Between Scylla and Charybdis
Monday, 25 March 2013
Embryo and fetal membranes of a marsupial
© 2011 Menzies et al; licensee BioMed Central Ltd.
This stunning photo of a tammar wallaby at 18 days gestation shows the vascular (TYS) and non-vascular (BYS) regions of the yolk sac. It will attach to the uterus and, as a yolk sac placenta, support fetal growth until delivery at 26-27 days. All marsupials have a yolk sac placenta. A few develop an additional chorioallantoic placenta (including the wombats).
Common wombat (Vombatus ursinus) Wikimedia Commons
Marsupials and their kin (Metatheria) share a common ancestor with “placental” mammals and their kin (Eutheria). Freyer et al. (2003) suggested that ancestor may have had both a yolk sac placenta and a chorioallantoic placenta.
As in placental mammals, the placenta of marsupials is more than an organ of maternal-fetal exchange. It is known to produce several hormones and recent studies extend the list to include growth hormone (GH), prolactin (PRL) and luteinizing hormone (LH). These hormones are expressed in the placentas of other mammals and in some elaborated as placental lactogens and chorionic gonadotropins. As argued by Menzies et al. (2011) this is further evidence of the affinity between placentas of “placental” mammals and marsupials.
There are sound reasons for distinguishing between Placentalia and Eutheria. The former is the clade including living mammals (minus marsupials and monotremes) and their common ancestor. The latter includes extinct lineages that share a number of characters with Placentalia. Similar reasoning discriminates between Marsupialia and Metatheria.
The problem arises with the vernacular terms “placentals” or “placental mammals.” Understandably this is anathema to students of marsupial placentation. However, if “eutherians” is used to represent this group, phylogeneticists are quick to take offence.