Preferences of newborn mice for odours indicating closer genetic relatedness: is experience necessary?

Differential fate between oxytocin and vasopressin cells in the developing mouse brain

Oxytocin (OXT) and arginine vasopressin (AVP), two neuropeptides involved in socio-emotional behaviors have been anatomically defined in the adult brain. Yet their spatial organization during postnatal development is not clearly defined. We built a developmental atlas using 3D imaging of cleared immunolabeled tissue over four early postnatal (P) stages, from birth (P0, P3, P7, P14) to young adulthood (≥P56). Our atlas-based mapping revealed that the number of OXT neurons doubles according to unique temporal dynamics in selective hypothalamic regions, namely, the periventricular and paraventricular nuclei, and in a novel location we named the antero-lateral preoptic. In the paraventricular nucleus, single-cell densities and fluorescence analysis demonstrated selective expansion of OXT cells in the antero-ventral division, whereas the postero-dorsal division contained cells present at birth. No changes were observed for AVP neurons. Our findings show the coexisting of innate and plastic OXT/AVP brain circuits probably triggered by environmental adaptation of the social brain.

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The OXT system continues to mature during early development but not the AVP system

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OXT hypothalamic nuclei emerge at different rates after birth

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PVN cells gradually acquire an oxytocinergic phenotype

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OXT cells are organized into innate and adaptive neural networks

Newborn mice form lasting CA2-dependent memories of their mothers

Some of the most enduring social connections begin when infants first recognize their caregivers, memories that form the basis of many family relationships. It remains unknown whether these early social memories persist into adulthood in mice and, if so, which brain regions support them. Here we show that mice form memories of their mother within days after birth and that these memories persist into adulthood. Pups display greater interest in the mother than in an unfamiliar dam before weaning, after which this preference reverses. Inhibition of CA2 neurons in the pup temporarily blocks the ability to discriminate between the mother and an unfamiliar dam, whereas doing so in adulthood prevents the formation of short-term memories about conspecifics, as well as social discrimination related to long-term memories of the mother. These results suggest that the CA2 supports memories of the mother during infancy and adulthood with a developmental switch in social preference.

First experimental evidence for olfactory species discrimination in two nocturnal primate species (Microcebus lehilahytsara and M. murinus)

Olfactory communication is highly important for nocturnal mammals, especially for solitary foragers, but knowledge is still limited for nocturnal primates. Mouse lemurs (Microcebus spp.) are nocturnal solitary foragers with a dispersed lifestyle and frequently use chemo-sensory signalling behaviour for governing social interactions. Different mouse lemur species can co-occur in a given forest but it is unknown whether olfaction is involved in species recognition. We first screened 24 captive mouse lemurs (9 M. murinus, 15 M. lehilahytsara) for their olfactory learning potential in an experimental arena and then tested the species discrimination ability with urine odour in an operant conditioning paradigm in four individuals. The majority of the screened animals (75%) did not pass the screening criteria within a 2-week test period. However, all four final test animals, two M. murinus and two M. lehilahytsara, were successfully trained in a 5-step-conditioning process to reliably discriminate conspecific from heterospecific urine odour (requiring an overall median of 293 trials). Findings complement previous studies on the role of acoustic signalling and suggest that olfaction may be an important additional mechanism for species discrimination.

Measuring Preweaning Sensorial and Motor Development in the Mouse

The immaturity at birth and the slowness of ontogenic processes in mice provide the opportunity to measure rates of development. We describe here 18 measures covering the sensorial and motor onset from birth to weaning. The measures are non-invasive, making a follow-up strategy possible. The first basic protocol indicates how to produce mice with known conceptional or chronological age, as the control of the age is a prerequisite to compare rates of development in groups of mice. The second basic protocol describes a set of methods for identifying the pups during a follow-up study. A third basic protocol describes testing newborn mice for the appearance of sensorial and motor abilities in a follow-up design. Taken together, the three protocols make possible the validation of potential murine models of interest for understanding human developmental disorders. © 2018 by John Wiley & Sons, Inc.

Testing smell when it is really vital: behavioral assays of social odors in the neonatal mouse

The initial interactions of mouse newborns with their mother are crucial for their survival. These interactions rapidly end in the pups reaching nipples and getting milk. While we realize that olfaction is clearly prevailing in the success of these first suckling episodes, we still understand little about the nature and range of the natural odorants involved. Here we non-exhaustively describe some experimental principles and methods to assay the behavior of newly born and infant mice exposed to different odor stimuli from conspecifics. Testing neonatal and young mice with chemostimuli which they are evolutionarily or developmentally canalized to detect may be a productive way to trace unanticipated odor signals. Moreover, testing neonates also may also lead to characterize unsuspected strategies of murine females to produce and release odor messages.

Paternal kin recognition in the high frequency / ultrasonic range in a solitary foraging mammal

BACKGROUND

Kin selection is a driving force in the evolution of mammalian social complexity. Recognition of paternal kin using vocalizations occurs in taxa with cohesive, complex social groups. This is the first investigation of paternal kin recognition via vocalizations in a small-brained, solitary foraging mammal, the grey mouse lemur (Microcebus murinus), a frequent model for ancestral primates. We analyzed the high frequency/ultrasonic male advertisement (courtship) call and alarm call.

RESULTS

Multi-parametric analyses of the calls' acoustic parameters and discriminant function analyses showed that advertisement calls, but not alarm calls, contain patrilineal signatures. Playback experiments controlling for familiarity showed that females paid more attention to advertisement calls from unrelated males than from their fathers. Reactions to alarm calls from unrelated males and fathers did not differ.

CONCLUSIONS

1) Findings provide the first evidence of paternal kin recognition via vocalizations in a small-brained, solitarily foraging mammal. 2) High predation, small body size, and dispersed social systems may select for acoustic paternal kin recognition in the high frequency/ultrasonic ranges, thus limiting risks of inbreeding and eavesdropping by predators or conspecific competitors. 3) Paternal kin recognition via vocalizations in mammals is not dependent upon a large brain and high social complexity, but may already have been an integral part of the dispersed social networks from which more complex, kin-based sociality emerged.

How does a newly born mouse get to the nipple? Odor substrates eliciting first nipple grasping and sucking responses

It is a mammalian female strategy to emit odor cues and signals that direct their inexperienced newborns to the nipple, and optimize their initial sucking success and, hence, viability. Here, natural odorous substrates that contribute to nipple grasping were investigated in mice, a species that has not been much scrutinized on this topic. The response of pups toward the nipples of lactating females (LF) versus nonlactating females (NLF) were first assessed right after watched birth, before and after the first suckling experience, and at 1 day old, after more extended suckling experience. It appeared that only nipples of LF induced grasping at these early ages, leading to take NLF as the baseline setting to present various odor substrates sampled from LF, viz. amniotic fluid, murine milk, LF saliva, pup saliva, LF urine, and an odorless control stimulus (water). Results indicate that: (1) only amniotic fluid and fresh milk induced nipple grasping before the first suckling experience; (2) LF saliva started inducing grasping after the first suckling experience; (3) pup saliva released grasping after 24-36 hr of suckling experience; finally (4) neither LF urine, nor water induced any nipple grasping. In conclusion, the activity of amniotic fluid and murine milk on neonatal pup behavior before any postnatal suckling experience suggests that either prenatal learning and/or predisposed olfactory mechanisms do operate, while the behavioral activation due to maternal and infantile salivas clearly depends on postnatal exposure.

Concordance in mate choice in female mound-building mice

Females must evaluate male quality to perform mate choice. Since females generally base their selection on different male features, individual females may differ in their choice. In this study, we show that concordance between females in mate choice decisions may arise without any experimental maximization of a particular attractive trait. Choice tests were performed in mound-building mice, Mus spicilegus, a monogamous species. Body odours of two male donors were presented to 12 female subjects individually. To determine female choice, the same pair of males was presented three times to a female. Four different pairs of male body odours were used. Male donors, not related to females, were selected at random in our polymorphic breeding stock. Using this two-way choice design, female mice displayed a clear choice and had a similar preference for particular males.

Effects of in utero odorant exposure on neuroanatomical development of the olfactory bulb and odour preferences

Human babies and other young mammals prefer food odours and flavours of their mother's diet during pregnancy as well as their mother's individually distinctive odour. Newborn mice also prefer the individual odours of more closely related--even unfamiliar--lactating females. If exposure to in utero odorants-which include metabolites from the mother's diet and the foetus's genetically determined individual odour-helps shape the neuroanatomical development of the olfactory bulb, this could influence the perception of such biologically important odours that are preferred after birth. We exposed gene-targeted mice during gestation and nursing to odorants that activate GFP-tagged olfactory receptors (ORs) and then measured the effects on the size of tagged glomeruli in the olfactory bulb where axons from olfactory sensory neurons (OSNs) coalesce by OR type. We found significantly larger tagged glomeruli in mice exposed to these activating odorants in amniotic fluid, and later in mother's milk, as well as significant preferences for the activating odour. Larger glomeruli comprising OSNs that respond to consistently encountered odorants should enhance detection and discrimination of these subsequently preferred odours, which in nature would facilitate selection of palatable foods and kin recognition, through similarities in individual odours of relatives.

Olfactory marker protein is critical for functional maturation of olfactory sensory neurons and development of mother preference

Survival of many altricial animals critically depends on the sense of smell. Curiously, the olfactory system is rather immature at birth and undergoes a maturation process, which is poorly understood. Using patch-clamp technique on mouse olfactory sensory neurons (OSNs) with a defined odorant receptor, we demonstrate that OSNs exhibit functional maturation during the first month of postnatal life by developing faster response kinetics, higher sensitivity, and most intriguingly, higher selectivity. OSNs expressing mouse odorant receptor 23 (MOR23) are relatively broadly tuned in neonates and become selective detectors for the cognate odorant within 2 weeks. Remarkably, these changes are prevented by genetic ablation of olfactory marker protein (OMP), which is exclusively expressed in mature OSNs. Biochemical and pharmacological evidence suggests that alteration in odorant-induced phosphorylation of signaling proteins underlie some of the OMP(-/-) phenotypes. Furthermore, in a novel behavioral assay in which the mouse pups are given a choice between the biological mother and another unfamiliar lactating female, wild-type pups prefer the biological mother, while OMP knock-out pups fail to show preference. These results reveal that OSNs undergo an OMP-dependent functional maturation process that coincides with early development of the smell function, which is essential for pups to form preference for their mother.

Reinforcement selection acting on the European house mouse hybrid zone

Behavioural isolation may lead to complete speciation when partial postzygotic isolation acts in the presence of divergent-specific mate-recognition systems. These conditions exist where Mus musculus musculus and M. m. domesticus come into contact and hybridize. We studied two mate-recognition signal systems, based on urinary and salivary proteins, across a Central European portion of the mouse hybrid zone. Introgression of the genomic regions responsible for these signals: the major urinary proteins (MUPs) and androgen binding proteins (ABPs), respectively, was compared to introgression at loci assumed to be nearly neutral and those under selection against hybridization. The preference of individuals taken from across the zone regarding these signals was measured in Y mazes, and we develop a model for the analysis of the transition of such traits under reinforcement selection. The strongest assortative preferences were found in males for urine and females for ABP. Clinal analyses confirm nearly neutral introgression of an Abp locus and two loci closely linked to the Abp gene cluster, whereas two markers flanking the Mup gene region reveal unexpected introgression. Geographic change in the preference traits matches our reinforcement selection model significantly better than standard cline models. Our study confirms that behavioural barriers are important components of reproductive isolation between the house mouse subspecies.

Signalling components of the house mouse mate recognition system

Subspecies-specific mate recognition may represent significant barrier to gene flow between diverged genomes potentially leading to speciation. In the house mouse, assortative mating involves the coevolution of several signals and receptors. We compared signalling ability of bedding material, faeces, urine, saliva, salivary androgen binding proteins (ABP) and combinations of urine with saliva and urine with ABP in mate choice in two wild-derived inbred strains (one of Mus musculus musculus and one of Mus musculus domesticus origin). We observed high levels of variation in assortative preferences between the two strains and sexes. The strongest preferences were observed in M. m. musculus-derived individuals in tests where urine was present either alone or as part of a composite signal target. M. m. domesticus-derived mice displayed strain-specific preferences for faeces. Saliva was the least preferred stimulus in both strains and sexes. No effect of two-compound cues was detected. We conclude that there is divergence across both the stimulus and preference parts of the recognition system for both house mouse strains. Of the tested stimuli, those that have the capacity to carry a signal for extended periods under natural conditions (such as urine and faeces) seem to be the most important substances in strain-specific recognition.

Variation in mate preference across a house mouse hybrid zone

Reproductive character displacement is known to occur at the borders of a secondary contact zone between the two European subspecies of the house mouse (in Jutland, Denmark), where selection against hybridization occurs. This study assessed patterns of mate preference in naturally occurring hybrids of the two subspecies. Mate odour choice was investigated in male and female mice sampled across the hybrid zone. Odour samples comprised urine (from the opposite sex to the test animal) obtained from populations geographically distant from the hybrid zone. Urine is known to carry subspecies recognition signals. The behavioural results changed across the hybrid zone, and were analysed by a model of clinal variation. This behavioural cline was compared with the allozyme cline across the same hybrid zone. Males on both sides of the hybrid zone showed an assortative preference, which shifted significantly and abruptly approximately 10 km from the genetic centre of the hybrid zone on the Mus musculus musculus side. Directional preference was not detected in females, which could relate to variation in sexual receptivity. Our model indicates that the peculiar pattern of male preference could involve several genes and be characterized by mild to strong epistasis favouring the expression of M. m. domesticus-like preference over a large portion of the hybrid zone. This study may provide the first picture of the genetic determination of mate preference in a mammal.