Parazoa Genomics

Cross reference:  Ediacaran Period    Placozoa    Porifera       
Genetics in General   Genomics in General   Genomic Analysis  
Ediacaran Genomics     Coelenterata Genomics        

The Trox-2 Hox/ParaHox Gene of Trichoplax (Goog) 
    "Hox and ParaHox genes are implicated in axial patterning of cnidarians and bilaterians, and are thought to have originated by tandem duplication of a single ldquoProtoHoxrdquo gene followed by duplication of the resultant gene cluster."  "Trox-2 is expressed in a ring around the periphery of Trichoplax, in small cells located between the outer margins of the upper and lower epithelial cell layers. Inhibition of Trox-2 function, either by uptake of morpholino antisense oligonucleotides or by RNA interference, causes complete cessation of growth and binary fission.

The Trichoplax Genome and the Nature of Placozoans (Goog)   
"Despite the apparent cellular and organismal simplicity of Trichoplax, its genome encodes a rich array of transcription factor and signaling pathway genes that are typically associated with diverse cell types and developmental processes in eumetazoans, motivating further searches for cryptic cellular complexity and/or as yet unobserved life history stages." 
    "The compact genome shows remarkable complexity, including conserved gene content, gene structure, and synteny relative to the human and other eumetazoan genomes. Despite the absence of any known developmental program and only a modest number of cell types, the Trichoplax genome encodes a rich array of transcription factors and signaling genes that are typically associated with embryogenesis and cell fate specification in eumetazoans, as well as other genes that are consistent with cryptic patterning of cells or unobserved life history stages and/or complex execution of biological processes such as fission and embryonic development in these enigmatic creatures." 
    "Analysis of the exon-intron structure of orthologous genes demonstrates a high degree of conservation in Trichoplax relative to other eumetazoans, extending the antiquity of many animal introns. For example, in conserved regions, 82% of human introns have orthologous counterparts with the same position and phase in Trichoplax."   

Analysis of the sponge [Porifera] gene repertoire: implications for the evolution of the metazoan body plan. (
PubMed) - 2003   
Only abstract available online.  
Sponges [phylum Porifera] form the basis of the metazoan kingdom and represent the evolutionary earliest phylum still extant. Hence, as living fossils, they are the taxon closest related to the hypothetical ancestor of all Metazoa, the Urmetazoa.  
    Until recently, it was still unclear whether sponges are provided with a defined body plan. Only after the cloning, expression and functional studies of characteristic metazoan genes, could it be demonstrated that these animals comprise the structural elements which allow the sponge cells to organize themselves according to a body plan." 
    "In addition, a neuronal receptor has been identified, which--together with the identified neuroactive molecules--indicate the existence of a primordial neuronal network already in Porifera.  
    The primmorph system, aggregated cells that retain the capacity to proliferate and differentiate, has been used to demonstrate that a homeobox-containing gene, Iroquois, is expressed during canal formation in primmorphs." 
    "Already at least one morphogen has been identified in sponges, myotrophin, which is likely to be involved in the axis formation. Taken together, these elements support the recent conclusions that sponges are not merely nonorganized cell aggregates, but already complex animals provided with a defined body plan.