Comparative Genomics and Transcriptomics in Placozoa
We have been sequencing and are currently analyzing several genomes and transcriptomes of placozoan species. We expect valuable insights into the phylogeny, genetic diversity and mechanisms of selection of an entire phylum. Placozoa have become an emerging model system for evo-devo research because of their simple bauplan and a basal position in the tree of life. We propose that placozoans are also well suited to become a model phylum in marine comparative genomics and ocean climate studies because: 1. they are abundant throughout the oceans in benthic communities 2. different lineages differ in their geographical distribution and associated tolerance for environmental factors 3. their compact genome is comparatively time and cost efficiently to sequence 4. assumed passive dispersal along oceanic currents allows to study effects of migration and local adaptation.
Placozoan genome or transcriptome sequencing projects in our group projected onto a 16S phylogenetic tree: A finished sequencing project of a placozoan 16S haplotype is marked with a green star. A sequencing project in progress is indicated by a yellow star. The coloration of the placozoan clades follows that of Eitel et al., 2013.
The Enigmatic Phylum Placozoa
The first placozoan species, Trichoplax adhaerens, was discovered by F.E. Schulze in 1883  in aquarium samples. More members (species, yet undescribed) of this phylum have been found in shallow tropical, sub-tropical and temperate waters [2,3]. They are irregular disc-shaped benthic animals of only a few millimeters which crawl over hard substrates. Placozoans exhibit the simplest morphology of all living metazoans, harboring only six somatic cell types [4,5] that are arranged in an upper epithelium, a lower epithelium and an intermediate layer (for review see ). This simple bauplan and a supposed basal position of placozoans in the tree of life has led to many speculations regarding the origin of Metazoa . Consequently the Trichoplax mitochondrial  and nuclear  genomes have been among the first sequenced non-bilaterian genomes and support the phylum's basal position .
(A) Schematic cross section of a placozoan. Placozoans possess two epithelia, a lower epithelium (le) facing the substrate and an upper epithelium (ue) facing the water. In between lie several layers of syncytial fiber cells (fc). Marginal cells (mc) are thought to represent pluripotent stem cells. The “shiny spheres” (ss) are lipid filled droplets in the upper epithelium probably used as a chemical defense mechanism (from Eitel et al., 2013). (B) Live placozoan on a rock grazing on algae. (C) Placozoan on a microscope slide.
A Case to Sequence the Phylum Placozoa
The fact that T. adhaerens has remained the only described species can mainly be attributed to the uniform morphology of the phylum and to the difficulty of observing placozoans in the field. This view changed when genetic studies by Voigt et al. and Eitel et al. [3,11,12] revealed substantial diversity within the phylum. Moreover, different lineages show remarkable differences in their geographical range and tolerance for environmental factors. For example, the cosmopolitan haplotype H2 has been found in tropical to temperate waters in all oceans, while the haplotype H3 seems to be restricted to the Caribbean. Currently the Placozoa consists of around 25 lineages, presently defined by their 16S haplotypes [3,12]. While their genetic distances as well as physiological differences leave no doubt that they represent valid species, attempts for species descriptions have only been partially successful, again, mainly due to the simple morphology of placozoans .
To resolve the tabula rasa Placozoa and to assess the phylum’s genetic diversity, the genomes and transcriptomes of the major lineages are currently sequenced, a process which is greatly facilitated by the small (100Mb) genome size of placozoans. In the future we plan to establish a genotyping-by-sequencing approach  to study global population dynamics of placozoans.
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