The placenta in Monoclea forsteri Hook. and Treubia lacunosa (Col.) Prosk: insights into placental evolution in liverworts

Placental morphology is remarkably diverse between major bryophyte groups, especially with regard to the presence and distribution of transfer cells in the sporophyte and gametophyte. In contrast, with the exception of metzgerialean liverworts, placental morphology is highly conserved within major bryophyte groups. Here we examine the ultrastructure of the placenta in Monoclea forsteri and Treubia lacunosa, basal members of the marchantialean and metzgerialean liverwort lineages, respectively. In both species several layers of transfer cells are found on both sides of the placenta, with sporophytic transfer cells exhibiting prominent wall labyrinths. Consistent with previous reports of a similar placenta in other putatively basal and isolated liverwort genera such as Fossombronia, Haplomitrium, Blasia and Sphaerocarpos, this finding suggests that this type of placenta represents the plesiomorphic (primitive) condition in liverworts. Distinctive ultrastructural features of placental cells in Monoclea include branched plasmodesmata in the sporophyte and prominent arrays of smooth endoplasmic reticulum, seemingly active in secretion in the gametophyte. These arrays contain a core of narrow tubules interconnected by electron-opaque rods, structures with no precedent in plants. Analysis of the distribution of different types of placenta in major bryophyte groups provides valuable insights into their inter-relationships and possible phylogeny.