Homeodomain containing transcription factors are vital for proper development and differentiation in animals, plants and fungi. This 60 amino acid conserved DNA binding motif is found in six different class of proteins: ANTP, PRD, POU, LIM, SIX and TALE. Huge gene diversity in Antennapedia class of homeodomain proteins account for evolution of body plans, regulation of body axis formation,symmetry and Nervous system.
The ANTP superclass consists of the Hox / ParaHox, extended Hox, and NKL genes (their relatedness complies with the above order). All Bilateria possess Hox /ParaHox, NKL, and extended Hox genes . and many of them serve conserved functions and most prominent example is of Hox genes,which pattern anterior posterior axis in all bilaterians in a similar manner. One can also site the famous NK 2 (tinman ) as an example for its role role in heart specification from annelids , flies to vertebrates.
It is estimated that cnidarians diverged from the bilateria at least 650 mya from a common ancestor .
Genome sequences from various organisms across animal kingdom provide valuable insights into the origin of the ANTP superclass genes , which are must for axis formation and hence body plans in all animals.Complete genome sequences are very valuable in assigning phylogenetic position for a particular organism , which is very much the case for only named Placozoan Trichoplax adherens . After the genome of this enigmatic organism , paving the way for identification of complete Antp class Homeodomain genes in these flat animals. One important aspect of Trichoplax genome is that genes are structured more or less the same as they are in vertebrates,which is also the case in recently sequenced other genomes like Nematostella (Cnidaria) and Amphioxus (Cephalochordate). 82% of human introns have orthologous counterparts with the same position and phase in Trichoplax, showing remarkably very little has changed as animal kingdom expandes in last 600 million years.
If the genome sequence helped out finding the evolutionary marvels in Platypus whereas coming to Trichoplax it added some more mysteries to this bizarre simple organism, while solving some like phylogenetic position.
First animal on earth ever to have Hox like gene,Trox-2:
Phylogenetic position of Trichoplax is always debated in the field. Results from Mito chondrial genome analysis placed it at the base of tree of animals Whereas early studies on the basis of a small number of genes suggested that placozoans could be secondarily simplified cnidarians.After complete genome sequence report by Mansi et.al, its becoming clear that divergence of Placozoa from other animals occured after the divergence of sponges but before (or close to) the cnidarian-bilaterian split. Whatever may be position of trichoplax but one thing for sure is that it is no longer a simplified Cindarian and lies below cnidarians (the more basal animal group before Trichoplax genome sequence with Hox genes).This makes Trichoplax the most primitive animal with Hox like gene in Trox-2.This hox like gene is expressed in scattered cells around the margin ,It is a kind of spatial localization; a change in gene regulation that involved a loss of expression over a part of its range would lead to an asymmetry that could be a precursor to defining an anterior-posterior axis.
Homeo domain genes in Trichoplax :
There are 37 Homeo box genes comprising of seven different classes that includes 14 Antp genes.Phylogenetics analysis grouped 11 out of 14 genes from Antp superclass ,which are Hox/ParaHox-like (Gsx), extended Hox (Not, Mnx),and NKL genes (Dlx, NK2, NK5, NK6, Hex and Dbx / Hlx).They lack Msx and Bar/Bsh related genes when compared to Amphimedon spone, indicating might have lost two NK genes during evolution. Results from the authors also confimed that Hox/ParaHox-like gene, Trox-2 (Gsx), may represent an ancestral as suggested by older data by jacob et. al (2004).The remaining three non clustered genes of Trichoplax (Hox / ext. Hox-related, NK related 1 & 2) ProtoHox gene may be result of recent duplication and rapid divergence.These data are consistent with a position of Placozoa ancestral to the cnidarian-bilaterian lineage.
Homeoox genes known from Amphimedon ,Trichoplax and Nematostella indicate that that the Placozoa possess significantly less homeobox diversity than Cnidaria but a similar diversity as found in Porifera.If you see there is not much difference in terms of sponge Homeobox genes except for presence of HOX/para Hox like gene (Gsx type gene, Trox-2) and extended Hox genes in Trichoplax.
Amphimedon has the smallest ANTP-class gene repertoire with 8 (all NK related) genes , while Trichoplax harbors 14 and Nematostella close to 80 ANTP class genes.
Comparison of the linkage patterns in sponges ,cnidarians and bilaterians suggests that most or all ANTP gene families have emerged from a single ancestral region and the remnants of linkage in Trichoplax conform to this scenario, some what similar to hypothetical Ureumetazoa.
1 ) Schierwater B, Kamm K, Srivastava M, Rokhsar D, Rosengarten RD, et al. (2008) The Early ANTP Gene Repertoire: Insights from the Placozoan Genome.
PLoS ONE 3(8): e2457. doi:10.1371/journal.pone.0002457
2 ) Srivastava M, Begovic E, Chapman J, Putnam NH, Hellsten U, Kawashima T, Kuo A, Mitros T, Salamov A, Carpenter ML, Signorovitch AY, Moreno MA, Kamm K, Grimwood J, Schmutz J, Shapiro H, Grigoriev IV, Buss LW, Schierwater B, Dellaporta SL, Rokhsar DS.
The Trichoplax genome and the nature of placozoans.
Nature. 2008 Aug 21;454(7207):955-60.