REPRODUCTION What is a placental anyway? Many developmental functions in and eutherian are accomplished by different tissues, but similar genes.


of the offspring and, unlike in eutherian mam- Related research article Guernsey MW, mals, the composition of this changes dra- Chuong EB, Cornelis G, Renfree MB, Baker matically as the young joeys grow. In essence, JC. 2017. Molecular conservation of marsu- the mammary glands of marsupials perform pial and eutherian placentation and lacta- many of the functions of the eutherian tion. eLife 6:e27450. DOI: (Renfree, 2010; Sharp et al., 2017; Figure 1). 10.7554/eLife.27450 And to upend what you may have learned in biology even more, marsupials do have a placenta after all, but it develops late in preg- nancy and from different tissues compared with ost of us learned in school that there eutherians. Thus, the difference between euthe- are three kinds of living mammals — rian mammals and marsupials is not the pres- eutherians, marsupials and monot- M ence or absence of a placenta, but rather the remes — and that the most obvious differences relative emphasis put on placentation and lacta- between them are how they reproduce. The tion to nurture offspring through development. eutherian or ’placental’ mammals, like , Now, in eLife, Julie Baker of Stanford Univer- make up the vast majority of today’s mammalian sity School of Medicine, Marilyn Renfree of the diversity. Eutherians all have a chorioallantoic University of Melbourne and co-workers — placenta, a remarkable that forms after including Michael Guernsey of Stanford as first conception at the site where the makes author, Edward Chuong of the University of contact with the lining of the mother’s Utah and Guillaume Cornelis (Stanford) — report (Langer, 2008). new details of the molecular mechanisms under- Marsupials and handle preg- lying placentation and lactation in eutherians nancy differently (Abbot and Rokas, 2017; Ren- and marsupials (Guernsey et al., 2017). The free, 2010). -laying monotremes, like the results were obtained by using a modified ver- duck-billed , have tiny ’puggles’ that sion of a technique called RNA-seq to measure hatch from leathery shells. Marsupials — the how the transcriptome (the complete set of RNA , , , and transcripts in a cell or set of cells) varied so on — have live , but their between different cells types during develop- are brief and their tiny joeys are developmen- ment (Rokas and Abbot, 2009). tally immature, and would seem to have little Guernsey et al. compared changes in gene Copyright Abbot and Capra. This need of a placenta. After , the joeys con- expression in two cell types in the placenta of a article is distributed under the terms tinue to develop outside of their mother’s tammar , a small Australian , of the Creative Commons Attribution body, often within folds and pouches on their during development. They found that gene License, which permits unrestricted expression differed between the two tissues use and redistribution provided that mother’s abdomen. the original author and source are In marsupials, the milk provided by the and, moreover, that it changed dynamically credited. mother after birth is central to the development over time, similar to what happens in

Abbot and Capra. eLife 2017;6:e30994. DOI: 1 of 3 Insight What is a placental mammal anyway?

Figure 1. The different reproductive strategies of eutherian mammals and marsupial mammals. In eutherians, the energy invested by the mother in rearing young before birth (via placentation) and after birth (via lactation) is roughly equally. In marsupials, is brief, the placenta forms late in , and lactation is extended. Guernsey et al. show that genetic features that regulate development via the placenta in eutherians are shared with the short-lived marsupial placenta (red arrows). They also show that some of the genes that underlie placental functions in eutherians are expressed during lactation in marsupials (blue arrows), including various conserved components of lactation itself (black arrow; Lefe`vre et al., 2010). Note: time scales are not absolute.

eutherians. Furthermore, among the transcripts conservation in both groups. But most remark- they found many that had critical functions in ably, they identified a number of genes eutherian , including members of the expressed in the mammary glands in the tammar Ig7 signaling pathways and GCM1, a transcrip- that are known to be functionally important in tion factor that is important in the formation the placenta in eutherians (Figure 1). These and development of the placenta. And it was genes included genes involved in nutrient trans- not simply the genes that were conserved, the port and several known to be required for patterns of gene expression in the wallaby pla- eutherian placentation (including GCM1). This centa resembled those seen in the mouse pla- conservation of gene expression argues that in centa in the early stages of pregnancy. This is marsupials the placenta manages early fetal exactly what we would expect to see if the pla- development and lactation manages late fetal centa performs early developmental functions in development, using some of the same genes the wallaby, with later functions being provided and molecular pathways as the eutherian post-natally. This suggests that an essential dif- placenta. ference between marsupials and eutherians is Those who study marsupials have long not in the early functions of the placenta, but argued that we need to correct our textbooks to rather in how placental functions have been acknowledge marsupisal placentas and their dis- compartmentalized over the course of the evo- tinctively complex lactation (Renfree, 1983). lution of eutherian pregnancy. Guernsey et al. strengthen the case by demon- Finally, Guernsey et al. characterized the pat- strating that both eutherians and marsupials terns of gene expression in the mammary glands express a conserved toolkit of genes that may of the tammar and several mammals. Both be localized to different tissues and organs, but mouse and wallaby shared similar patterns of serve common purposes in fetal development. gene expression, underscoring the theme of This surprising conservation underscores the functional compartmentalization and importance of identifying the genes underlying

Abbot and Capra. eLife 2017;6:e30994. DOI: 2 of 3 Insight Reproduction What is a placental mammal anyway?

functional changes during evolution Langer P. 2008. The phases of maternal investment in (Rausher and Delph, 2015). eutherian mammals. Zoology 111:148–162. Looking ahead, it is worth noting that marsu- DOI:, PMID: 18222662 pials vary tremendously in reproductive traits Lefe` vre CM, Sharp JA, Nicholas KR. 2010. Evolution (Tyndale-Briscoe, 2005), and that characterizing of lactation: ancient origin and extreme adaptations of more in the way that Guernsey et al. the lactation system. Annual Review of Genomics and have done for the will provide a Genetics 11:219–238. DOI: 1146/annurev-genom-082509-141806, richer understanding of the evolution and diver- PMID: 20565255 sity of marsupial pregnancy itself. However, Ostrovsky AN, Lidgard S, Gordon DP, Schwaha T, more work is needed to develop appropriate Genikhovich G, Ereskovsky AV. 2016. and statistical methods for quantifying the conserva- placentation in : a new paradigm. tion of transcriptome profiles between species. Biological Reviews 91:673–711. DOI: And looking beyond mammals, forms of placen- 10.1111/brv.12189, PMID: 25925633 Rausher MD, Delph LF. 2015. When does tation are found in everything from lizards, to understanding phenotypic evolution require , to , and preliminary studies identification of the underlying genes? Evolution 69: indicate that many of the genes or traits involved 1655–1664. DOI:, are shared (Ostrovsky et al., 2016; PMID: 25973520 Whittington et al., 2015). It will be fascinating Renfree MB. 1983. Marsupial reproduction: the choice between placentation and lactation. In: Finn C. A (Ed). to learn how deeply we can trace the origins of Oxford Reviews of Reproductive Biology. Vol. 5. the pregnancy toolkit. Oxford: Oxford University Press. Renfree MB. 2010. Marsupials: placental mammals with a difference. Placenta 31 Suppl:S21–S26. Patrick Abbot is in the Department of Biological DOI:, Sciences, Vanderbilt University, Nashville, United PMID: 20079531 States Rokas A, Abbot P. 2009. Harnessing genomics for [email protected] evolutionary insights. Trends in Ecology & Evolution 24:192–200. DOI: 11.004, PMID: 19201503 John A Capra is in the Department of Biological Sharp JA, Wanyonyi S, Modepalli V, Watt A, Sciences, Vanderbilt University, Nashville, United Kuruppath S, Hinds LA, Kumar A, Abud HE, Lefevre C, States Nicholas KR. 2017. The tammar wallaby: a marsupial model to examine the timed delivery and role of Competing interests: The authors declare that no bioactives in milk. General and Comparative competing interests exist. Endocrinology 244:164–177. DOI: 1016/j.ygcen.2016.08.007, PMID: 27528357 Published 12 September 2017 Tyndale-Briscoe H. 2005. Life of Marsupials. Collingwood, : CSIRO Publishing. References Whittington CM, Griffith OW, Qi W, Thompson MB, Abbot P, Rokas A. 2017. Mammalian pregnancy. Wilson AB. 2015. brood transcriptome Current Biology 27:R127–R128. DOI: reveals common genes associated with 10.1016/j.cub.2016.10.046, PMID: 28222284 pregnancy. Molecular Biology and Evolution 32: Guernsey MW, Chuong EB, Cornelis G, Renfree MB, msv177–3131. DOI: Baker JC. 2017. Molecular conservation of marsupial msv177 and eutherian placentation and lactation. eLife 6: e27450. DOI:

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