Effects of Paternal Predation Risk and Rearing Environment on Maternal Investment and Development of Defensive Responses in the Offspring

Compensatory enhancement of paternal care in maternally neglected mice family

Parental care strategies, ranging from biparental to uniparental, evolve based on factors affecting sexual conflict over care. Plasticity in how parents respond to reduction in each other's care effort is thus proposed to be important in the evolution of parental care behaviors. Models predict that 'obligate' biparental care is stable when a parent responds to reduced partner effort with 'partial' compensation, trading-off current and future reproduction. A meta-analysis of experimental studies on biparental birds also revealed partial compensation, supporting coevolution of parental care type and plasticity pattern. However, few studies have addressed this issue across different taxa and different parental care types. In laboratory mice, a female-biased 'facultative' biparental species, fathers paired with a competent mother rarely provide care. We show that, when mated with a pup-neglecting mutant mother, fathers increased care effort to 'fully' compensate for the lost maternal care in both pup survival rate and total care amount. Pup retrieval latency was significantly shorter, and neural activity in relevant brain regions twice as high, suggesting enhanced motivation. This study with mice not only opens a road to explore the neural correlates of paternal plasticity but will also help understand how behavioral plasticity contributes to adaptive evolution of parental care behaviors.

A comparative study of Western, high-carbohydrate, and standard lab diet consumption throughout adolescence on metabolic and anxiety-related outcomes in young adult male and female Long-Evans rats

Anxiety and obesity are prevalent health concerns that are affected by diet in rodents and humans. How diet influences the development and maintenance of anxiety and obesity has been challenging to characterize, in part, due to methodological differences in chosen experimental and control diets. Within the same experiment, anxiety- and obesity-related effects were characterized in rats fed a Western diet (WD) relative to two control diets. Sixty Long-Evans rats split equally by sex were given standard diet (SD), control (i.e., high-carbohydrate) diet (HCD), or WD from weaning until sacrifice in early adulthood. Anxiety-related behavior was characterized in a modified open field test (mOFT) that allowed for the measurement of defensive behaviors (e.g., hiding within a refuge area), in addition to traditional OF measures (e.g., time in center). Both anxiety-related behaviors and hippocampal CA3 BDNF revealed specific sex differences. Neither adolescent weight gain of male and female rats, nor total body weight in early adulthood, were dependent on administration of HCD or WD, although the WD group consumed the most calories. In males only, administration of either WD or HCD resulted in elevated leptin levels relative to administration of the SD. Results indicate that SDs and HCDs are two distinct types of control diets that can affect comparability of studies and that using an SD might reveal more subtle metabolic changes. Control diet choice should be strongly considered during study design and interpretation, depending on specific research goals. Such studies should include both males and females as these effects are sex-specific.

Effects of paternal high-fat diet and maternal rearing environment on the gut microbiota and behavior

Exposing a male rat to an obesogenic high-fat diet (HFD) influences attractiveness to potential female mates, the subsequent interaction of female mates with infant offspring, and the development of stress-related behavioral and neural responses in offspring. To examine the stomach and fecal microbiome's potential roles, fecal samples from 44 offspring and stomach samples from offspring and their fathers were collected and bacterial community composition was studied by 16 small subunit ribosomal RNA (16S rRNA) gene sequencing. Paternal diet (control, high-fat), maternal housing conditions (standard or semi-naturalistic housing), and maternal care (quality of nursing and other maternal behaviors) affected the within-subjects alpha-diversity of the offspring stomach and fecal microbiomes. We provide evidence from beta-diversity analyses that paternal diet and maternal behavior induced community-wide shifts to the adult offspring gut microbiome. Additionally, we show that paternal HFD significantly altered the adult offspring Firmicutes to Bacteroidetes ratio, an indicator of obesogenic potential in the gut microbiome. Additional machine-learning analyses indicated that microbial species driving these differences converged on Bifidobacterium pseudolongum. These results suggest that differences in early-life care induced by paternal diet and maternal care significantly influence the microbiota composition of offspring through the microbiota-gut-brain axis, having implications for adult stress reactivity.

Deciphering the RNA universe in sperm in its role as a vertical information carrier

The inheritance of neurophysiologic and neuropsychologic complex diseases can only partly be explained by the Mendelian concept of genetic inheritance. Previous research showed that both psychological disorders like post-traumatic stress disorder and metabolic diseases are more prevalent in the progeny of affected parents. This could suggest an epigenetic mode of transmission. Human studies give first insight into the scope of intergenerational influence of stressors but are limited in exploring the underlying mechanisms. Animal models have elucidated the mechanistic underpinnings of epigenetic transmission. In this review, we summarize progress on the mechanisms of paternal intergenerational transmission by means of sperm RNA in mouse models. We discuss relevant details for the modelling of RNA-mediated transmission, point towards currently unanswered questions and propose experimental considerations for tackling these questions.

Mapping the past, present and future research landscape of paternal effects

BACKGROUND

Although in all sexually reproducing organisms an individual has a mother and a father, non-genetic inheritance has been predominantly studied in mothers. Paternal effects have been far less frequently studied, until recently. In the last 5 years, research on environmentally induced paternal effects has grown rapidly in the number of publications and diversity of topics. Here, we provide an overview of this field using synthesis of evidence (systematic map) and influence (bibliometric analyses).

RESULTS

We find that motivations for studies into paternal effects are diverse. For example, from the ecological and evolutionary perspective, paternal effects are of interest as facilitators of response to environmental change and mediators of extended heredity. Medical researchers track how paternal pre-fertilization exposures to factors, such as diet or trauma, influence offspring health. Toxicologists look at the effects of toxins. We compare how these three research guilds design experiments in relation to objects of their studies: fathers, mothers and offspring. We highlight examples of research gaps, which, in turn, lead to future avenues of research.

CONCLUSIONS

The literature on paternal effects is large and disparate. Our study helps in fostering connections between areas of knowledge that develop in parallel, but which could benefit from the lateral transfer of concepts and methods.

Intergenerational effects of paternal predator cue exposure on behavior, stress reactivity, and neural gene expression

Predation threat impacts prey behavior, physiology, and fitness. Stress-mediated alterations to the paternal epigenome can be transmitted to offspring via the germline, conferring a potential advantage to offspring in predator-rich environments. While intergenerational epigenetic transmission of paternal experience has been demonstrated in mammals, how paternal predator exposure might alter offspring phenotypes across development is unstudied. We exposed male mice to a predator odor (2,4,5-trimethylthiazoline, TMT) or a neutral odor (banana extract) prior to mating and measured offspring behavioral phenotypes throughout development, together with adult stress reactivity and candidate gene expression in the prefrontal cortex, hippocampus, amygdala, and hypothalamus. We predicted that offspring of TMT-exposed males would be less active, would display elevated anxiety-like behaviors, and would have a more efficient stress response relative to controls, phenotypes that should enhance predator avoidance in a high predation risk environment. Unexpectedly, we found that offspring of TMT-exposed males are more active, exhibit less anxiety-like behavior, and have decreased baseline plasma corticosterone relative to controls. Effects of paternal treatment on neural gene expression were limited to the prefrontal cortex, with increased mineralocorticoid receptor expression and a trend towards increased Bdnf expression in offspring of TMT-exposed males. These results suggest that fathers exposed to predation threat produce offspring that are buffered against non-acute stressors and, potentially, better adapted to a predator-dense environment because they avoid trade-offs between predator avoidance and foraging and reproduction. This study provides evidence that ecologically relevant paternal experience can be transmitted through the germline, and can impact offspring phenotypes throughout development.

Non-genetic inheritance via the male germline in mammals

Numerous studies in humans and in animal models have demonstrated that exposure to adverse environmental conditions in early life results in long-term structural and functional changes in an organism, increasing the risk of cardiometabolic, neurobehavioural and reproductive disorders in later life. Such effects are not limited to the first generation offspring but may be transmitted to a second or a number of subsequent generations, through non-genomic mechanisms. While the transmission of ‘programmed’ effects through the maternal line could occur as a consequence of multiple influences, for example, altered maternal physiology, the inheritance of effects through the male line is more difficult to explain and there is much interest in a potential role for transgenerational epigenetic inheritance. In this review, we will discuss the mechanisms by which induced effects may be transmitted through the paternal lineage, with a particular focus on the role of epigenetic inheritance.

This article is part of the theme issue ‘Developing differences: early-life effects and evolutionary medicine’.

Environmental influences on placental programming and offspring outcomes following maternal immune activation

Adverse experiences during pregnancy induce placental programming, affecting the fetus and its developmental trajectory. However, the influence of 'positive' maternal experiences on the placenta and fetus remain unclear. In animal models of early life stress, environmental enrichment (EE) has ameliorated and even prevented associated impairments in brain and behavior. Here, using a maternal immune activation (MIA) model in rats, we test whether EE attenuates maternal, placental and/or fetal responses to an inflammatory challenge, thereby offering a mechanism by which fetal programming may be prevented. Moreover, we evaluate life-long EE exposure on offspring development and examine a constellation of genes and epigenetic writers that may protect against MIA challenges. In our model, maternal plasma corticosterone and interleukin-1β were elevated 3 h after MIA, validating the maternal inflammatory response. Evidence for developmental programming was demonstrated by a simultaneous decrease in the placental enzymes Hsd11b2 and Hsd11b2/Hsd11b1, suggesting disturbances in glucocorticoid metabolism. Reductions of Hsd11b2 in response to challenge is thought to result in excess glucocorticoid exposure to the fetus and altered glucocorticoid receptor expression, increasing susceptibility to behavioral impairments later in life. The placental, but not maternal, glucocorticoid implications of MIA were attenuated by EE. There were also sustained changes in epigenetic writers in both placenta and fetal brain as a consequence of environmental experience and sex. Following MIA, both male and female juvenile animals were impaired in social discrimination ability. Life-long EE mitigated these impairments, in addition to the sex specific MIA associated disruptions in central Fkbp5 and Oprm1. These data provide the first evidence that EE protects placental functioning during stressor exposure, underscoring the importance of addressing maternal health and well-being throughout pregnancy. Future work must evaluate critical periods of EE use to determine if postnatal EE experience is necessary, or if prenatal exposure alone is sufficient to confer protection.

Parental pre-conception stress status and risk for anxiety in rat offspring: specific and sex-dependent maternal and paternal effects

Prenatal stressful events have long-lasting consequences on behavioral responses of offspring. While the effects of gestational and maternal stress have been extensively studied on psychological alterations in the progeny, little is known about effects of each parent's pre-conception life events on emotional responses in offspring. Here, the effect of maternal and/or paternal pre-conception stress was investigated on anxiogenic responses of offspring. Male and female adult rats were subjected to predatory stress (contactless exposure to a cat for 1 + 1 h per day) for 50 (male, n: 12) and 15 (female, n: 24) consecutive days; controls were not exposed. After the stress procedure, the control and stressed rats were mated to create four types of breeding pairs: control female/control male, stressed female/control male, control female/stressed male, and stressed female/stressed male. On postnatal days 30-31, the offspring were tested on the elevated plus maze and plasma corticosterone concentration was measured. Half of the pups were exposed to acute predatory stress before the elevated plus maze test. In most subgroups, corticosterone and anxiety-like behaviors in the offspring with both or only one parent exposed to pre-gestational stress increased compared to their control counterparts. However, under acute stress conditions, a different sex-dependent pattern of anxiety responses emerged. The combined effects of maternal and paternal stress were not additive. Hence, individual offspring behaviors can be influenced by the former life stress experiences of either parent. Incorporation of genetic and epigenetic aspects in development of neurobehavioral abnormalities and reprograming of the hypothalamic-pituitary-adrenal axis may contribute to this phenomenon. Lay summary Early life stress (including during pregnancy) is known to have long-lasting effects on offspring, including emotional behaviors. Whether individual anxiety behaviors can be influenced by stress experiences of each parent even before a pregnancy is less well-understood. Our findings from this study on rats exposed to predator stress before mating suggest that maternal or paternal adult life events prior to pregnancy can lead to maladaptive behavior in their offspring later in life.

Resilience priming: Translational models for understanding resiliency and adaptation to early life adversity

Despite the increasing attention to early life adversity and its long-term consequences on health, behavior, and the etiology of neurodevelopmental disorders, our understanding of the adaptations and interventions that promote resiliency and rescue against such insults are underexplored. Specifically, investigations of the perinatal period often focus on negative events/outcomes. In contrast, positive experiences (i.e. enrichment/parental care//healthy nutrition) favorably influence development of the nervous and endocrine systems. Moreover, some stressors result in adaptations and demonstrations of later-life resiliency. This review explores the underlying mechanisms of neuroplasticity that follow some of these early life experiences and translates them into ideas for interventions in pediatric settings. The emerging role of the gut microbiome in mediating stress susceptibility is also discussed. Since many negative outcomes of early experiences are known, it is time to identify mechanisms and mediators that promote resiliency against them. These range from enrichment, quality parental care, dietary interventions and those that target the gut microbiota.

Effects of paternal high-fat diet and rearing environment on maternal investment and development of defensive responses in the offspring

Paternal preconception risk factors (e.g. stress, diet, drug use) correlate with metabolic dysfunction in offspring, which is often comorbid with depressive and anxiety-like phenotypes. Detection of these risk factors or deleterious phenotypes informs a female about prevailing ecological demands, in addition to potential adverse environment-induced phenotypes that may be disseminated to her offspring. We examined whether a F₀ male rat's prior exposure to an obesogenic high-fat diet (HFD) influences a female's attraction towards a male, subsequent mother-infant interactions and the development of defensive (emotional) responses in the F₁ offspring. Females displayed less interest in the HFD exposed F₀ males relative to control diet-exposed F₀ males. Dams that reared F₁ offspring in larger, semi-naturalistic housing provided more licking and grooming and active arched-back-nursing behavior. However, some of these effects interacted with paternal experience. F₀ HFD and maternal rearing environment revealed sex-dependent, between group differences in F₁ offspring wean weight, juvenile social interactions and anxiety-like behavior in adolescence. Our results show for the first time in mammals that male exposure to HFD may contribute to stable behavioral variation among females in courtship, maternal care, even when the females are not directly exposed to a HFD, and anxiety-like behavior in F₁ offspring. Furthermore, when offspring were exposed to a predatory threat, hypothalamic Crf gene regulation was influenced by early housing. These results, together with our previous findings, suggest that paternal experience and maternal rearing conditions can influence maternal behavior and development of defensive responses of offspring.

Biostimulation and reproductive performance of artificially inseminated rabbit does (Oryctolagus cuniculus)

Biostimulation is a non-hormonal and practical technique that has not yet been widely utilised when applied immediately before insemination to improve reproductive efficiency in livestock species. This study was conducted to determine the influence of short-term male biostimulation on behavioural and reproductive performance of inseminated rabbit does. A total of 142 female New Zealand White rabbits were randomly assigned to 3 groups. Females were either exposed to male odour (Odour group) or an adult aproned male (Male group), while the remaining does that were neither exposed to the male odour nor the adult male are considered the control group. All females were inseminated after the 2 h exposure session. Conception rates were determined by abdominal palpation 12 d after insemination. The results showed that conception rate of the male odour group (79.59%) was greater than that of male presence group (76.09%) and that of the control group (68.09%). Moreover, biostimulated does showed significant behavioural activities during the 2 h exposure session compared to the control group. Although no significant differences were recognised, litter size at birth and at weaning was slightly increased in biostimulated compared to control females. Nor were there any significant difference in serum oestradiol concentrations between treated groups. Conclusively, short-term 2 h biostimulation of rabbit does resulted in the appearance of various behavioural responses followed by differences in conception rates between groups after routine artificial insemination.