Data from: Minimal ProtoHox cluster inferred from bilaterian and cnidarian Hox complements

Bilaterian animals have a Hox gene cluster essential for patterning the main body axis, and a ParaHox gene cluster. Comparison of Hox and ParaHox genes has led workers to postulate that both clusters originated from the duplication of an ancient cluster named ProtoHox, which contained up to four genes with at least the precursors of anterior and posterior Hox/ParaHox genes. However, the way in which genes diversified within the ProtoHox, Hox and ParaHox clusters remains unclear because no systematic study of non-bilaterian animals exists. Here we characterize the full Hox/ParaHox gene complements and genomic organization in two cnidarian species (Nematostella vectensis and Hydra magnipapillata), and suggest a ProtoHox cluster simpler than originally thought on the basis of three arguments. First, both species possess bilaterian-like anterior Hox genes, but their non-anterior genes do not appear as counterparts of either bilaterian central or posterior genes; second, two clustered ParaHox genes, Gsx and a gene related to Xlox and Cdx, are found in Nematostella vectensis; and third, we do not find clear phylogenetic support for a common origin of bilaterian Cdx and posterior genes, which might therefore have appeared after the ProtoHox cluster duplication. Consequently, the ProtoHox cluster might have consisted of only two anterior genes. Non-anterior genes could have appeared independently in the Hox and ParaHox clusters, possibly after the separation of bilaterians and cnidarians.

两侧对称动物（Bilaterian animals）拥有对于体轴模式建成至关重要的Hox基因簇（Hox gene cluster）与ParaHox基因簇（ParaHox gene cluster）。对Hox与ParaHox基因的比较研究促使研究者推测，两类基因簇均起源于一个被称为ProtoHox基因簇（ProtoHox gene cluster）的古老基因簇的复制事件，该古老基因簇最多包含4个基因，至少包含前体轴与后体轴Hox/ParaHox基因的祖先序列。然而，由于尚未对非两侧对称动物（non-bilaterian animals）开展系统性研究，ProtoHox、Hox以及ParaHox基因簇内的基因分化模式仍不明确。本研究对两种刺胞动物（cnidarian）——星状海葵（Nematostella vectensis）与大乳头水螅（Hydra magnipapillata）的完整Hox/ParaHox基因全集与基因组组织结构进行了解析，并基于三项论据提出，ProtoHox基因簇的结构比此前设想的更为简洁。其一，两种刺胞动物均拥有类似两侧对称动物的前体轴Hox基因，但其非前体轴基因既不对应两侧对称动物的中枢Hox基因，也不对应其后体轴Hox基因；其二，星状海葵中存在两个成簇排列的ParaHox基因：Gsx基因以及一个与Xlox基因、Cdx基因同源的基因；其三，本研究未发现明确的系统发育学证据支持两侧对称动物的Cdx基因与后体轴Hox基因具有共同起源，因此二者可能是在ProtoHox基因簇复制事件之后才出现的。综上，ProtoHox基因簇或许仅包含两个前体轴基因。非前体轴基因或可独立起源于Hox与ParaHox基因簇，其出现时间大概率晚于两侧对称动物与刺胞动物的演化分化节点。