Parent-progeny recognition as a function of MHC odortype identity

No evidence for a role of MHC class II genotype in the chemical encoding of heterozygosity and relatedness in Antarctic fur seals

Despite decades of research, surprisingly little is known about the mechanism(s) by which an individual's genotype is encoded in odour. Many studies have focused on the role of the major histocompatibility complex (MHC) owing to its importance for survival and mate choice. However, the salience of MHC-mediated odours compared to chemicals influenced by the rest of the genome remains unclear, especially in wild populations where it is challenging to quantify and control for the effects of the genomic background. We addressed this issue in Antarctic fur seals by analysing skin swabs together with full-length MHC DQB II exon 2 sequences and data from 41 genome-wide distributed microsatellites. We did not find any effects of MHC relatedness on chemical similarity and there was also no relationship between MHC heterozygosity and chemical diversity. However, multilocus heterozygosity showed a significant positive association with chemical diversity, even after controlling for MHC heterozygosity. Our results appear to rule out a dominant role of the MHC in the chemical encoding of genetic information in a wild vertebrate population and highlight the need for genome-wide approaches to elucidate the mechanism(s) and specific genes underlying genotype-odour associations.

Lymphocytic Choriomeningitis Virus Alters the Expression of Male Mouse Scent Proteins

Mature male mice produce a particularly high concentration of major urinary proteins (MUPs) in their scent marks that provide identity and status information to conspecifics. Darcin (MUP20) is inherently attractive to females and, by inducing rapid associative learning, leads to specific attraction to the individual male’s odour and location. Other polymorphic central MUPs, produced at much higher abundance, bind volatile ligands that are slowly released from a male’s scent marks, forming the male’s individual odour that females learn. Here, we show that infection of C57BL/6 males with LCMV WE variants (v2.2 or v54) alters MUP expression according to a male’s infection status and ability to clear the virus. MUP output is substantially reduced during acute adult infection with LCMV WE v2.2 and when males are persistently infected with LCMV WE v2.2 or v54. Infection differentially alters expression of darcin and, particularly, suppresses expression of a male’s central MUP signature. However, following clearance of acute v2.2 infection through a robust virus-specific CD8 cytotoxic T cell response that leads to immunity to the virus, males regain their normal mature male MUP pattern and exhibit enhanced MUP output by 30 days post-infection relative to uninfected controls. We discuss the likely impact of these changes in male MUP signals on female attraction and mate selection. As LCMV infection during pregnancy can substantially reduce embryo survival and lead to lifelong infection in surviving offspring, we speculate that females use LCMV-induced changes in MUP expression both to avoid direct infection from a male and to select mates able to develop immunity to local variants that will be inherited by their offspring.

Patterns of MHC-dependent sexual selection in a free-living population of sheep

The MHC is one of the most polymorphic gene clusters in vertebrates and play an essential role in adaptive immunity. Apart from pathogen-mediated selection, sexual selection can also contribute to the maintenance of MHC diversity. MHC-dependent sexual selection could occur via several mechanisms but at present there is no consensus as to which of these mechanisms are involved and their importance. Previous studies have often suffered from limited genetic and behavioural data and small sample size, and were rarely able to examine all the mechanisms together, determine whether signatures of MHC-based non-random mating are independent of genomic effects or differentiate whether MHC-dependent sexual selection takes place at the pre- or post-copulatory stage. In this study, we use Monte Carlo simulation to investigate evidence for non-random MHC-dependent mating patterns by all three mechanisms in a free-living population of Soay sheep. Using 1710 sheep diplotyped at the MHC class IIa region and genome-wide SNPs, together with field observations of consorts, we found sexual selection against a particular haplotype in males at the pre-copulatory stage and sexual selection against female MHC heterozygosity during the rut. We also found MHC-dependent disassortative mating at the post-copulatory stage, along with strong evidence of inbreeding avoidance at both stages. However, results from generalized linear mixed models suggest that the pattern of MHC-dependent disassortative mating could be a by-product of inbreeding avoidance. Our results therefore suggest that while multiple apparent mechanisms of non-random mating with respect to the MHC may occur, some of them have alternative explanations.

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.

Olfactory discrimination between litter mates by mothers and alien adult cats: lump or split?

Mother cats can discriminate between their own and alien kittens using kittens' body odour. Here we ask whether they can also distinguish between body odours of kittens from the same litter. We conducted three experiments using the habituation-dishabituation technique with the odour of 1- and 7-week-old kittens of both sexes. In Experiment 1, we found no evidence that mothers discriminated among their own kittens of either age when presented three times with the odour of one individual (habituation trials) and then with the odour of a different individual (dishabituation or discrimination trial), even when the donor kittens were of different sex. In Experiment 2, alien adults of both sexes distinguished between 7 but not between 1-week-old litter mates. In Experiment 3, mothers distinguished between unknown litter mates in a similar and age-dependent manner to the animals of Experiment 2. We conclude that litter mates possess individual odour signatures that can be discriminated by adult cats, that these cues take some time to develop, but are not discriminated by their own mother, at least not during the pre-weaning period. Mothers possibly perceive and respond to a learned "nest"/litter odour shared by all litter mates or categorize the individual odours of their kittens as belonging to an "own kitten" category. That mothers did not discriminate between the odours of their own kittens but did so between individual kittens of alien litters suggests that different levels of processing olfactory information exist in mothers' ability to cognitively partition and differentially respond to such odours.

Cross-fostering: Elucidating the effects of gene×environment interactions on phenotypic development

Cross-fostering of litters from soon after birth until weaning is a valuable tool to study the ways in which gene×environment interactions program the development of neural, physiological and behavioral characteristics of mammalian species. In laboratory mice and rats, the primary focus of this review, cross-fostering of litters between mothers of different strains or treatment groups (intraspecific) or between mothers of different species (interspecific) has been conducted over the past 9 decades. Areas of particular interest have included maternal effects on emotionality, social preferences, responses to stressful stimulation, nutrition and growth, blood pressure regulation, and epigenetic effects on brain development and behavior. Results from these areas of research highlight the critical role of the postnatal maternal environment in programming the development of offspring phenotypic characteristics. In addition, experimental paradigms that have included cross-fostering have permitted investigators to tease apart prenatal versus postnatal effects of various treatments on offspring development and behavior.

The Odour of Sex: Sex-Related Differences in Volatile Compound Composition among Barn Swallow Eggs Carrying Embryos of Either Sex

Avian communication has been traditionally believed to be mainly mediated by visual and auditory channels. However, an increasing number of studies are disclosing the role of olfaction in the interaction of birds with their social environment and with other species, as well as in other behaviors such as nest recognition, food location and navigation. Olfaction has also been suggested to play a role in parent-offspring communication not only in the post- but also in the pre-hatching period. Volatile compounds produced during embryogenesis and passively released through the eggshell pores may indeed represent the only cue at parents' disposal to assess offspring quality, including the sex composition of their clutch before hatching. In turn, sex identification before hatching may mediate adaptive strategies of allocation to either sex. In the present study, we analyzed odour composition of barn swallow eggs incubated in their nest in order to identify any sex-related differences in volatile compounds emitted. For the first time in any bird species, we also investigated whether odour composition is associated with relatedness. The evidence of differences in odour composition among eggs containing embryos of either sex indicates that parents have a cue to identify their brood sex composition even before hatching which can be used to modulate their behavior accordingly. Moreover, odour similarity within nests may represent the prerequisite for kin recognition in this species.

Major Histocompatibility Complex Alleles, Sexual Responsivity, and Unfaithfulness in Romantic Couples

Preferences for mates that possess genes dissimilar to one's own at the major histocompatibility complex (MHC), a polymorphic group of loci associated with the immune system, have been found in mice, birds, fish, and humans. These preferences may help individuals choose genetically compatible mates and may adaptively function to prevent inbreeding or to increase heterozy-gosity and thereby immunocompetence of offspring. MHC-dissimilar mate preferences may influence the psychology of sexual attraction. We investigated whether MHC similarity among romantically involved couples (N = 48) predicted aspects of their sexual relationship. All women in our sample normally ovulated, and alleles at three MHC loci were typed for each person. As the proportion of MHC alleles couples shared increased, women's sexual responsivity to their partners decreased, their number of extrapair sexual partners increased, and their attraction to men other than their primary partners increased, particularly during the fertile phase of their cycles.

Sex-specific offspring discrimination reflects respective risks and costs of misdirected care in a poison frog

The ability to differentiate between one's own and foreign offspring ensures the exclusive allocation of costly parental care to only related progeny. The selective pressure to evolve offspring discrimination strategies is largely shaped by the likelihood and costs of offspring confusion. We hypothesize that males and females with different reproductive and spatial behaviours face different risks of confusing their own with others' offspring, and this should favour differential offspring discrimination strategies in the two sexes. In the brilliant-thighed poison frog, Allobates femoralis, males and females are highly polygamous, terrestrial clutches are laid in male territories and females abandon the clutch after oviposition. We investigated whether males and females differentiate between their own offspring and unrelated young, whether they use direct or indirect cues and whether the concurrent presence of their own clutch is essential to elicit parental behaviours. Males transported tadpoles regardless of location or parentage, but to a lesser extent in the absence of their own clutch. Females discriminated between clutches based on exact location and transported tadpoles only in the presence of their own clutch. This sex-specific selectivity of males and females during parental care reflects the differences in their respective costs of offspring confusion, resulting from differences in their spatial and reproductive behaviours.

Cross-Fostering of Male Mice Subtly Affects Female Olfactory Preferences

The maternal environment has been shown to influence female olfactory preferences through early chemosensory experience. However, little is known about the influence of the maternal environment on chemosignals. In this study, we used two inbred mouse strains, C57BL/6 (C57) and BALB/c (BALB), and explored whether adoption could alter male chemosignals and thus influence female olfactory preferences. In Experiment 1, C57 pups were placed with BALB dams. Adult BALB females then served as the subjects in binary choice tests between paired male urine odours (BALB vs. C57, BALB vs. adopted C57 and C57 vs. adopted C57). In Experiment 2, BALB pups were placed with C57 dams, and C57 females served as the subjects in binary choice tests between paired male urine odours (C57 vs. BALB, C57 vs. adopted BALB, and BALB vs. adopted BALB). In both experiments, we found that females preferred the urine of males from different genetic backgrounds, suggesting that female olfactory preferences may be driven by genetic compatibility. Cross-fostering had subtle effects on female olfactory preferences. Although the females showed no preference between the urine odours of adopted and non-adopted males of the other strain, the BALB females preferred the urine odour of BALB males to that of adopted C57 males, whereas the C57 females showed no preference between the urine odour of C57 and adopted BALB males. Using gas chromatography-mass spectrometry (GC-MS) and stepwise discriminant analysis, we found that the ratios of volatile chemicals from urine and preputial gland secretions were altered in the fostered male mice; these changes may have resulted in the behavioural changes observed in the females. Overall, the results suggest that female mice prefer urine odours from males with different genetic backgrounds; this preference may be driven by genetic compatibility. The early maternal environment influences the chemosignals of males and thus may influence the olfactory preferences of females. Our study provides additional evidence in support of genotype-dependent maternal influences on phenotypic variability in adulthood.

Chemical olfactory signals and parenthood in mammals

This article is part of a Special Issue "Chemosignals and Reproduction". In mammalian species, odor cues emitted by the newborn are essential to establish maternal behavior at parturition and coordinate early mother-infant interactions. Offspring odors become potent attractive stimuli at parturition promoting the contact with the young to ensure that normal maternal care develops. In some species odors provide a basis for individual recognition of the offspring and highly specialized neural mechanisms for learning the infant signals have evolved. Both the main and the accessory olfactory systems are involved in the onset of maternal care, but only the former contributes to individual odor discrimination of the young. Electrophysiological and neurochemical changes occur in the main olfactory bulb leading to a coding of the olfactory signature of the familiar young. Olfactory neurogenesis could also contribute to motherhood and associated learning. Parturition and interactions with the young influence neurogenesis and some evidence indicates a functional link between olfactory neurogenesis and maternal behavior. Although a simple compound has been found which regulates anogenital licking in the rat, studies identifying the chemical nature of these odors are lacking. Neonatal body odors seem to be particularly salient to human mothers who are able to identify their infant's odors. Recent studies have revealed some neural processing of these cues confirming the importance of mother-young chemical communication in our own species.

Mammalian-specific sequences in pou3f2 contribute to maternal behavior

Various mutations have occurred during evolution among orthologs, genes in different species that diverged from a common ancestral gene by speciation. Here, we report the remarkable deterioration of a characteristic mammalian maternal behavior, pup retrieval, in nonmammalized mice, in which the transcription factor Pou3f2 was replaced with the Xenopus ortholog lacking all of the homopolymeric amino acid repeats of mammalian POU3F2. Most of the pups born to the nonmammalized mice died within days after birth, depending on the dam genotype alone. Quantitative immunohistochemical analysis revealed decreases in the rate-limiting enzymes of dopamine and serotonin synthesis in various brain structures. Similar results were obtained in knock-in mice in which all of the homopolymeric amino acid repeats of mammalian POU3F2 were removed. Pup retrieval behavior in mammals is thus strongly related to monoamine neurotransmitter levels via the acquisition of homopolymeric amino acid repeats during mammalian evolution.

Crozier's paradox revisited: maintenance of genetic recognition systems by disassortative mating

Background

Organisms are predicted to behave more favourably towards relatives, and kin-biased cooperation has been found in all domains of life from bacteria to vertebrates. Cooperation based on genetic recognition cues is paradoxical because it disproportionately benefits individuals with common phenotypes, which should erode the required cue polymorphism. Theoretical models suggest that many recognition loci likely have some secondary function that is subject to diversifying selection, keeping them variable.

Results

Here, we use individual-based simulations to investigate the hypothesis that the dual use of recognition cues to facilitate social behaviour and disassortative mating (e.g. for inbreeding avoidance) can maintain cue diversity over evolutionary time. Our model shows that when organisms mate disassortatively with respect to their recognition cues, cooperation and recognition locus diversity can persist at high values, especially when outcrossed matings produce more surviving offspring. Mating system affects cue diversity via at least four distinct mechanisms, and its effects interact with other parameters such as population structure. Also, the attrition of cue diversity is less rapid when cooperation does not require an exact cue match. Using a literature review, we show that there is abundant empirical evidence that heritable recognition cues are simultaneously used in social and sexual behaviour.

Conclusions

Our models show that mate choice is one possible resolution of the paradox of genetic kin recognition, and the literature review suggests that genetic recognition cues simultaneously inform assortative cooperation and disassortative mating in a large range of taxa. However, direct evidence is scant and there is substantial scope for future work.

Linking adult olfactory neurogenesis to social behavior

In the adult brain, new neurons are added to two brain areas: the olfactory bulb (OB) and the hippocampus. Newly-generated neurons integrate into the preexisting circuits, bringing a set of unique properties, such as increased plasticity and responsiveness to stimuli. However, the functional implications of the constant addition of these neurons remain unclear, although they are believed to be important for learning and memory. The levels of neurogenesis are regulated by a variety of environmental factors, as well as during learning, suggesting that new neurons could be important for coping with changing environmental demands. Notably, neurogenesis has been shown to be physiologically regulated in relation to reproductive behavior: neurogenesis increases in female mice upon exposure to cues of the mating partners, during pregnancy and lactation, and in male mice upon exposure to their offspring. In this scenario, and because of the key contribution of olfaction to maternal behavior, we sought to investigate the contribution of adult-generated neurons in the olfactory system to maternal behavior and offspring recognition. To do so, we selectively disrupted neurogenesis in the olfactory pathway of female mice using focal irradiation. Disruption of adult neurogenesis in the OB did not affect maternal behavior, or the ability of female mice to discriminate familiar from unfamiliar pups. However, reduction of olfactory neurogenesis resulted in abnormal social interaction of female mice, specifically with male conspecifics. Because the olfactory system is crucial for sex recognition, we suggest that the abnormal interaction with males could result from the inability to detect or discriminate male-specific odors and could therefore have implications for the recognition of potential mating partners. Here, I review the results of our study and others, and discuss their implications for our understanding of the function of adult neurogenesis.

Citronellal ingestion decreases the appeal of male mouse urinary pheromone for female mice

To determine whether ingestion of citronellal decreases the attractive power of the male mouse urinary odor, female mice were used in preference tests. A series of tests revealed that the female mice preferred voided urine odors from aged mice over those from younger adult mice. However, exogenous citronellal directly inhibited the advantage of the aged males with regard to attraction.

The smell of age: perception and discrimination of body odors of different ages

Our natural body odor goes through several stages of age-dependent changes in chemical composition as we grow older. Similar changes have been reported for several animal species and are thought to facilitate age discrimination of an individual based on body odors, alone. We sought to determine whether humans are able to discriminate between body odor of humans of different ages. Body odors were sampled from three distinct age groups: Young (20–30 years old), Middle-age (45–55), and Old-age (75–95) individuals. Perceptual ratings and age discrimination performance were assessed in 41 young participants. There were significant differences in ratings of both intensity and pleasantness, where body odors from the Old-age group were rated as less intense and less unpleasant than body odors originating from Young and Middle-age donors. Participants were able to discriminate between age categories, with body odor from Old-age donors mediating the effect also after removing variance explained by intensity differences. Similarly, participants were able to correctly assign age labels to body odors originating from Old-age donors but not to body odors originating from other age groups. This experiment suggests that, akin to other animals, humans are able to discriminate age based on body odor alone and that this effect is mediated mainly by body odors emitted by individuals of old age.

MHC signaling during social communication

The major histocompatibility complex (MHC) has been known to play a critical role in immune recognition since the 1950s. It was a surprise, then, in the 1970s when the first report appeared indicating MHC might also function in social signaling. Since this seminal discovery, MHC signaling has been found throughout vertebrates and its known functions have expanded beyond mate choice to include a suite of behaviors from kin-biased cooperation, parent-progeny recognition to pregnancy block. The widespread occurrence of MHC in social signaling has revealed conserved behavioral-genetic mechanisms that span vertebrates and includes humans. The identity of the signal's chemical constituents and the receptors responsible for the perception of the signal have remained elusive, but recent advances have enabled the identification of the key components of the behavioral circuit. In this chapter we organize recent findings from the literature and discuss them in relation to four nonmutually exclusive models wherein MHC functions as a signal of (i) individuality, (ii) relatedness, (iii) genetic compatibility and (iv) quality. We also synthesize current mechanistic studies, showing how knowledge about the molecular basis of MHC signaling can lead to elegant and informative experimental manipulations. Finally, we discuss current evidence relating to the primordial functions of the MHC, including the possibility that its role in social signaling may be ancestral to its central role in adaptive immunity.

Affiliative behavior requires juvenile, but not adult neurogenesis

The capacity to interact with conspecifics is essential for stable social networks, reproduction, and survival in mammals. In rodents, social exploration and play behavior increase during the juvenile period, suggesting that this timeframe represents an important window for socialization. However, the cellular and molecular mechanisms necessary to support this developmental process have not been elucidated. Neurogenesis during the juvenile period, like that in adults, is mainly confined to the subgranular and subventricular zones. Nevertheless, the levels of neurogenesis are significantly higher during the juvenile period, suggesting unique functions not shared with adult neurogenesis. Here we use a transgenic mouse approach that allows for ablation of neurogenesis during different developmental phases. We find that ablating neurogenesis during either juvenile or adult phases altered anxiety and memory in adult female mice, demonstrating an age-independent function of new neurons for certain behaviors. Blocking neurogenesis during the juvenile period resulted in a profound impairment in the ability of these mice to interact with other adult females or to retrieve pups, without causing gross olfactory deficits. Interestingly, ablating neurogenesis in adult females had no effect on these social behaviors. This work defines a novel role for juvenile neurogenesis in establishing brain circuits necessary for socialization, and demonstrates that juvenile and adult neurogenesis make different contributions to social competency in adult female mice. Additional work is needed to determine whether ablation of juvenile neurogenesis in the subgranular zone and/or the subventricular zone is responsible for the social abnormalities seen after global elimination of juvenile neurogenesis.

Loss of ceramide synthase 3 causes lethal skin barrier disruption

The stratum corneum as the outermost epidermal layer protects against exsiccation and infection. Both the underlying cornified envelope (CE) and the intercellular lipid matrix contribute essentially to these two main protective barriers. Epidermis-unique ceramides with ultra-long-chain acyl moities (ULC-Cers) are key components of extracellular lipid lamellae (ELL) and are bound to CE proteins, thereby contributing to the cornified lipid envelope (CLE). Here, we identified human and mouse ceramide synthase 3 (CerS3), among CerS1-6, to be exclusively required for the ULC-Cer synthesis in vitro and of mouse CerS3 in vivo. Deficiency of CerS3 in mice results in complete loss of ULC-Cers (≥C26), lack of continuous ELL and a non-functional CLE. Consequently, newborn mutant mice die shortly after birth from transepidermal water loss. Mutant skin is prone to Candida albicans infection highlighting ULC-Cers to be pivotal for both barrier functions. Persistent periderm, hyperkeratosis and deficient cornification are hallmarks of mutant skin demonstrating loss of Cers to trigger a keratinocyte maturation arrest at an embryonic pre-barrier stage.

Opposite-sex siblings decrease attraction, but not prosocial attributions, to self-resembling opposite-sex faces

Contextual cues of genetic relatedness to familiar individuals, such as cosocialization and maternal-perinatal association, modulate prosocial and inbreeding-avoidance behaviors toward specific potential siblings. These findings have been interpreted as evidence that contextual cues of kinship indirectly influence social behavior by affecting the perceived probability of genetic relatedness to familiar individuals. Here, we test a more general alternative model in which contextual cues of kinship can influence the kin-recognition system more directly, changing how the mechanisms that regulate social behavior respond to cues of kinship, even in unfamiliar individuals for whom contextual cues of kinship are absent. We show that having opposite-sex siblings influences inbreeding-relevant perceptions of facial resemblance but not prosocial perceptions. Women with brothers were less attracted to self-resembling, unfamiliar male faces than were women without brothers, and both groups found self-resemblance to be equally trustworthy for the same faces. Further analyses suggest that this effect is driven by younger, rather than older, brothers, consistent with the proposal that only younger siblings exhibit the strong kinship cue of maternal-perinatal association. Our findings provide evidence that experience with opposite-sex siblings can directly influence inbreeding-avoidance mechanisms and demonstrate a striking functional dissociation between the mechanisms that regulate inbreeding and the mechanisms that regulate prosocial behavior toward kin.

Differential endocrine responses to infant odors in common marmoset (Callithrix jacchus) fathers

Olfactory cues can exert priming effects on many mammalian species. Paternally experienced marmosets, Callithrix jacchus, exposed to direct isolated olfactory contact with their own infant's scent show rapid decreases in testosterone levels within 20 min, whereas paternally inexperienced males do not. The following study tests whether there is a differential steroid response to exposure of infant scent from dependent infants (own and novel) and independent infants (own and novel). We examined the serum levels of estradiol, estrone, testosterone, dihydrotestosterone (DHT), and combined estrogens and androgens in eight male marmosets 20 min after exposure to isolated infant scent. Testosterone and androgen levels combined were significantly lower with exposure to own infant scent than a novel infant scent when the infants were at a dependent age but not at an independent age. Estrogen levels elevated significantly in response to own infant scent when the infants were at a dependent age but not at an independent age. These results suggest that marmoset fathers are more responsive to priming cues from related infants and hormonal responses from fathers are greatest when the infant is at a dependent age.

Odour signals major histocompatibility complex genotype in an Old World monkey

The major histocompatibility complex (MHC) is an extraordinarily diverse cluster of genes that play a key role in the immune system. MHC gene products are also found in various body secretions, leading to the suggestion that MHC genotypes are linked to unique individual odourtypes that animals use to assess the suitability of other individuals as potential mates or social partners. We investigated the relationship between chemical odour profiles and genotype in a large, naturally reproducing population of mandrills, using gas chromatography-mass spectrometry and MHC genotyping. Odour profiles were not linked to the possession of particular MHC supertypes. Sex influenced some measures of odour diversity and dominance rank influenced some measures of odour diversity in males, but not in females. Odour similarity was strongly related to similarity at the MHC, and, in some cases, to pedigree relatedness. Our results suggest that odour provides both a cue of individual genetic quality and information against which the receiver can compare its own genotype to assess genetic similarity. These findings provide a potential mechanism underlying mate choice for genetic diversity and MHC similarity as well as kin selection.

Disruption of Adult Neurogenesis in the Olfactory Bulb Affects Social Interaction but not Maternal Behavior

Adult-born neurons arrive to the olfactory bulb (OB) and integrate into the existing circuit throughout life. Despite the prevalence of this phenomenon, its functional impact is still poorly understood. Recent studies point to the importance of newly generated neurons to olfactory learning and memory. Adult neurogenesis is regulated by a variety of factors, notably by instances related to reproductive behavior, such as exposure to mating partners, pregnancy and lactation, and exposure to offspring. To study the contribution of olfactory neurogenesis to maternal behavior and social recognition, here we selectively disrupted OB neurogenesis using focal irradiation of the subventricular zone in adult female mice. We show that reduction of olfactory neurogenesis results in an abnormal social interaction pattern with male, but not female, conspecifics; we suggest that this effect could result from the inability to detect or discriminate male odors and could therefore have implications for the recognition of potential mating partners. Disruption of OB neurogenesis, however, neither impaired maternal-related behaviors, nor did it affect the ability of mothers to discriminate their own progeny from others.

Alteration of mouse urinary odor by ingestion of the xenobiotic monoterpene citronellal

Body odors provide a rich source of sensory information for other animals. There is considerable evidence to suggest that short-term fluctuations in body odor can be caused by diet; however, few, if any, previous studies have demonstrated that specific compounds can directly mask or alter mouse urinary odor when ingested and thus alter another animal's behavior. To investigate whether the ingestion of citronellal, a monoterpene aldehyde that produces an intense aroma detected by both humans and mice, can alter mouse urinary odor, mice (C57BL6J) were trained in a Y maze to discriminate between the urinary odors of male donor mice that had ingested either citronellal in aqueous solution or a control solution. Trained mice could discriminate between urinary odors from the citronellal ingestion and control groups. A series of generalization tests revealed that citronellal ingestion directly altered mouse urinary odor. Moreover, trained mice that had successfully discriminated between urinary odors from donor mice of different ages failed to detect age-related changes in urine from male mice that had ingested 50 ppm of citronellal. This study is the first to show that ingestion of a xenobiotic can alter mouse urinary odor and confuse the behavioral responses of trained mice to age-related scents.

Paternal recognition of adult offspring mediated by newly generated CNS neurons

In mammals, olfaction is often used to distinguish individuals on the basis of their unique odor types (genetically programmed body odors). Parental-offspring recognition behavior is mediated, in part, by learning and processing of different odor types and is crucial for reproductive success. Maternal recognition behavior and associated brain plasticity has been well characterized, but paternal recognition behavior and brain plasticity is poorly understood. We found that paternal-adult offspring recognition behavior in mice was dependent on postnatal offspring interaction and was associated with increased neurogenesis in the paternal olfactory bulb and hippocampus. Newly generated paternal olfactory interneurons were preferentially activated by adult offspring odors. Disrupting prolactin signaling abolished increased paternal neurogenesis and adult offspring recognition. Rescuing this neurogenesis restored recognition behavior. Thus, neurogenesis in the paternal brain may be involved in offspring recognition.

Functional neuronal processing of human body odors

Body odors carry informational cues of great importance for individuals across a wide range of species, and signals hidden within the body odor cocktail are known to regulate several key behaviors in animals. For a long time, the notion that humans may be among these species has been dismissed. We now know, however, that each human has a unique odor signature that carries information related to his or her genetic makeup, as well as information about personal environmental variables, such as diet and hygiene. Although a substantial number of studies have investigated the behavioral effects of body odors, only a handful have studied central processing. Recent studies have, however, demonstrated that the human brain responds to fear signals hidden within the body odor cocktail, is able to extract kin specific signals, and processes body odors differently than other perceptually similar odors. In this chapter, we provide an overview of the current knowledge of how the human brain processes body odors and the potential importance these signals have for us in everyday life.

The source of self: genetic parasites and the origin of adaptive immunity

Stable colonization of the host by viruses (genetic parasites) can alter the systems of host identity and provide immunity against related viruses. To attain the needed stability, some viruses of prokaryotes (P1 phage) use a strategy called an addiction module. The linked protective and destructive gene functions of an addiction module insures both virus persistence but will also destroy cells that interrupt this module and thereby prevent infection by competitors. Previously, I have generalized this concept to also include persistent and lytic states of virus infection, which can be considered as a virus addiction module. Such states often involve defective viruses. In this report, I examine the origin of the adaptive immune system from the perspective of a virus addiction module. The likely role of both endogenous and exogenous retroviruses, DNA viruses, and their defective elements is considered in the origin of all the basal components of adaptive immunity (T-cell receptor, RAG-mediated gene rearrangement, clonal lymphocyte proliferation, antigen surface presentation, apoptosis, and education of immune cells). It is concluded that colonization by viruses and their defectives provides a more coherent explanation for the origin of adaptive immunity.

Change in maternal environment induced by cross-fostering alters genetic and epigenetic effects on complex traits in mice

The interaction between maternally provided environment and offspring genotype is a major determinant of offspring development and fitness in many organisms. Recent research has demonstrated that not only genetic effects, but also epigenetic effects may be subject to modifications by the maternal environment. Genomic imprinting resulting in parent-of-origin-dependent gene expression is among the best studied of epigenetic effects. However, very little is known about the degree to which genomic imprinting effects can be modulated by the maternally provided environment, which has important implications for phenotypic plasticity. In this study, we investigated this unresolved question using a cross-fostering design in which mouse pups were nursed by either their own or an unrelated mother. We scanned the entire genome to search for quantitative trait loci whose effects depend on cross-fostering and detected 10 of such loci. Of the 10 loci, 4 showed imprinting by cross-foster interactions. In most cases, the interaction effect was due to the presence of an effect in either cross-fostered or non-cross-fostered animals. Our results demonstrate that genomic imprinting effects may often be modified by the maternal environment and that such interactions can impact key fitness-related traits suggesting a greater plasticity of genomic imprinting than previously assumed.

Birth spacing in the mouse communal nest shapes adult emotional and social behavior

The interactions with the mother and with peers are among the most relevant early environmental factors shaping adult brain function and behavior. In order to investigate the role of these factors, we exploited a novel early manipulation, the Communal Nest (CN), consisting in a single nest where three mothers give birth, keep their pups and share care-giving behavior from birth to weaning. In particular, we reared CD-1 swiss mice in three different CN conditions, each one characterized by a different interval between the three deliveries (Birth Spacing) of 3, 5 or 7 days (respectively, CN+/-3, CN+/-5, CN+/-7). Length of birth spacing affected maternal behavior, CN+/-7 pups receiving the highest levels. At adulthood, mice reared in the different conditions showed differences in emotional response and social skills. In the plus maze test, short birth spacing was found to be associated with enhanced emotionality, CN+/-3 mice showing highest levels of anxiety-like responses in the plus maze compared to the other two CN groups. In the social interaction test, the strategies to achieve dominance differed among the three groups. While CN+/-3 mice appeared to have a more aggressive strategy, displaying high levels of attack behavior in the first encounter, CN+/-5 and CN+/-7 mice displayed a more affiliative strategy based on social investigation. Overall, these findings show that birth spacing shapes the early mouse social environment and, in turn, affects the development of social skills and emotional responses.

Self-referent MHC type matching in frog tadpoles

Self/non-self recognition mechanisms underlie the development, immunology and social behaviour of virtually all living organisms, from bacteria to humans. Indeed, recognition processes lie at the core of how social cooperation evolved. Much evidence suggests that the major histocompatibility complex (MHC) both facilitates nepotistic interactions and promotes inbreeding avoidance. Social discrimination based on MHC differences has been demonstrated in many vertebrates but whether the labels used in discrimination are directly associated with the MHC, rather than with other genes with which it covaries, has remained problematic. Furthermore, effects of familiarity on natural preferences have not been controlled in most previous studies. Here we show that African clawed frog (Xenopus laevis) tadpoles discriminate among familiar full siblings based on MHC haplotype differences. Subjects (N=261) from four parental crosses preferred siblings with which they shared MHC haplotypes to those with no MHC haplotypes in common. Using only full siblings in experimental tests, we controlled for genetic variation elsewhere in the genome that might influence schooling preferences. As test subjects were equally familiar with stimulus groups, we conclude that tadpole discrimination involves a self-referent genetic recognition mechanism whereby individuals compare their own MHC type with those of conspecifics.

Psychology of Human Kin Recognition: Heuristic Cues, Erroneous Inferences, and Their Implications

Humans possess explicit, rule-based, and culturally determined systems for identifying kin, but kinship inferences are also influenced implicitly by cue-based mechanisms found commonly across the animal kingdom. These mechanisms are fallible. An evolutionarily informed signal-detection analysis suggests that (a) cue-based kin recognition may sometimes be biased in favor of false-positive errors, resulting in implicit kinship inferences even in the presence of nonkin, and (b) the tendency toward this inferential error may vary predictably in response to specific developmental and contextual circumstances. This analysis has important implications for a wide variety of psychological phenomena (especially in the realms of person perception, interpersonal attraction, and prosocial behavior) and leads to the deduction of many novel hypotheses.

Scent marking behavior as an odorant communication in mice

In rodents, where chemical signals play a particularly important role in determining intraspecies interactions including social dominance and intersexual relationships, various studies have shown that behavior is sensitive to conspecific odor cues. Mice use urinary scent marks for communication with individual conspecifics in many social contexts. Urinary scent involves genetic information about individuals such as species, sex, and individual identity as well as metabolic information such as social dominance, and reproductive and health status, which are mediated by chemical proteins in scent marks including the major histocompatibility complex and the major urinary proteins. The odor of the predator which can be considered to be a threatening signal for the prey also modulate mouse behavior in which scent marking is suppressed in response to the cat odor exposure in mice. These odorant chemicals are detected and recognized through two olfactory bulbs, the role of which in detection of chemosignals with biological relevant appears to be differential, but partly overlapped. Mice deposit scent marks toward conspecifics to maintain their social relationships, and inhibit scent marking in a context where natural predator, cat odor is contained. This suppression of scent marking is long-lasting (for at least 7 days) and context-dependent, while the odorant signaling to conspecifics tends to appear frequently (over 24h but less than 7 days intervals) depending on the familiarity of each signal-recipient. It has been discussed that scent marking is a communicative behavior associated with territoriality toward conspecifics, indicating that the social signaling within species are sensitive to predator odor cues in terms of vulnerability to predation risk.