Abstract
Recently it was shown that the closest invertebrate relatives to vertebrates are the tunicates rather than the much more morphologically similar cephalochordates [1]; while tunicate adult morphologies are very diverse their embryonic morphology displays a typical chordate body plan much like that of cephalochordates. In support of this position of tunicates on the phylogenetic tree of life tunicates also possess putative primitive vertebrate innovations. True vertebrate innovations are a set of structures exclusive to vertebrates, the gain of which are thought to have contributed to the transition of the ancestral vertebrate from sessile filter feeding to active predation [2, 3]. The accompanying increase in the organismal complexity seen in the early vertebrate lineage is thought to have been driven by the large amount of raw genetic material generated by the whole genome duplications at the base of the vertebrate lineage [4, 5].
Whole genome duplications create a large amount of redundant genetic material; for the most part the redundant genes are lost, however those genes that are retained undergo structural and functional evolution [6]. Certain types of genes are more likely to be retained after whole genome duplications than others, among these are the highly conserved set of developmental regulators known as the developmental toolbox [7]. The Sox superfamily are members of the developmental toolbox, they are divided into several conserved subgroups (SoxB1, SoxB2, SoxC, SoxD, SoxE and SoxF), these are involved in almost every aspect of vertebrate development, including the development of vertebrate innovations [8].
We compared developmental Sox expression patterns in a tunicate (Ciona savignyi) and vertebrates and were able to show that, potentially, there are several conserved expression patterns of different Sox genes between C. savignyi and vertebrates. This includes conservation of SoxB1 in early neuroectoderm, SoxC in mesenchyme, SoxE in neural tissue and SoxF in endoderm. In adult C. savignyi gonadal tissue SoxE expression was shown to be expressed in the testis, which is also found in vertebrates, and all of the C. savignyi Sox genes examined were found to be maternally expressed in Ciona.
This thesis sheds light on the expression patterns of the conserved C. savignyi Sox genes during C. savignyi development and in adult gonads. By comparing these expression patterns to the already well-studied expression of homologous Sox genes during vertebrate development we can infer that some of the expression patterns seen in during C. savignyi development have been potentially conserved from the last common ancestor of chordates. However more information about the expression patterns of Sox genes in other tunicate species is needed to substantiate our conjectures.