterozygosity or specific alleles. Nevertheless, of the 11 alleles identified, we found that the frequencies of 2 alleles, Rhro-DRB2 (P < 0.01) and Rhro-DRB5 (P < 0.05) were higher in offspring than in their parents. These findings suggest that MHC-DRB in this population of R. roxellana is unlikely to be associated with mating preferences. Limited female opportunities for mate choice are likely due, in part, to the harem breeding structure present in R. roxellana, and the relatively small number of resident adult males in our study band (N = 4-6). In addition, we suggest that differences in the frequency of particular alleles across generations may be linked to parasite resistance in a fluctuating environment; however, confirmation of this finding requires further study.

Preen secretions encode information on MHC similarity in certain sex-dyads in a monogamous seabird

Animals are known to select mates to maximize the genetic diversity of their offspring in order to achieve immunity against a broader range of pathogens. Although several bird species preferentially mate with partners that are dissimilar at the major histocompatibility complex (MHC), it remains unknown whether they can use olfactory cues to assess MHC similarity with potential partners. Here we combined gas chromatography data with genetic similarity indices based on MHC to test whether similarity in preen secretion chemicals correlated with MHC relatedness in the black-legged kittiwake (Rissa tridactyla), a species that preferentially mates with genetically dissimilar partners. We found that similarity in preen secretion chemicals was positively correlated with MHC relatedness in male-male and male-female dyads. This study provides the first evidence that preen secretion chemicals can encode information on MHC relatedness and suggests that odor-based mechanisms of MHC-related mate choice may occur in birds.