Post-natal imprinting: evidence from marsupials

Abstract

Genomic imprinting has been identified in therian (eutherian and marsupial) mammals but not in prototherian (monotreme) mammals. Imprinting has an important role in optimising pre-natal nutrition and growth, and most imprinted genes are expressed and imprinted in the placenta and developing fetus. In marsupials, however, the placental attachment is short-lived, and most growth and development occurs post-natally, supported by a changing milk composition tailor-made for each stage of development. Therefore there is a much greater demand on marsupial females during post-natal lactation than during pre-natal placentation, so there may be greater selection for genomic imprinting in the mammary gland than in the short-lived placenta. Recent studies in the tammar wallaby confirm the presence of genomic imprinting in nutrient-regulatory genes in the adult mammary gland. This suggests that imprinting may influence infant post-natal growth via the mammary gland as it does pre-natally via the placenta. Similarly, an increasing number of imprinted genes have been implicated in regulating feeding and nurturing behaviour in both the adult and the developing neonate/offspring in mice. Together these studies provide evidence that genomic imprinting is critical for regulating growth and subsequently the survival of offspring not only pre-natally but also post-natally.

Figure 1

Relative phases of maternal investment from conception to weaning. The percentage of time spent during gestation, lactation (milk-only period) and mixed feeding (milk and solids) for elephants, macaques, pigs, rats (Langer, 2008) and tammar wallabies. There are four classes by which eutherians are grouped based on the characteristics of the young or litter at birth. Elephants represent group 4 and are described as precocial (open eyes and haired) and nidifugous (leaves the nest shortly after hatching or birth). Macaques represent group 3 and are described as precocial and transported (young are supported or carried). Pigs represent group 2 and are described as precocial and nidicolous (dependent on parent for feeding, care and protection). Rats represent group 1 and are described as altricial (closed eyes and naked) and nidicolous. Tammars are classified as altricial, nidicolous and transported, spending >270–300 days of the 350 days of lactation in the pouch totally dependent on milk. Also see Table 2 for gestation length in days.

Figure 2

Comparative IGF2 gene structure. Schematic of human, mouse, tammar and opossum IGF2 (not to scale). Mouse has four promoters and three DMRs, while human has five promoters. There is no mouse homologue for human P1 (HuP1). The P0 promoters and non-coding exons are homologous to each other as are mouse P1–P3 to human P1–P3. Tammar has three promoters homologous to mouse and human P1–P3 and a putative DMR (pDMR) homologous to mouse and human DMR2. The opossum has one promoter and one non-coding exon and a putative DMR located at the transcription start site. The coding region of mammalian IGF2 is located in the last three exons (black boxes). Transcription start sites are indicated with turned arrows. Homologous non-coding exons are represented by coloured boxes: P0 (orange), P1 (blue), P2 (Red) and P3 (green); white boxes: non-homologous non-coding exons. A full color version of this figure is available at the Heredity journal online.

Figure 3

Milk composition and pouch young growth. Changes in protein, carbohydrate and fat content of milk (redrawn from Green, 1984). There are four stages of lactation in the tammar (Tyndale-Biscoe and Renfree, 1987). Phase 1 encompasses the initiation of lactogenesis in late gestation. Phase 2A spans the first 100–125 days of pouch life when the young is permanently attached to the teat, followed by Phase 2B to day 200 postpartum, when the young can relinquish the teat and sucking becomes more intermittent. Phases 2A and 2B are characterised by milk that is high in carbohydrate and low in fat. Phase 3 of lactation (days 200–350) includes the period of rapid growth of the young when it begins to exit the pouch and starts to eat grass, up until weaning. This phase is characterised by low-carbohydrate, high-fat milk. Growth curve data provided by Renfree and Shaw (unpublished data). A full color version of this figure is available at the Heredity journal online.

Figure 4

Schematic of predicted tammar TH and INS genes and the TH-INS and INS transcripts (not to scale). Predicted coding exons (grey), verified coding exons (black) and non-coding exons (white) are represented by boxes. Transcription start sites identified are indicated by turned arrows. The putative DMR is shown with individual bisulphite sequences underneath: open and closed circles are unmethylated and methylated CpGs, respectively. Each row represents the methylation pattern on a separate DNA fragment from the same sample. Both methylated and unmethylated alleles were present in the liver and mammary gland tissues at the TH-INS transcription start site. TH-INS and INS chromatogram traces (viewed in FinchTV version 5.1) for genomic DNA (gDNA) and complementary DNA (cDNA) derived from the pouch young liver and the adult mammary gland. The single-nucleotide polymorphism identified in the gDNA was used to determine monoallelic expression in the liver and mammary gland.

Figure 5

Maternal–infant co-adaptation. (a) Pre-natal and post-natal function of paternally expressed (maternally imprinted) genes in the hypothalamus and placenta in eutherians (adapted from Keverne and Curley (2008) and predicted functions (b) in the hypothalamus, placenta and mammary gland of marsupials.