The Early ANTP Gene Repertoire: Insights from the Placozoan Genome

Abstract

The evolution of ANTP genes in the Metazoa has been the subject of conflicting hypotheses derived from full or partial gene sequences and genomic organization in higher animals. Whole genome sequences have recently filled in some crucial gaps for the basal metazoan phyla Cnidaria and Porifera. Here we analyze the complete genome of Trichoplax adhaerens, representing the basal metazoan phylum Placozoa, for its set of ANTP class genes. The Trichoplax genome encodes representatives of Hox/ParaHox-like, NKL, and extended Hox genes. This repertoire possibly mirrors the condition of a hypothetical cnidarian-bilaterian ancestor. The evolution of the cnidarian and bilaterian ANTP gene repertoires can be deduced by a limited number of cis-duplications of NKL and “extended Hox” genes and the presence of a single ancestral “ProtoHox” gene.

Never Ending Analysis of a Century Old Evolutionary Debate: “Unringing” the Urmetazoon Bell

“…Canning and Okamura, 2004). Another open question concerns the use of key developmental genes as indicators of phylogenetic position, such as the Hox gene system (Kuhn et al, 1999;Schierwater and Desalle, 2001;Schierwater et al, 2008) or the mesodermal gene system (Ryan et al, 2013). These approaches will need to be considered if the now classical view of evolution and development, where a genetic toolbox exists in the common ancestor of organisms, is a valid one for this part of the tree of life.…”

“…The subject of whether cnidarians possess true Hox genes remains controversial, analysis of genome synteny suggesting that Nematostella vectensis has a set of genes Hox genes, many of which are the result of independent duplications, and a separate set orthologous to ParaHox genes Ryan et al, 2006;Hui et al, 2008). The simplest explanation of these data is that the oldest ANTP class genes are the NK-like homeobox genes, which date to the origin of animals, and that the Hox and ParaHox genes arose rather later in evolution by tandem duplication and divergence from NK-like genes (Castro and Holland, 2003;Larroux et al, 2007;Schierwater et al, 2008). In this scenario, the birth of Hox genes occurred in the evolutionary lineage leading to cnidarians and bilateria ns, after this lineage had diverged from sponges and ctenophores.…”

“…In this scenario, the birth of Hox genes occurred in the evolutionary lineage leading to cnidarians and bilateria ns, after this lineage had diverged from sponges and ctenophores. The placozoan Trichoplax adhaerens could occupy a pivotal position in this important evolutionary transition, because current data indicates it contains just a single Hox/ParaHox-related gene, Trox-2 (Schierwater and Kuhn, 1998;Monteiro et al, 2006;Schierwater et al, 2008;Srivastava et al, 2008). This gene has a protein sequence similar to the ParaHox gene Gsx, but could conceivably be representative of the elusive progenitor gene, ancestral to Hox and ParaHox, denoted the ProtoHox gene.…”

The cytochrome P450 genesis locus: the origin and evolution of animal cytochrome P450s

“…In Trichoplax. MYB is next to the trox-2 (Gsx) gene, the only Hox/ParaHoxtype gene in Trichoplax and proposed as a possible ancestral ProtoHox gene [84]. The linkage provides another tie between Clan 7 and the Hox genes.…”

Nuclear Receptor Genes: Evolution

“…The general consensus places Porifera before Cnidaria; however, the placement of Placozoa is hotly debated and may constitute the first metazoan or the first triploblast (Schierwater et al, 2008;Srivastava et al, 2008). These characterized branches of the metazoan tree demonstrate 2 NRs in living sponges (Porifera) ( Figure 1a and Table 1), 4 NRs in the Placozoan Trichoplax and the 21 NRs currently (middle of 2009) available for the phylum Cnidaria.…”