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Figure 10 | BMC Evolutionary Biology

Figure 10

From: A clustered set of three Sp-family genes is ancestral in the Metazoa: evidence from sequence analysis, protein domain structure, developmental expression patterns and chromosomal location

Figure 10

Evolution and orthology of the Sp genes in the Metazoa. The ancestral state in the Metazoa (metazoan grade) is hypothesized to have been a cluster of three Sp genes (one of each clade, color coded as in Fig. 9) linked to a single Hox gene (small red box; no Hox cluster is present yet). This is consistent with the data from T. adhaerens. Linkage data for the Sp1-4 gene, however, are missing and the nature of Trox2 gene is debated (indicated by the dashed lines and the question mark). In addition, data from the most basal metazoan group, the Porifera, are not yet available. The Sp gene cluster is conserved in the Eumetazoa (eumetazoan grade) and is linked to the Hox cluster. This is consistent with data from N. vectensis, and is further supported by comparative genomics [50]. Further evolution in the Vertebrata lineage lead to the multiplication of the Sp gene cluster along with the Hox gene cluster. The number of Sp gene clusters, their chromosomal location and Sp gene complement is fully compatible with the sequence (D(A(C, B))) of vertebrate Hox gene cluster duplication proposed by Bailey et al. [88]. We propose that the ancestral Sp gene cluster was duplicated, the duplicate lost the Sp5/btd gene, and this reduced cluster served as template for two additional Hox/Sp duplications. In the Insecta lineage the ancestral linkage of the Sp cluster with the Hox cluster was partially disbanded by the relocation of the Sp5/btd and Sp6-9 genes.

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