Recruitment of 5′ Hoxa genes in the allantois is essential for proper extra-embryonic function in placental mammals

The Hox gene family is well known for its functions in establishing morphological diversity along the anterior-posterior axis of developing embryos. In mammals, one of these genes, Hoxa13, is crucial for embryonic survival, as its function is required for the proper expansion of the fetal vasculature in the placenta. Thus, it appears that the developmental strategy specific to placental mammals is linked, at least in part, to the recruitment of Hoxa13 function in developing extra-embryonic tissues. Yet, the mechanism underlying this extra-embryonic recruitment is unknown. Here, we provide evidence that this functional novelty is not exclusive to Hoxa13 but is shared with its neighboring Hoxa11 and Hoxa10 genes. We show that the extra-embryonic function of these three Hoxa genes stems from their specific expression in the allantois, an extra-embryonic hallmark of amniote vertebrates. Interestingly, Hoxa10-13 expression in the allantois is conserved in chick embryos, which are non-placental amniotes, suggesting that the extra-embryonic recruitment of Hoxa10, Hoxa11 and Hoxa13 most likely arose in amniotes, i.e. prior to the emergence of placental mammals. Finally, using a series of targeted recombination and transgenic assays, we provide evidence that the regulatory mechanism underlying Hoxa expression in the allantois is extremely complex and relies on several cis-regulatory sequences.