Extended Data Fig. 10: Comparison of two possible scenarios for the evolution of trophoblast syncytialization.
From: Cell type and cell signalling innovations underlying mammalian pregnancy
a. In the convergent syncytiotrophoblast origins scenario, endogenous retroviral (ERV) envelope genes are into the genome of different mammalian lineages in locations which fall under the control of regulatory genes (MR) in different placental cell types’ gene-regulatory networks (GRN). This leads to non-similarity between syncytial cell types in cross-species transcriptomic comparison (for example SAMap) which should be interpreted as true non-homology. b. in the single syncytiotrophoblast origin scenario, ERV envelope genes were recruited into the GRNs of the same (homologous) progenitor cell type, which subsequently underwent transcriptomic divergence and/or functional specialization in different lineages (recruitment of genes E and F in the figure), leading to lack of similarity in extant species which should be interpreted as functional divergence rather than non-homology. Silhouettes from PhyloPic under a Creative Commons licence: human, NASA (PDM 1.0); tenrec, Yan Wong (CC0 1.0); macaque, Jane Whitehouse (CC0 1.0); mouse, Michael Keesey (PDM 1.0).
