Cas9-mediated excision of Nematostella brachyury disrupts endoderm development, pharynx formation and oral-aboral patterning.

The mesoderm is a key novelty in animal evolution, although we understand little of how the mesoderm arose. brachyury, the founding member of the T-box gene family, is a key gene in chordate mesoderm development. However, the brachyury gene was present in the common ancestor of fungi and animals long before mesoderm appeared. To explore ancestral roles of brachyury prior to the evolution of definitive mesoderm, we excised the gene using CRISPR/Cas9 in the diploblastic cnidarian Nematostella vectensisNvbrachyury is normally expressed in precursors of the pharynx, which separates endoderm from ectoderm. In knockout embryos, the pharynx does not form, embryos fail to elongate, and endoderm organization, ectodermal cell polarity and patterning along the oral-aboral axis are disrupted. Expression of many genes both inside and outside the Nvbrachyury expression domain is affected, including downregulation of Wnt genes at the oral pole. Our results point to an ancient role for brachyury in morphogenesis, cell polarity and the patterning of both ectodermal and endodermal derivatives along the primary body axis.

Expression of a lung developmental cassette in the adult and developing zebrafish swimbladder.

The presence of an air-filled organ (AO), either lungs or a swimbladder, is a defining character of the Osteichthyes (bony vertebrates, including tetrapods). Despite the functional and structural diversity of AOs, it was not previously known whether the same group of developmental regulatory genes are involved in the early development of both lungs and swimbladders. This study demonstrates that a suite of genes (Nkx2.1, FoxA2, Wnt7b, GATA6), previously reported to be co-expressed only in the tetrapod lung, is also co-expressed in the zebrafish swimbladder. We document the expression pattern of these genes in the adult and developing zebrafish swimbladder and compare the expression patterns to those in the mouse lung. Early-acting genes involved in endoderm specification are expressed in the same relative location and stage of AO development in both taxa (FoxA2 and GATA6), but the order of onset and location of expression are not completely conserved for the later acting genes (Nkx2.1 and Wnt7b). Co-expression of this suite of genes in both tetrapod lungs and swimbladders of ray-finned fishes is more likely due to common ancestry than independent co-option, because these genes are not known to be co-expressed anywhere except in the AOs of Osteichthyes. Any conserved gene product interactions may comprise a character identity network (ChIN) for the osteichthyan AO.