Evolution of Viviparity in Mammals: What Genomic Imprinting Tells Us About Mammalian Placental Evolution

CSIRO PUBLISHING Reproduction, Fertility and Development, 2019, 31, 1219–1227 Review https://doi.org/10.1071/RD18127

Evolution of viviparity in mammals: what genomic imprinting tells us about mammalian placental evolution

Tomoko Kaneko-IshinoA,C and Fumitoshi Ishino B,C

ASchool of Health Sciences, Tokai University, 143 Shimokasuya, Isehara, Kanagawa 259-1193, Japan. BDepartment of Epigenetics, Medical Research Institute, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan. CCorresponding authors. Emails: [email protected]; [email protected]

Abstract. Genomic imprinting is an epigenetic mechanism of regulating parent-of-origin-specific monoallelic expression of imprinted genes in viviparous therian mammals such as eutherians and marsupials. In this review we discuss several issues concerning the relationship between mammalian viviparity and genomic imprinting, as well as the domestication of essential placental genes: why has the genomic imprinting mechanism been so widely conserved despite the evident developmental disadvantages originating from monoallelic expression? How have genomic imprinted regions been established in the course of mammalian evolution? What drove the evolution of mammalian viviparity and how have genomic imprinting and domesticated genes contributed to this process? In considering the regulatory mechanism of imprinted genes, reciprocal expression of paternally and maternally expressed genes (PEGs and MEGs respectively) and the presence of several essential imprinted genes for placental formation and maintenance, it is likely that complementary, thereby monoallelic, expression of PEGs and MEGs is an evolutionary trade-off for survival. The innovation in novel imprinted regions was associated with the emergence of imprinting control regions, suggesting that genomic imprinting arose as a genome defence mechanism against the insertion of exogenous DNA. Mammalian viviparity emerged in the period when the atmospheric oxygen concentration was the lowest (,12%) during the last 550 million years (the Phanerozoic eon), implying this low oxygen concentration was a key factor in promoting mammalian viviparity as a response to a major evolutionary pressure. Because genomic imprinting and gene domestication from retrotransposons or retroviruses are effective measures of changing genomic function in therian mammals, they are likely to play critical roles in the emergence of viviparity for longer gestation periods.

Additional keywords: complementation hypothesis, domesticated genes, evolutionary trade-off, genome defence mechanism, Peg10, Peg11/Rtl1, reciprocal monoallelic expression, retrotransposons.

Received 5 April 2018, accepted 1 October 2018, published online 10 January 2019

Why has genomic imprinting been widely conserved in discovery. For example, the prevention of parthenogenetic viviparous mammals? development may be advantageous for mammalian reproduc- This question may be put alternatively as ‘What is the merit of tion because females can avoid childbearing in seasons unfit for genomic imprinting in the therian mammals that overwhelms child rearing (Solter 1988). Another advantage is the prevention its developmental disadvantages originating from monoallelic of the development of trophoblast cells, placental cells with an expression of a series of essential imprinted genes?’ All the invasive nature, to avoid developing malignant ovarian ter- human genomic imprinting diseases, such as Silver–Russell, atocarcinomas that arise during parthenogenesis (Varmuza and Beckwith–Wiedemann, Prader–Willi, Angelman, Albright, Mann 1994). However, it is difficult to test such hypotheses by Kagami–Ogata and Temple syndromes (Kalish et al. 2014), as purely experimental procedures. To address these types of well as their related developmental, growth and behavioural issues, it is reasonable to expect that it will be necessary to at abnormalities observed in mice, are attributable to the mono- least advance our knowledge of the regulatory mechanism allelic expression of imprinted genes. This is because (partial) underlying genomic imprinting, the evolutionary relationship uniparental chromosome duplications induce a lack and/or between genomic imprinting and viviparity, the environmental overexpression of relevant imprinted genes (Cattanach and pressure that drove their evolution and the genetic factors Beechey 1990; Blake et al. 2010). Several advantages conferred that enabled these processes. Knowledge accumulated so far by genomic imprinting have been proposed since its initial warrants further discussion.

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Maternally imprinted region Methylated DMRs In the two versions of ‘the conflict hypothesis’, the weak Non-metylated DMRs version has come to be widely accepted as the concept of MEG ‘parental conflict’ (Moore and Haig 1991). This version states PEG that in viviparous organisms, such as therian mammals, under polygamous conditions in which females are able to accept more than one male, the genes from the paternal alleles tend to PEG promote embryo growth, whereas genes from the maternal MEG alleles tend to inhibit embryo growth as a consequence of the Paternally imprinted region genetic conflict between the paternal and maternal alleles. A Maternally derived Paternally derived substantial number of paternally and maternally expressed genome genome genes (PEGs and MEGs respectively) fit well with this predic- Fig. 1. Reciprocal monoallelic expression of paternally and maternally tion, such as the paternally expressed insulin-like growth factor expressed genes (PEGs and MEGs respectively). In both paternally or 2(IGF2) and maternally expressed insulin-like growth factor 2 maternally imprinted regions, PEGs and MEGs exhibit reciprocal mono- receptor (IGF2R) genes (Killian et al. 2000; Reik and Walter allelic expression. The DNA methylation status of differentially methylated 2001; Wilkins and Haig 2003; Hore et al. 2007; Renfree et al. regions (DMRs) is indicated by the black (methylated) and white (non- 2009). The former encodes the embryonic growth factor IGF2, methylated) circles. whereas IGF2R protein binds IGF2 and degrades it by transport- ing it to the lysosome. Conversely, the so-called strong version Maternally derived chromosome of the conflict hypothesis predicts that under the continuous Insulator-binding pressure exerted by genetic conflict, the genomic imprinting protein (CTCF) mechanism inevitably originates in viviparous organisms during the course of evolution (Wilkins and Haig 2003). Through a process known as evolutionary equilibrium, a locus that was MEGX initially expressed in both alleles gradually comes to be Paternally derived chromosome expressed from a single allele, with the other allele made silent by means of natural selection. This process involves the increased expression of one allele and the decreased expression DMR of the other. For example, a growth factor gene initially showing PEGZ PEGY biallelic expression is gradually changed to paternal allele- Promoters Insulator Enhancer specific expression over a long period, when fitness is determined by the level of the growth factor and natural selection Fig. 2. The differentially methylated region (DMR) acting as an imprinting regulator: an insulator model. In this case of a paternally imprinted favours higher production when an allele is paternally derived region, the paternal DMR is fully methylated whereas the maternal DMR is than when it is maternally derived. It is known that genomic not methylated. The DNA methylation status of the DMR regulates recipro- imprinting is also observed in plants, such as angiosperms, cal monoallelic expression of paternally and maternally expressed genes providing strong support for this hypothesis (Haig and Westoby (PEGs and MEGs respectively). Importantly, expression of PEGs requires 1991). However, the emergence of genomic imprinting regions DNA methylation of the DMR. CTCF, CCCTC-Binding Factor. They are in mammals is likely to arise as a genome defence mechanism model examples of one MEG and two PEG genes, numbered X, Y, Z. against the insertion of exogenous DNA, suggesting that target chromosomal regions and/or genes were at first randomly mutated. Some were then selected as imprinted regions and/or Hark et al. 2000) or transcriptional start site of antisense RNA imprinted genes as a result of developmental advantages, which (antisense of IGF2R non-protein coding RNA (AIRN) in the has also been predicted by the weak version of the conflict IGF2R region; Sleutels et al. 2002; Latos et al. 2012; Fig. 2). hypothesis. This is the mechanism by which the methylated DMR sequence In considering the regulatory mechanism of imprinted gene not only represses certain imprinted genes, but also induces expression, we have proposed ‘the complementary hypothesis’ expression of other imprinted genes over a substantially wide (Kaneko-Ishino et al. 2003, 2006; Kaneko-Ishino and Ishino imprinted region, sometimes up to 500 Mb. In the maternally 2015), in which the monoallelic expression mechanism emerged imprinted regions, DMR DNA methylation represses PEGs but as an evolutionary trade-off for survival. Usually, each induces MEGs. Conversely, DMR DNA methylation in pater- imprinted region comprises both PEGs and MEGs and their nally imprinted regions represses MEGs while inducing PEGs reciprocal expression is regulated by the DNA methylation (Figs 1, 2). Thus, it is clear that PEGs and MEGs cannot be status of each differentially methylated region (DMR; also expressed from the individual parental chromosomes simulta- known as an imprinting control region (ICR); Fig. 1). The neously; that is, two chromosomes of different DNA methyla- imprinted regions are divided into two groups, namely pater- tion status (i.e. two different epigenotypes) are necessary for the nally and maternally imprinted regions, with DNA methylation simultaneous expression of PEGs and MEGs in somatic cells. in the paternal and maternal alleles respectively. The DMR Using somatic cloning techniques, cloned embryos gener- sequences seem to function as cis-regulatory units, such as ated from the nuclei of primordial germ cells on embryonic acting as an insulator (IGF2–H19, imprinted maternal Day 12 represent the default state of genomic imprinting; that is, expressed transcript (H19) region; Bell and Felsenfeld 2000; both parental alleles lose all DMR DNA methylation and thus Evolution of viviparity and genomic imprinting Reproduction, Fertility and Development 1221

Paternal imprinting

Erasure of imprinting

Fertilisation

Imprinted state in Imprinted state in Default state in somatic cells somatic cells germ line cells (next generation)

Maternal imprinting

Fig. 3. How are paternally and maternally expressed genes (PEGs and MEGs respectively) regulated during mammalian reproductive cycle? Expression profile of imprinted genes in somatic cells (left). Erasure of imprinted memories occurred in PGCs on embryonic Day (E) 12.5 (no differentially methylated region (DMR) DNA methylation in either parental chromosome). Expression profiles of imprinted genes in E12.5 PGC cloned embryos are shown as the default state of genomic imprinting (leftmost but one). It should be noted that PEGs in paternally imprinted regions and MEGs in maternally imprinted regions are repressed in the default state. Paternal imprinting changes the expression patterns of only paternally imprinted regions (top centre), whereas maternal imprinting changes the expression only of maternally imprinted regions (bottom centre). In the next generation, the somatic cells are comprised of one of paternally and one maternally derived chromosome; thus, the normal imprinted state is recovered (right). (), methylated DMRs; (J), non-methylated DMRs. have the same status. Importantly, they exhibit early embryonic how imprinted regions emerged during evolution came from a lethality like parthenogenetic and androgenetic embryos, sup- series of comprehensive comparative studies of imprinted genes porting the concept that two different parental epigenotypes are and/or imprinted regions between eutherians and marsupials. In essential for development (Lee et al. 2002). Under this condi- marsupials, only six imprinted genes, namely IGF2, IGF2R, tion, only PEGs are expressed from the maternally imprinted paternally expressed 1 (PEG1)/mesoderm specific transcript regions and only MEGs are expressed from the paternally (MEST), paternally expressed 10 (PEG10), insulin (INS) and imprinted regions, so they lack expression of several essential H19, have thus far been identified, whereas many other ortho- imprinted genes (Fig. 3). logues of eutherian imprinted genes are not imprinted (Renfree This provides a notion of perceptual shift, because most et al. 2009, 2012). Of these imprinted genes, the PEG10 and hypotheses on genomic imprinting presuppose that PEGs and IGF2–H19 regions have conserved DMRs (PEG10 and H19 MEGs exhibit monoallelic expression under the control of DMRs) in both eutherians and marsupials, demonstrating that at genomic imprinting and are expressed from both parental alleles least these two regions existed in a common therian ancestor equally (biallelic expression) without any particular regulation. (Suzuki et al. 2007; Smits et al. 2008). These observations also As discussed later, a substantial number of PEGs and MEGs demonstrate that those DMRs were present at the beginning of play essential roles in development, growth and behaviour, the genomic imprinting history, and that several imprinted genes indicating that complementary monoallelic expression is essen- and imprinted regions were considerably increased later in the tial for survival in the current adaptive form of therian mam- eutherian lineage. In addition to their DMRs, the PEG10 and mals. Thus, it is likely that it arose as an evolutionary trade-off H19 genes first emerged in the therian genomes, because there for survival against the disadvantageous monoallelic expression are no corresponding DNA sequences present in platypus of several essential and/or important developmental genes. (monotremes) or other non-mammal vertebrates, such as the chicken (birds) and fugu (fish). The evidence further indicates How have genomic imprinting regions been established in that it was newly inserted exogenous DNA in these regions from mammals? which the PEG10 and H19 DMRs originated (Renfree et al. More than 15 imprinted regions exist in the genomes of humans 2012; Kaneko-Ishino and Ishino 2015)(Figs 4, 5). and mice, and each region has at least one DMR. Unexpectedly, The same is true for most of the DMRs in the eutherian- no sequence homology was identified among the DMRs in either specific imprinted regions (Suzuki et al. 2011; Renfree et al. the paternally or maternally imprinted regions. This has ham- 2012; Fig. 6), except for small nuclear ribonucleoprotein poly- pered understanding of the common features of the DMRs, peptide N (SNRPN) and solute carrier family 38 member 4 particularly their regulatory mechanisms. An important clue to (SLC38A4) DMRs, in which the DNA sequences pre-existed in 1222 Reproduction, Fertility and Development T. Kaneko-Ishino and F. Ishino

DNA methylation Human CpG profile (eutherians) in CpG island SGCE PEG10 PPP1R9A 20% Mouse Human (eutherians) (eutherians) Sgce Peg10 Ppp1r9a 0% Wallaby and SGCE PEG10 opossum 20% (marsupials) SGCE PEG10 PPP1R9A Mouse (eutherians) Platypus 0% (monotremes) Sgce Peg10 SGOE PPP1R9A 20% Chicken Wallaby (birds) SGCE PPP1R9A (marsupials) 0% Fugu SGCE PEG10 (fish) 20% SGCE PPP1R9A Platypus (monotremes) 0 k

50 k 0% 100 k 150 k 200 k 250 k SGCE 20% Fig. 4. Emergence of paternally expressed 10 (PEG10) in marsupials and Chicken eutherians. Comparison of genome sequences around the PEG10 region (bird) 0% among several vertebrate species. The pink arrow indicates insertion of the SGCE PEG10 original retrotransposon after the spilt of the therians and mono- 1 kb tremes. Modified from fig. 1 in Suzuki et al. (2007). Sgce/SGCE, sarcogly- can epsilon; Peg10/PEG10, paternally expressed 10; Ppp1R9A, guanine Fig. 5. Emergence of the paternally expressed 10 (PEG10) differentially nucleotide binding protein (G protein), alpha protein phosphatase 1 regula- methylated region (DMR) from a newly inserted DNA sequence in marsu- tory subunit 9A. pials and eutherians. PEG10 DMR is corresponds to a novel cytosine– phosphorous–guanine (CpG) island that appeared in eutherians and marsupials. Modified from fig. 5 in Suzuki et al. (2007). GC content of a common mammalian and therian ancestor respectively. In the SGCE-PEG10 region is shown as %. both cases, eutherian-specific genomic imprinting regulation was associated with eutherian-specific chromosomal translocation that occurred in the neighbourhood of each DMR (Rapkins genes that were inserted by cDNA retrotransposition and et al. 2006; Suzuki et al. 2011). Some of the imprinted regions, their promoter regions became their DMRs (Cowley and such as malignant T cell amplified sequence 2 (Mcts2), pater- Oakey 2010). nally expressed 13 (Peg13), Ras protein specific guanine nucle- There are no conserved DMRs in the marsupial PEG1/MEST otide releasing factor 1 (Rasgrf1), zinc finger (CCCH type), or IGF2R region (Suzuki et al. 2005; Weidman et al. 2006). RNA binding motif and serine/arginine rich 1 (Zrsrt)/U2 small It is of considerable interest to elucidate how these genes were nuclear ribonucleoprotein auxiliary factor 35 kDa subunit- imprinted, although the latter may be regulated by a different related protein 1 (U2af1-rs1) and impact, RWD protein DMR than that in eutherians. One possibility is histone modifi- (Impact), emerged in a lineage-specific manner, in which the cation, which functions as an alternative regulatory mechanism insertion of their unique DMR sequences was confirmed. Thus, of genomic imprinting. Both eutherians and marsupials have one conclusion that could be drawn is that the emergence of another epigenetic mechanism of X chromosome inactivation genomic imprinting regions occurred in association with major (XCI; Lyon 1963; and 1986). One of two female X chromo- chromosomal changes (i.e. the insertion of exogenous DNA somes is inactivated by expression of X inactive specific and/or chromosome translocation). This implies that genomic transcript (XIST), a non-coding RNA specifically expressed imprinting arose as a genome defence mechanism against the from an inactivated X chromosome. At least in eutherians, disturbance of the local genetic network by cis-regulatory units, H3K27 trimethylation, not DNA methylation, inactivates XIST probably due to either the loss or addition of such unit(s). As expression from an active X chromosome, leading to mono- mentioned above, the complementary monoallelic expression allelic XIST expression, similar to that in genomic imprinting system of genomic imprinting may be a useful means to (Inoue et al. 2017a). Both histone modification and DNA overcome these situations, which could otherwise result in methylation result in genomic imprinting (Li et al. 2008; Inoue developmental abnormalities and/or lethality. Thus, the number et al. 2017b), but in the eutherians the control of genomic of imprinted regions may only represent a list of successful imprinting must have shifted to the latter, probably because a recovery of genomic imprinting. Many more insertion and/or strict all-or-none-type of regulation was more advantageous. chromosomal rearrangement events must have occurred at the In this regard, acquisition of DNA methyltransferase 3 like level of the complete genome, but harmful loci were lost, (DNMT3L) in the therian lineage may be of critical importance whereas non-harmful loci have remained. It should be noted (Yokomine et al. 2006) because it regulates DNA methylation in that several imprinted genes that reside in the introns of other germ cells in cooperation with DNA methyltransferase 3A and B genes, such as Mcts2, nucleosome assembly protein 1 like (DNMT3A and DNMT3B), thus establishing the DNA methyla- 5(Nap1L5), inositol polyphosphate-5-phosphatase F variant 2 tion status of genome imprinting (Bourc’his et al. 2001; (Inpp5f_v2), U2af1-rs1 and neuronatin (Nnat), are duplicated Hata et al. 2002). Evolution of viviparity and genomic imprinting Reproduction, Fertility and Development 1223

Peg5/Nnat (dis2) Gnas/Nesp (dis2) Rasgrf1 (dis9) Peg1/Mest (prox6) Mcts2 (2) U2af1-rs1 (prox11) Nap1/5 (prox6) Peg13 (15) Impact (cent18) Peg3 (prox7) Mouse Inpp5f_v2 (7) Rodentia Lit1/Kcnq1ot1 (dis7) Plagl1/Zac1 (proc10) Rabbit Meg1/Grb10 (prox11) Lagomorpha Dlk1-Meg3 IG (dis12) Human Airn-lgf 2r (prox17) Euarchonta Peg 10 (prox6) Snrpn*(cent7) Cow Eutherians Laurasiatheria H19 (dis7) Elephant Afrotheria Slc38a4*(15) Opossum and wallaby Marsupials

Platypus Monotremes

Fig. 6. Emergence of differentially methylated regions (DMRs) as newly inserted DNA sequences in mammalian linages. DMRs in paternally and maternally imprinted regions are shown in blue and red respectively. The locations of the mouse chromosomes are shown in parentheses. Paternally expressed 10 (PEG10) and H19 DMRs are common to eutherians and marsupials. *The imprinted regulation of these two regions started in eutherians in association with chromosome translocations near the DMRs. Modified from fig. 6 in Suzuki et al. (2011). Slc38a4, solute carrier family 38 member 4; Peg10, paternally expressed 10; Snprn, small nuclear ribonucleoprotein polypeptide N; H19, H19, imprinted maternally expressed transcript; Peg5/Nnat, paternally expressed 5/neuronatin; Gnas/Nesp, guanine nucleotide binding protein (G protein), alpha complex locus/neuroendocrine secretory protein; Peg1/Mest, paternally expressed 1/mesoderm specific transcript; Nap1/5, nucleosome assembly protein 1 like 5; Peg3, paternally expressed 3; Inpp5f_v2, inositol polyphosphate-5-phosphatase F variant 2; Lit1/Kcnq1ot1, long QT intronic transcript 1/potassium voltage-gated channel subfamily Q member 1 (Kcnq1) opposite strand/antisense transcript 1; Plagl1/Zac1, pleiomorphic adenoma gene 1(Plag1) like zinc finger 1/zinc-finger gene involved in apoptosis and cell-cycle control; Meg1/Grb10, maternally expressed 1/growth factor receptor bound protein 10; Dlk1/Meg3 IG, delta like non-canonical Notch ligand 1-maternally expressed 3 intergenic region; Airn-Igf2r, antisense of Igf2r non-protein coding RNA- insulin like growth factor 2 receptor; Mcts2, malignant T cell amplified sequence 2; Peg13, paternally expressed 13; Rasgrf1, Ras protein specific guanine nucleotide releasing factor 1; U2af1-rs1, U2 small nuclear ribonucleoprotein auxiliary factor 35 kDa subunit-related protein 1; Impact, impact, RWD protein.

Relationship between genomic imprinting and viviparous relationship between genomic imprinting and placental for- reproduction mation and function. As mentioned earlier, among the vertebrates, genomic Androgenetic death is attributable to at least two imprinted imprinting is unique to viviparous mammals. It was originally genes, paternally expressed Igf2 and maternally expressed discovered in 1984 from experiments showing that partheno- achaete-scute family b HLH transcription factor 2 (Ascl2)/ genetic and androgenetic embryos with either two maternal or murine achaete-scute homolog 2 (Mash2), which are located in two paternal pronuclei exhibited early embryonic lethality, two closely neighbouring imprinted regions on the mouse albeit with different morphological anomalies in the placenta distal chromosome 7 (Cattanach and Beechey 1990; Blake (Surani et al. 1984; Mann and Lovell-Badge 1984; McGrath et al. 2010; Fig. 8). Ascl2/Mash2 encodes a placenta-specific and Solter 1984; Fig. 7). The former died at an early embry- transcription factor essential for the formation of placental onic stage due to severe placental defects, even though the spongiotrophoblast and labyrinth layers (Guillemot et al. 1994, embryos appeared normal. The latter also exhibited early 1995). A lack of Ascl2/Mash2 combined with overproduction embryonic lethality due to severe growth retardation along of embryonic growth hormone Igf2 (Constaˆncia et al. 2002) with overgrowth of the impaired placenta. This not only explains the placental phenotype of the androgenetic embryos. demonstrates the functional differences between the paternal Ascl2/Mash2 emerged as a duplicated gene of achaete- and maternal genomes, but also suggests a profound scute family b HLH transcription factor 1 (Ascl1)/murine 1224 Reproduction, Fertility and Development T. Kaneko-Ishino and F. Ishino

Parthenogenetic embryo

Early embryonic lethal Unfertilised egg

Fertilisation Embryo Placenta Spermatozoon Early embryonic lethal

Androgenetic embryo

Fig. 7. Development of parthenogenetic and androgenetic embryos in mice. Pronuclear transplantation experiments demonstrate that both parthenogenetic and androgenetic embryos exhibit early embryonic lethality with totally different morphological defects in embryos and placentas on developmental Day 10.5.

26711121718 ϽϽ

Peg10 ϽϽ ϾϾ ϽϽ ϽϽ ϽϽ ϽϽ ϾϾ ϾϾ ϾϾ Neonatal lethality Early embryonic lethality Fetal/placental growth growth Fetal/placental rowth retardation? rowth Fetal/placental growth growth Fetal/placental growth Fetal/placental Fetal growth growth Fetal growth Fetal g Postnatal growth/viability growth/viability Postnatal Fetal/placental growth growth Fetal/placental Postnatal lethality Postnatal Placental growth Placental growth Fetal growth retardation? growth Fetal Fetal

Peg11/Rtl1 Late embryonic lethality and growth Late embryonic lethality and growth Late fetal lethality Late fetal lgf2 Late embryonic/neonatal lethality and growth Decreased cerebllar folding Decreased cerebllar Early embryonic lethality Ascl2/Mash2 Regions affected by paternal duplication Regions affected by maternal duplication Neonatal lethal with hyperactivity Neonatal lethal with hypoactivity

Fig. 8. Genomic imprinting map in mice. The abnormal phenotypes observed in mice with partial uniparental duplications are shown. Modified from data in Blake et al. (2010). Paternally expressed 10 (Peg10) is an imprinted gene responsible for an early embryonic lethal phenotype due to severe placental defects upon maternal duplication of proximal chromosome 6 as well as parthenogenetic embryos; acaete-scute family bHLH transcription factor 2 (Ascl2)/murine acaete-scute homolog 2 (Mash2) is responsible for an early embryonic lethal phenotype upon paternal duplication of distal chromosome 7. Overgrown impaired placenta associated with androgenetic embryos is attributable to a lack of Ascl2/Mash2 expression combined with overexpression of insulin-like growth factor 2 (Igf2). Lack of Peg11 or retrotransposon Gaglike 1 (Rtl1) expression is responsible for late embryonic or neonatal lethality associated with growth retardation upon maternal duplication of distal chromosome 12. ,, and .. in the figure means growth retardation and promotion, respectively. achaete-scute homolog 1 (Mash1), encoding a brain-specific Mouse proximal chromosome 6 is an imprinted region, the transcription factor. In chickens, both are expressed in the maternal duplication of which causes early embryonic lethality. brain, whereas Ascl2/Mash2 exhibits placenta-specific expres- Peg10 in this region is the major gene responsible for the early sion, at least in humans and mice, suggesting that it was embryonic lethal phenotype. Peg10-knockout (KO) embryos recruited for placenta formation in the course of eutherian die due to severe placental defects, as for parthenogenetic evolution (Yokomine et al. 2005). embryos (Ono et al. 2006). The placentas of Peg10-KO embryos Evolution of viviparity and genomic imprinting Reproduction, Fertility and Development 1225 lack spongiotrophoblast and labyrinth layers, as seen in Ascl2/ at least partly because a short-lived chorioallantoic organ may Mash2-KO mice. It should be noted that this relationship not have enough advantage to override the adoption of the yolk between Peg10 and Ascl2/Mash2 is an apparent exception to sac placenta. Thus, elucidation of the biochemical function of the ‘parental conflict’ model, because both the PEGs and MEGs PEG11/RTL1 is important to understanding the evolutionary exhibit very similar embryonic growth promotion. As described history of how the viviparous systems diverged in eutherians and earlier, PEG10 is a therian-specific gene as well as one of the marsupials. first imprinted genes to appear in a common therian ancestor. Importantly, PEG10 is a long terminal repeat (LTR) retro- What drove the evolution of viviparity in mammals? transposon-derived gene encoding an LTR retrotransposon As mentioned above, the emergence of therian mammals Gag- and Pol-like protein (Ono et al. 2001). Thus, this is a very 168 million years before the present (Ma) coincided with the good example of a therian-specific gene playing an essential role appearance of genomic imprinting and domestication of one of in the formation of the placenta, a therian-specific organ for the essential placental genes, PEG10. Both events may have had viviparous reproduction. a considerable effect on genomic function, especially in asso- Viviparous reproduction is widely observed in vertebrates ciation with the acquisition of viviparity. The divergence of the and non-vertebrates, such as squamates, sharks and scorpions. eutherians and marsupials 148 Ma coincided with the emer- However, a unique feature of the mammalian placenta is the gence of a series of new imprinted regions and the domestication inclusion of trophoblast cells (Roberts et al. 2016). Eutherians of another essential placental gene, PEG11/RTL1. Both events and marsupials developed different types of placentas (i.e. may have affected a wide range of eutherian and marsupial chorioallantoic and choriovitelline (yolk sac) placentas respec- genomic functions relating to the diversification of the vivipa- tively) and both contain trophoblast cells in addition to the rous reproduction systems. Thus, genomic imprinting and gene chorion, allantois or yolk sac membranes. It is highly probable domestication may have played critical roles in the evolution that PEG10 plays an essential role in the differentiation of such and diversification of the viviparous reproduction systems as trophoblast cells in the spongiotrophoblast and labyrinth layers. effective adaptive measures. However, in the first place, what If this is the case, domestication of PEG10 was a critical event in kind of environmental factor(s) drove the evolution of mam- the establishment of the current therian viviparous systems. malian viviparity? That is, what was the major evolutionary Another important question concerning therian viviparity is pressure that forced the therian mammals to innovate a new style why eutherians and marsupials respectively adopted chorioal- of reproduction system? lantoic and yolk sac placentas (Renfree 2010). Some eutherian One possibility is the low atmospheric oxygen concentration species, such as dogs and cats, also use yolk sac placentas in the (12–13%) approximately 190–150 Ma in the Phanerozoic eon early stage of gestation, whereas some marsupial species, such (from 550 Ma to the present; Berner et al. 2007), because oxygen as bandicoots, develop chorioallantoic placentas, which they use concentration is a major factor affecting development and for a short period in the very last stage of gestation (Tyndale- growth in a wide range of organisms, including plants and Biscoe and Renfree 1987). This suggests that both groups had an animals. However, it is also possible that this was a coincidence. equal chance to widely adopt the chorioallantoic placenta. Two consecutive mass extinction events occurred at the PEG11/retrotransposon Gaglike 1 (RTL1), another paternally Permian–Triassic and Triassic–Jurassic boundaries 250 and expressed imprinted gene, may key to addressing this question 200 Ma respectively as a result of low atmospheric oxygen, (Charlier et al. 2001). Mouse Peg11/Rtl1 located on the distal most of the therapsids, except for the ancestors of mammals, chromosome 12 is a major gene responsible for late embryo or became extinct. Dinosaurs, ancestors of birds, adapted to these neonatal lethality and is associated with late embryo growth conditions using an air sac system with high oxygen exchange retardation upon maternal duplication of this region (Sekita efficiency and dominated the world for a long time. It is likely et al. 2008). Peg11/Rtl1 plays an essential role in ‘the feto– that an efficient oxygen supply to the embryo via a placental maternal interface’ in the placenta; that is, in fetal capillary blood circulating system along with an effective nutrient supply endothelial cells, where nutrients and oxygen are exchanged would provide a developmental advantage. between the fetal and maternal blood. The fetal capillary endothelial cells are surrounded by three layers of trophoblast cells, two syncytiotrophoblast layers and one cytotrophoblast Conclusion: evolutionary trade-off in mammals layer. The trophoblast cells could harm the maternal tissues Viviparous reproduction exerts an inordinate burden on because of their invasive nature. In the Peg11/Rtl1-KO placenta, females. Even today, many women die due to problems related the fetal capillary endothelial cells were found to be severally to pregnancy and childbirth. Humans and rodents have the most damaged by trophoblast cells, whereas the trophoblast them- invasive type of haemochorial placentas. Interestingly, recent selves were severally damaged when Peg11/Rtl1 was over- phylogenetic analysis suggests that the haemochorial or produced (Sekita et al. 2008). Therefore, Peg11/Rtl1 is endotheliochorial placenta that has a discoid shape with essential for the maintenance of the eutherian-type of chorioal- labyrinths, but not villi, represents the ancestral type of the lantoic placenta for the entire gestation period. Importantly, eutherian placenta, whereas a non-invasive type of epithe- PEG11/RTL1 is another LTR retrotransposon-derived gene tiochorial placenta is an evolution-derived phenotype for specific to eutherians that is absent from marsupials (Charlier avoiding immunological problems between the mother and et al. 2001; Edwards et al. 2008). This explains why chorioal- hemiallogeneic fetus (Wildman et al. 2006; Roberts et al. lantoic placentas were not widely adopted in marsupial species, 2016). It is likely that the adoption of viviparity using the 1226 Reproduction, Fertility and Development T. Kaneko-Ishino and F. Ishino

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