A link between thrifty phenotype and maternal care across two generations of intercrossed mice.

Maternal effects are causal influences from mother to offspring beyond genetic information, and have lifelong consequences for multiple traits. Previously, we reported that mice whose mothers did not nurse properly had low birth weight followed by rapid fat accumulation and disturbed development of some organs. That pattern resembles metabolic syndromes known collectively as the thrifty phenotype, which is believed to be an adaptation to a stressful environment which prepares offspring for reduced nutrient supply. The potential link between maternal care, stress reactivity, and the thrifty phenotype, however, has been poorly explored in the human and animal literature: only a couple of studies even mention (much less, test) these concepts under a cohesive framework. Here, we explored this link using mice of the parental inbred strains SM/J and LG/J-who differ dramatically in their maternal care-and the intercrossed generations F1 and F2. We measured individual differences in 15 phenotypes and used structural equation modeling to test our hypotheses. We found a remarkable relationship between thrifty phenotype and lower quality of maternal behaviors, including nest building, pup retrieval, grooming/licking, and nursing. To our knowledge, this is the first study to show, in any mammal, a clear connection between the natural variation in thrifty phenotype and maternal care. Both traits in the mother also had a substantial effect on survival rate in the F3 offspring. To our surprise, however, stress reactivity seemed to play no role in our models. Furthermore, the strain of maternal grandmother, but not of paternal grandmother, affected the variation of maternal care in F2 mice, and this effect was mediated by thrifty phenotype in F2. Since F1 animals were all genetically identical, this finding suggests that maternal effects pass down both maternal care and thrifty phenotype in these mice across generations via epigenetic transmission.

Milk ejection in mice LG/J x SM/J.

In mammals, milk provision is crucial to offspring survival and growth from birth to weaning. Milk deficiency early in life may cause death or changes in the progeny metabolism that later may lead to obesity and metabolic disorders. This study investigates milk ejection (ME) the first day after birth (D1) in F(2) females from the intercross of LG/J and SM/J inbred mice strains. The absence of milk in F(3) pups' stomach at D1 is directly associated with their survival (p < 0.001) and growth pattern (p < 0.001) in the early stages of life. Furthermore, late growth pattern is also affected by this lack of nutrients at D1 because pups that survive this absence, mostly males, are heavier at weaning (p < 0.001) which, after necropsy, is shown to be due to significant higher total fat deposition (p < 0.01). We performed QTL analysis for ME at D1 in these F(2) females. Maternal performance of ME revealed a complex genetic architecture which even though it contains only a single QTL (accounting for 8 % of the variation in ME), it is totally context-dependent on the genetic background. We discovered many regions involved in epistatic interactions that together with the single QTL explain 19 % of the genetic variation for this trait. Milk ejection is an important component of maternal care, and understanding the mechanisms modulating its variation, along with other maternal features, may help to disentangle the complexity that is the mother/offspring relationship.

Hypothalamic expression of Peg3 gene is associated with maternal care differences between SM/J and LG/J mouse strains.

Maternal care is essential in mammals, and variations in the environment provided by mothers may directly influence the viability of newborns and emotional behavior later in life. A previous study investigated genetic variations associated with maternal care in an intercross of LG/J and SM/J inbred mouse strains and identified two single-locus QTLs (quantitative trait loci). Here, we selected three candidate genes located within these QTLs intervals; Oxt on chromosome 2, and FosB and Peg3 on chromosome 7 and tested their association with maternal care. LG/J females showed impaired postpartum nest building and pup retrieval, a one-day delay in milk ejection, reduced exploratory activity, and higher anxiety-like behavior when compared to SM/J females. The nucleotide sequences of Oxt and FosB were similar between strains, as were their hypothalamic expression levels. Conversely, Peg3 nucleotide sequences showed four nonsynonymous replacement substitutions on LG/J dams, T11062G, G13744A, A13808G, and G13813A, and a 30 base pair (10 aa) in tandem repeat in the coding region with three copies in SM/J and five copies in LG/J. Maternal care impaired LG/J mothers express 37% lower Peg3 mRNA levels in the hypothalamus on the second postpartum day. We also found an association of the Peg3 repeat-variant and poor maternal care in F(2) heterozygote females derived from a LG/J × SM/J intercross. These results may suggest that the maternally imprinted Peg3 gene is responsible for the single-locus QTL on chromosome 7 that has been shown to influence maternal care in these strains. Furthermore, these data provide additional support for an epigenetic regulation of maternal behavior.

Quantitative trait loci for maternal performance for offspring survival in mice.

Maternal performance refers to the effect that the environment provided by mothers has on their offspring's phenotypes, such as offspring survival and growth. Variations in maternal behavior and physiology are responsible for variations in maternal performance, which in turn affects offspring survival. In our study we found females that failed to nurture their offspring and showed abnormal maternal behaviors. The genetic architecture of maternal performance for offspring survival was investigated in 241 females of an F(2) intercross of the SM/J and LG/J inbred mouse strains. Using interval-mapping methods we found two quantitative trait loci (QTL) affecting maternal performance at D2Mit17 + 6 cM and D7Mit21 + 2 cM on chromosomes 2 and 7, respectively. In a two-way genome-wide epistasis scan we found 15 epistatic interactions involving 23 QTL distributed across all chromosomes except 12, 16, and 17. These loci form several small sets of interacting QTL, suggesting a complex set of mechanisms operating to determine maternal performance for offspring survival. Taken all together and correcting for the large number of significant factors, QTL and their interactions explain almost 35% of the phenotypic variation for maternal performance for offspring survival in this cross. This study allowed the identification of many possible candidate genes, as well as the relative size of gene effects and patterns of gene action affecting maternal performance in mice. Detailed behavior observation of mothers from later generations suggests that offspring survival in the first week is related to maternal success in building nests, grooming their pups, providing milk, and/or manifesting aggressive behavior against intruders.

Genetic influences on maternal care.

The basis of social evolution in mammals is the mother-offspring relationship. It is also the primary and most important instance of indirect genetic effects, where genetic variation in one individual affects phenotypic variation among others. This relationship is so important in mammals that often the major factor determining the life or death of newborns is the environment provided by their mother. Variations in these environments can be due to variations in maternal genotypes. In our work with the intercross of two mouse inbred strains, LG/J and SM/J, we uncovered a very severe variation in maternal performance. These females failed to nurture their offspring and showed abnormal maternal behaviors leading to loss of their litter. Rather than this being due to a single gene variant as in knockout mice, we uncovered a complex genetic basis for this trait. The effects of genes on maternal performance are entirely context dependent in our cross. They depend on the alleles present at the same or other epistatically interacting loci. Genomic locations identified in this study include locations of candidate genes whose knockouts displayed similar aberrant maternal behavior. Behaviors significantly associated with maternal performance in this study include suckling, nest building, placentophagia, pup grooming, and retrieval of pups after disturbance.