Uterine epithelial remodelling during pregnancy in the marsupial Monodelphis domestica (Didelphidae): Implications for mammalian placental evolution

Mammalian pregnancy involves remodelling of the uterine epithelium to enable placentation. In marsupials, such remodelling has probably played a key role in the transition from ancestral invasive placentation to non‐invasive placentation. Identifying uterine alterations that are unique to marsupials with non‐invasive placentation can thus elucidate mechanisms of marsupial placental evolution. We identified apical alterations to uterine epithelial cells prior to implantation in Monodelphis domestica, a member of the least derived living marsupial clade (Didelphidae) with invasive (endotheliochorial) placentation. We then compared these traits with those of Macropus eugenii (Macropodidae) and Trichosurus vulpecula (Phalangeridae), both with non‐invasive placentation, to identify which alterations to the uterine epithelium are ancestral and which facilitate secondarily evolved non‐invasive placentation. In M. domestica, remodelling of the uterine epithelium involves reduced cellular heterogeneity and development of uterodome‐like cells, suggesting that similar alterations may also have occurred in the marsupial common ancestor. These alterations also overlap with those of both T. vulpecula and Ma. eugenii, suggesting that the placental shift from invasive to non‐invasive placentation in marsupials involves essential, conserved characteristics, irrespective of placental mode. However, unique apical alterations of both T. vulpecula and Ma. eugenii, relative to M. domestica, imply that lineage‐specific alterations underpin the evolutionary shift to non‐invasive placentation in marsupials. Invasive placentation in Monodelphis domestica (A, this study) requires a homogeneous epithelium with uterodome‐like cells that may also have formed in the marsupial common ancestor. Comparison with secondarily evolved non‐invasive placentation in Trichosurus vulpecula (B, Laird et al., 2017b) and Macropus eugenii (C, Freyer et al., 2002) reveals that both conserved and lineage‐specific traits, relative to M. domestica, underpin the evolutionary shift to non‐invasive placentation in marsupials.