Platypus Pou5f1 reveals the first steps in the evolution of trophectoderm differentiation and pluripotency in mammals

Abstract

Uterine nourishment of embryos by the placenta is a key feature of mammals. Although a variety of placenta types exist, they are all derived from the trophectoderm (TE) cell layer of the developing embryo. Egg-laying mammals (platypus and echidnas) are distinguished by a very short intrauterine embryo development, in which a simple placenta forms from TE-like cells. The Pou5f1 gene encodes a class V POU family transcription factor Oct3/4. In mice, Oct3/4 together with the highly conserved caudal-related homeobox transcription factor Cdx2, determines TE fate in pre-implantation development. In contrast to Cdx2, Pou5f1 has only been identified in eutherian mammals and marsupials, whereas, in other vertebrates, pou2 is considered to be the Pou5f1 ortholog. Here, we show that platypus and opossum genomes contain a Pou5f1 and pou2 homolog, pou2-related, indicating that these two genes are paralogues and arose by gene duplication in early mammalian evolution. In a complementation assay, we found that platypus or human Pou5f1, but not opossum or zebrafish pou2, restores self-renewal in Pou5f1-null mouse ES cells, showing that platypus possess a fully functional Pou5f1 gene. Interestingly, we discovered that parts of one of the conserved regions (CR4) is missing from the platypus Pou5f1 promoter, suggesting that the autoregulation and reciprocal inhibition between Pou5f1 and Cdx2 evolved after the divergence of monotremes and may be linked to the development of more elaborate placental types in marsupial and eutherian mammals.

Identification of a pou2 ortholog in Chinese sturgeon, Acipenser sinensis and its expression patterns in tissues, immature individuals and during embryogenesis

“…Due to the deficiency of sequence information for the primitive fish, the exact time when pou5f1 appeared in the evolutionary tree was uncertain. Niwa et al (2008) suggested that pou5f1 and pou2 genes are paralogs and arise by gene duplication in primitive mammalian evolution. However, as it was credibly shown by Frankenberg et al (2010), the duplication of pou5f1 and pou2 occurred at least in the early tetrapod lineage.…”

“…Nevertheless, there were differences in the linker, such as the length of a-helix and the number of the turn and b-sheet, which might be the reason for their divergence of functions. For instance, result from some differences of POU domain sequence, self-renewal function of the class V POU domain transcription factors was affected (Niwa et al 2008). Due to the deficiency of sequence information for the primitive fish, the exact time when pou5f1 appeared in the evolutionary tree was uncertain.…”

“…In chicken, the class V POU gene, cPouV, was required for the maintenance of pluripotency and self-renewal of ESC (Lavial et al 2007). From the current point of view, a common ancestor of gnathostomes has a single POU V gene, syntenic to zebrafish pou5f1/pou2 (Niwa et al 2008;Frankenberg et al 2010). Because only a single pou2 had been identified in all five sequenced fish species, it was said that pou2 gave rise to Pou5f1 by duplication early in the evolution of tetrapods (Frankenberg et al 2010).…”

Pluripotency and lineages in the mammalian blastocyst: An evolutionary view

“…17,18,21,22 However, recent reports claim that in the early vertebrate lineage the Pou5 class was represented by a single gene, which was duplicated before the mammalian radiation to give rise to the two Pou5 genes found today in noneutherian mammals. 35,36 One copy was subsequently lost in eutherian mammals, resulting in the single Oct4 gene present in mouse and humans. Therefore the pluripotency gene Oct4 is not the ortholog of chicken Pou2-related (Pou2-r, following the nomenclature of Niwa 36 ) and other vertebrate Pou5 genes, but a paralog.…”

“…35,36 One copy was subsequently lost in eutherian mammals, resulting in the single Oct4 gene present in mouse and humans. Therefore the pluripotency gene Oct4 is not the ortholog of chicken Pou2-related (Pou2-r, following the nomenclature of Niwa 36 ) and other vertebrate Pou5 genes, but a paralog. The frequently result from local or global duplication events.…”