Trichoplax adhaerens

Today I learn a species/ a clade that is thought to be the most primitive metazoan form.


Greek “tricha" = ‘hair’ and “plax" = ‘plate’, Latin “adhaerere" = ‘to stick’

Figure 1 from "Chasing the urmetazoon: Striking a blow for quality data?" by Osigus et al,, 2013 ( doi:10.1016/j.ympev.2012.05.028) (A) Photograph of Trichoplax adhaerens, Schulze (1883). For additional images of placozoan specimens see www.trichoplax.com. (B) Modern placula hypothesis of metazoan origin (for details see Schierwater et al., 2009a). (from Schierwater et al., 2009a).

And the phylogenetic location of this clade:

Maximum likelihood phylogenetic tree of metazoan relationships using a concatenated data matrix. Figure 3 from "Global Diversity of the Placozoa" by Eitel et al., 2013 (doi: 10.1371/journal.pone.0057131)

The body is in this structure:

Revised schematic cross sections of a Placozoon.  Figure 2 from "Global Diversity of the Placozoa" by Eitel et al., 2013 (doi: 10.1371/journal.pone.0057131)

And total ~10 types of cells are identified. 

Trichoplax has a small genome (smallest metazoan genome?) in comparison to other animals, nearly 87% of its 11,514 predicted protein-coding genes are identifiably similar to known genes in other animals.

wiki https://en.wikipedia.org/wiki/Trichoplax

Some interesting references:

• Global Diversity of the Placozoa PLoS One. 2013; 8(4): e57131. 
• My favorite animal, Trichoplax adhaerens  BioEssays 27:129 4–13 02, ß 2005 
• Genome of T. adhaerens (by JGI)  @ NCBI

Trichoplax adhaerens

Representative genome: Trichoplax adhaerens (assembly v1.0)
Download sequences in FASTA format for genome, transcript, protein
Download genome annotation in GFF, GenBank or tabular format
BLAST against Trichoplax adhaerens genome, transcript, protein 

T. Driscoll, J. J. Gillespie, E. K. Nordberg, A. F. Azad, B. W. Sobral, Bacterial DNA Sifted from the Trichoplax adhaerens (Animalia: Placozoa) Genome Project Reveals a Putative Rickettsial Endosymbiont, Genome Biology and Evolution, 2013, 5, 4, 621CrossRef

Tracing the published BioProject of Placozoa, I found this paper:

• Early origins and evolution of microRNAs and Piwi-interacting RNAs in animals.", Nature, 2008 Oct 1;455(7217):1193-7

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Digest from  the 8x assembly manuscript

The Trichoplax genome and the nature of placozoans

by Srivastava M et al., 2008. Nature 454, 955-960 (21 August 2008) | doi:10.1038/nature07191; 

[Backgrounds]

• Ecological Location:
  throughout tropical and subtropical oceans in nearshore habitats, particularly mangrove communities
• Classifications and Nomenclature:
  The only named species in the phylum is Trichoplax adhaerens, while the morphologically indistinguished individuals from various locations shows great divergency in DNA level, suggesting cryptic species may exist.
• Body Structure:
  a flat disc of cells consisting of two epithelial layers, which sandwich a layer of multinucleate fibre cells. Only four cell types have been described previously. Nerves, sensory cells and muscle cells are apparently absent. 
  No evident body axes other than top versus bottom and periphery versus interior are defined. They show no regular directionality in their movement.
• Known Behaviors:
• Moving:
  the animals move by cilia on the bottom surface and by the fibre cell layer. 
• Feeding:
  Trichoplax climbs atop its food using the bottom surface as a temporary extraorganismal gastric cavity; digestion is both extracellular and phagocytic.
• Breeding:
  In culture, Trichoplax reproduces by fission, whereby two (sometimes three) parts of the animal move away from each other until their connection is ruptured.
  Sexual reproduction is suggested but has not been observed.  Putative oocyte formation in degenerating animals is routinely seen. These large cells have been observed to undergo cleavage up to a 256-cell stage before degenerating (unpublished observations). Sperm have been described once without other investigators proofs. Population genetic analyses, however, demonstrate allelic variation and evidence for genetic recombination in animals in the wild that is consistent with sex.

[genome]

• karyotype study (from another paper: On the Karyotype of Trichoplax sp. by Birstein, V.J 1989 )
  
  • 2n=12.  3 pairs of bi-armed (meta- or submetacentrics) and 3 pairs of acrocentrics.
  • dorsal / ventral layers are in 2n; intermediate are possibly tetraploid.
  • Comparing to other primitive metazoans:
    ** Turbellaria (渦蟲): 2n=10~20, C ~0.4 to 1.2 pg. and often bi-armed.
    ** Sponges and polyp (Cnideria) C = 0.5~0.6 pg, 10x to Trichoplax. Other Cnideria shows even larger C value.  Chromosome number of sponges may be double to Trichoplax (2n=24) or even in polyploid way (4n ~44 or 6n ~66)
  • The genome size and chromosome number are comparable to Protozoans.
  ** Karyotypes of sp in Protomonadida, Polymastigida are in 6~10.
  ** some parasitic flagellates (Trypanosoma, Leishmania) are in 0.1~0.2pg/nucleus (2n ?)
  
• Nucleus genome
  The genome size was estimated at 0.08 pg/haploid genome using Feulgen staining.
  
  Because there are at present no genetic or physical maps of Trichoplax, we could not reconstruct entire chromosomes, but the completeness of the draft assembly (98% of the 14,571 expressed sequence tags (ESTs) align) and its long-range linkage (19 scaffolds longer than 1 Mb represent 80% of the assembly) make it an excellent substrate for annotation and comparative analysis (Supplementary Information).
  
  source: The "Grell" strain, a single clonal lineage kept from 1969. Animals were grown on a monoculture of the cryptophyte alga Pyrenomonas helgolandii. 12,000 individual Trichoplax specimens were picked by hand, washed individually and subjected to DNA extraction using a standard protocol


• Mitochondria (reported previously)

An estimated 8 million metazoan species currently inhabit the Earth's aquatic and terrestrial environments, a predicted 86% of which have not been characterized (Mora, Tittensor, Adl, Simpson, & Worm, 2011).