Abstract
Group B of the Sox transcription factor family is crucial in embryo development in the insects and vertebrates. Sox group B, unlike the other Sox groups, has an unusually enlarged functional repertoire in insects, but the timing and mechanism of the expansion of this group were unclear. We collected and analyzed data for Sox group B from 36 species of 12 phyla representing the major metazoan clades, with an emphasis on arthropods, to reconstruct the evolutionary history of SoxB in bilaterians and to date the expansion of Sox group B in insects. We found that the genome of the bilaterian last common ancestor probably contained one SoxB1 and one SoxB2 gene only and that tandem duplications of SoxB2 occurred before the arthropod diversification but after the arthropod-nematode divergence, resulting in the basal repertoire of Sox group B in diverse arthropod lineages. The arthropod Sox group B repertoire expanded differently from the vertebrate repertoire, which resulted from genome duplications. The parallel increases in the Sox group B repertoires of the arthropods and vertebrates are consistent with the parallel increases in the complexity and diversification of these two important organismal groups.
Highlights
Sox (Sry-related high-mobility-group box) group B belongs to the Sox family of proteins, which are transcription factors essential in diverse developmental processes [1,2], including neurogenesis [3,4], gonadogenesis [5], and lymphopoiesis [6]
After earlier phylogentic analyses on the HMG superfamily involving the Sox family [10,11], the analysis conducted by Bowles et al (2000) based on the HMG domain sequences and other structural indicators, including intron positions, suggested that the Sox family can be classified into groups A–J [12]: A refers to the Sry proteins restricted to some mammals; B, C, D, E, and F are the major groups expressed by a broad range of metazoan taxa [13,14]; and G–J are particular lineage-specific proteins
Phylogenetic origin of the Sox subgroup B1/B2 Larroux et al (2008) suggests that the metazoan last common ancestor (LCA) had one or two proto-SoxB members, and because the genomes of the fungi and choanoflagellates, the closest relatives of metazoans, contain no Sox sequences, SoxB must have originated after the divergence of the metazoan and choanoflagellate lineages [13]
Summary
Sox (Sry-related high-mobility-group box) group B belongs to the Sox family of proteins, which are transcription factors essential in diverse developmental processes [1,2], including neurogenesis [3,4], gonadogenesis [5], and lymphopoiesis [6]. The Sox family was initially identified in relation to the mammalian testisdetermining factor, SRY, based on the sequence conservation of the single HMG (high-mobility group) domain, which is a domain of about 79 residues [7] that functions in DNA binding, DNA bending, protein interactions, and nuclear transport [1]. After earlier phylogentic analyses on the HMG superfamily involving the Sox family [10,11], the analysis conducted by Bowles et al (2000) based on the HMG domain sequences and other structural indicators, including intron positions, suggested that the Sox family can be classified into groups A–J [12]: A refers to the Sry proteins restricted to some mammals; B, C, D, E, and F are the major groups expressed by a broad range of metazoan taxa [13,14]; and G–J are particular lineage-specific proteins This transcription factor family first emerged in the stem of the metazoa, and the bilaterian last common ancestor (LCA) already contained all the major Sox groups in its genome [13,14]. SoxB1- and SoxB2-like proteins have been identified in the cnidarians [18] and demosponges [13,19], with much less confidence, which implies that the division into subgroups B1 and B2 might have taken place before the demosponges diverged from the eumetazoans [13]
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