The evolution of class V POU domain transcription factors in vertebrates and their characterisation in a marsupial.

POU5F1 (OCT4) encodes a master regulator of pluripotency that is present in all mammals. A paralogue, POU2, is also present in the genomes of marsupials and monotremes and is an orthologue of zebrafish pou2 and chicken POUV. We explored the evolution of class V POU domain transcription factors and show that POU5F1 arose by gene duplication of pou2 early in the evolution of tetrapods and is not mammal-specific, as previously thought. Instead, either POU5F1 or POU2/POUV has become extinct independently in various lineages, although all gnathostomes appear to possess at least one or the other. In the tammar wallaby, POU5F1 expression is limited to pluripotent cell types (embryonic tissues and germ cells). POU2 is similarly expressed in pluripotent tissues but is also expressed in a broad range of adult tissues. Thus, unlike POU5F1, the role of POU2 may not be restricted to pluripotent cell types but could have a related function such as maintaining multipotency in adult stem cells.