Calcisponges have a ParaHox gene and dynamic expression of dispersed NK homeobox genes

摘要

Sponges are simple animals with few cell types, but their genomes paradoxically contain a wide variety of developmental transcription factors, including homeobox genes belonging to the Antennapedia (ANTP) class, which in bilaterians encompass Hox, ParaHox and NK genes. In the genome of the demosponge Amphimedon queens-landica, no Hox or ParaHox genes are present, but NK genes are linked in a tight cluster similar to the NK clusters of bilaterians. It has been proposed that Hox and ParaHox genes originated from NK cluster genes after divergence of sponges from the lineage leading to cnidarians and bilaterians. On the other hand, synteny analysis lends support to the notion that the absence of Hox and ParaHox genes in Amphimedon is a result of secondary loss (the ghost locus hypothesis). Here we analysed complete suites of ANTP-class homeoboxes in two calcareous sponges, Sycon ciliatum and Leucosolenia complicata. Our phylogenetic analyses demonstrate that these calcisponges possess orthologues of bilaterian NK genes (Hex, Hmx and Msx), a varying number of additional NK genes and one ParaHox gene, Cdx. Despite the generation of scaffolds spanning multiple genes, we find no evidence of clustering of Sycon NK genes. All Sycon ANTP-class genes are developmentally expressed, with patterns suggesting their involvement in cell type specification in embryos and adults, metamorphosis and body plan patterning. These results demonstrate that ParaHox genes predate the origin of sponges, thus confirming the ghost locus hypothesis, and highlight the need to analyse the genomes of multiple sponge lineages to obtain a complete picture of the ancestral composition of the first animal genome.%海绵简单的身体构造掩饰了它们基因组的复杂性，其基因组包含各种在更复杂动物的身体成形中所涉及的转录因子，如“两侧对称动物”(前后侧明显不同的动物)的“触角足”类基因，其中包括Hox、ParaHox和NK。“寻常海绵”Amphimedon queenslandica的基因组包含—簇紧密的NK基因，但没有Hox或ParaHox基因。这导致以下假说：Hox和ParaHox基因是在海绵从导致“刺胞动物”(水母、海葵和其他)和“两侧对称动物”出现的类别分化出来之后才形成的。然而，对海绵基因组的整体结构所做的研究表明，Hox和ParaHox也许曾经存在过、但后来消失了，这就是所谓的“幽灵位点”假说。Maja Adamska及同事研究了两种“钙质海绵”(与“寻常海绵”不同的一个类别)的基因组，发现虽然NK基因没有成簇，但至少有一个ParaHox基因(Cdx)。这个发现为“幽灵位点”假说增加了说服力，也说明需要对很多不同海绵类别的基因组进行研究，以便对先祖动物的“触角足”基因有更全面的认识。