Abstract
Hox gene transcription factors are important regulators of positional identity along the anterior–posterior axis in bilaterian animals. Cnidarians (e.g., sea anemones, corals, and hydroids) are the sister group to the Bilateria and possess genes related to both anterior and central/posterior class Hox genes. Here we report a previously unrecognized domain of Hox expression in the starlet sea anemone, Nematostella vectensis, beginning at early blastula stages. We explore the relationship of two opposing Hox genes (NvAx6/NvAx1) expressed on each side of the blastula during early development. Functional perturbation reveals that NvAx6 and NvAx1 not only regulate their respective expression domains, but also interact with Wnt genes to pattern the entire oral–aboral axis. These findings suggest an ancient link between Hox/Wnt patterning during axis formation and indicate that oral–aboral domains are likely established during blastula formation in anthozoan cnidarians.
Highlights
The axial patterning role of cnidarian Hox genes was long debated[21,31,32,33,34], yet the functional analysis of Hox genes in Nematostella indicate that some aspects of genomic organization and A–P patterning were present in the cnidarian ancestor
It will be interesting if the study of other cnidarian species reveal an additional role for Hox genes during early development, which will help resolve the shared and derived characters of cnidarian and bilaterian Hox genes
All experiments exhibited greater than 50% penetrance of phenotypes, with 75% of the data containing greater than 75% penetrance
Summary
Hox genes are expressed prior to germ layer segregation. We analyzed the expression of Hox and neighboring non-Hox homeobox genes in Nematostella throughout early development (Fig. 2a), using a highly sensitive method (quantitative PCR) and verified expression by in situ hybridization. mRNA of both anterior and central/posterior Hox genes were detected early in development (Fig. 2b). The endomesodermal marker, NvSnailA, is reduced in NvAx6 knockdown experiments and is lost in NvAx1 overexpression treatments (Fig. 5a, c–e) In these two treatments, embryos continue to gastrulate (Fig. 3l–m), fail to form the pharynx, and do not express the pharyngeal marker NvFoxA or the oral marker NvBrachyury that is expressed at the ectodermal/. Controls were injected with either NvAx6 or NvAx1 morpholinos with a standard control morpholino and produced phenotypes identical to those seen with normal single gene-specific morpholino injections (Fig. 6) Embryos treated with both NvAx6 and NvAx1 morpholinos undergo gastrulation to form an outer ectoderm (Ec) and an inner endomesoderm (En) that expresses the maker NvSnailA, but fail to form a pharynx(P) or express the pharyngeal markers NvFoxA and NvBrachyury (Fig. 6). NvFoxA NvWnt[3] NvBrachyury, NvWntA, NvWnt[1], NvWnt[4] NvWnt[2] NvSnailA NvSix3/6 NvFgf2A, NvSfrp1/5, NvDkk1/2/4
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