Pax-6 origins – implications from the structure of two coral Pax genes

Abstract

Vertebrate Pax-6 and its Drosophila homolog eyeless play central roles in eye specification, although it is not clear if this represents the ancestral role of this gene class. As the most "primitive" animals with true nervous systems, the Cnidaria may be informative in terms of the evolution of the Pax gene family. For this reason we surveyed the Pax gene complement of a representative of the basal cnidarian class (the Anthozoa), the coral Acropora millepora. cDNAs encoding two coral Pax proteins were isolated. Pax-Aam encoded a protein containing only a paired domain, whereas Pax-Cam also contained a homeodomain clearly related to those in the Pax-6 family. The paired domains in both proteins most resembled the vertebrate Pax-2/5/8 class, but shared several distinctive substitutions. As in most Pax-6 homologs and orthologs, an intron was present in the Pax-Cam locus at a position corresponding to residues 46/47 in the homeodomain. We propose a model for evolution of the Pax family, in which the ancestor of all of the vertebrate Pax genes most resembled Pax-6, and arose via fusion of a Pax-Aam-like gene (encoding only a paired domain) with an anteriorly-expressed homeobox gene resembling the paired-like class.

Paxgene diversity in the basal cnidarianAcropora millepora(Cnidaria, Anthozoa): Implications for the evolution of thePaxgene family

“…The identification of a Pax-3͞7 gene in Acropora indicates that substantial diversification of the Pax family predates the Cnidaria͞higher Metazoa split, and the presence of common intron positions is consistent with the monophyly of the Pax family. We have elsewhere argued that Pax diversification accompanied and facilitated the elaboration of the nervous system (2,18,21). Consistent with this argument, we show that Pax-Cam is expressed in presumed neurons.…”

“…The construction of the cDNA library is described elsewhere (22). Details of the PCR primers, amplification conditions, and methods used for screening cDNA libraries have been described (18).…”

“…AJ249563) as well as C. quinquecirrha (19), H. littoralis, and H. magnipapillata (19,20). Although these genes have provided novel perspectives on Pax gene evolution (18)(19)(20), some speculation is involved in relating the known cnidarian genes to the Pax gene classes identified in higher animals. Additionally, nothing is known about the roles of these genes in lower Metazoa.…”

“…The rationale behind this assumption is that, in a representative basal animal, genes should be performing more restricted functions, and hence, ancestral roles may be more clearly seen. We previously identified two Pax genes in Acropora (18), one of which has orthologs in the hydrozoans Hydra littoralis (19) and Hydra magnipapillata (20) as well as the scyphozoan Chrysaora quinquecirrha (19). A third cnidarian Pax gene is known from the hydrozoan Podocoryne carnea; EMBL accession no.…”

Comparative genomic analysis reveals the evolutionary conservation of Pax gene family

“…Based on the overall structure, Catmull et al (1998) assumed that the Pax gene family was originated from PaxA precursor. Alternatively, the fact that in the Porifera and Placozoa only PaxB genes have been found led some authors to suggest that the first divergence of this family was PaxB gene (Miller et al, 2000).…”