Placozoans
The Simplest of All Known
Animals
Placozoans
• Discovered in the late 1880's living on the glass walls of
an aquarium in a European laboratory and rediscovered
in the 1970’s.
• Found throughout tropical and subtropical oceans in
nearshore habitats, particularly mangrove
communities.
• Placozoans are readily collected in the wild and can be
maintained in the laboratory on diverse food sources.
Placozoans
• Although placozoans found in diverse locations are
morphologically indistinguishable, they show
surprising diversity at the DNA level, suggesting that
cryptic species may exist.
• The only named species in the phylum is Trichoplax
adhaerens F. E. Schulze.
• Not surprisingly, given their small size and squishy
nature, fossil placozoans have yet to be discovered.
Bauplan
• Exclusively marine
• Unique
• Simple, irregular sandwich organization
– An upper and a lower epithelium surround a loose
network (not an epithelium) of so-called fiber
cells
• Looks like an irregular “hairy plate”
(“Tricho plax”)
Bauplan
• Known from only a
single species
– Trichoplax adhaerens.
– Looks like a very large
amoeba, BUT it is
composed of a few
thousand cells
– No organs or tissues
– Has a top and a bottom
but is otherwise
asymmetrial
Bauplan
• Trichoplax appears as a flat disc of cells consisting
of two epithelial layers, which sandwich a layer of
multinucleate fibre cells (Fig. 1a).
– a. Trichoplax in laboratory culture; scale bar, 200 m.
• To feed, Trichoplax climbs atop its food using the
bottom surface as a temporary extraorganismal
gastric cavity; digestion is both extracellular and
phagocytic.
• When not feeding, the animals move by cilia on the
bottom surface and by the fibre cell layer.
Placozoan body plan and reproduction.
M Srivastava et al. Nature 454, 955-960 (2008) doi:10.1038/nature07191
Exodigestion in Trichoplax adhaerens
Bauplan
• No organs
• No specialized nerve or muscular cells
• No basal lamina
• No extracellular matrix
• Body shape is irregular and changes constantly.
• No evident body axes other than top versus
bottom and periphery versus interior
• No oral-aboral or even a dorso-ventral polarity
exists.
Morphology
• Trichoplax adhaerens.
– 2 cell layers thick
• Bottom epithelium
– Columnar cells each with a single flagellum
– Glandular cells secrete digestive enzymes
– Digestion is extracellular
• Top epithelium also has flagellated cells
– In between layers is a fluid-filled space with dense
network of contractile fibrous cells
– Fully mobile
• Swims (planktonic)
• Crawls on substrate
cilium
• Size: up to 2mm
• Only four cell types have been described in Trichoplax
– Upper epithelial cells
– Lower epithelial cells
– Gland cells within the lower, feeding epithelium,
– Fiber cells sandwiched between the epithelia
Phylogeny
• Phylum (Placozoa)
– Genus (Trichoplax)
– species (adhaerens).
• A recent study using a group of genes from the
completed nuclear genome placed Trichoplax basal to
Eumetazoa with Porifera branching off first (Srivastava
et al. 2008).
– Contains signature sequences for gene regulation
found in more complex animals and humans
– Trichoplax shares over 80 percent of its genes with
humans
Metazoan phylogeny and Trichoplax.
Bayesian phylogeny of metazoans places Trichoplax as the sister
group to cnidarians and bilaterians.
M Srivastava et al. Nature 454, 955-960 (2008) doi:10.1038/nature07191
Phylogeny
• In contrast to the commonly proposed ‘Poriferabasal-model’ another comprehensive study found an
early evolutionary split into two sister clades: the
‘bilateria’ (or triploblasts) and the ‘non-bilaterian’
(or diploblasts).
– The latter comprises the basal metazoan clade
Placozoa, Porifera, Cnidaria and Ctenophora,
with placozoans being basal within this clade.
– American Museum of Natural History (2009, January 26). New Tree
Of Life Divides All Lower Metazoans From Higher Animals,
Molecular Research Confirms. ScienceDaily. Retrieved February
15, 2012, from http://www.sciencedaily.com/releases/2009/01/090126203157.htm
Amphimedon queenslandica
Demospongia
•
More recent paper looks at genome of a, Amphimedon queenslandica
adult. Scale bar, 5 cm. b, Embryos in a brood chamber. Scale bar,
1 mm. c, Larva. Scale bar, 100 μm. d, Animal phylogeny based on
whole-genome data. This unrooted tree is inferred from 229
concatenated nuclear protein-coding genes with 44,616 amino acids
using Bayesian inference. All clades are supported with a posterior
probability of 1. Coloured boxes mark the nodes for which origins
of genes are inferred in Figs 3 and 4. The same topology is
supported by the nuclear gene data sets generated by alternative
methods as well as by other inference methods (Supplementary
Note 7). The metazoan stem leading to the animal radiation is
shown in bold. Contrary to the current consensus of eukaryotic
relationships, Amoebozoa are not a sister-group to Opisthokonta in
this tree (Supplementary Note 7).
Amphimedon life history and metazoan phylogeny.
choanoflagellate
M Srivastava et al. Nature 466, 720-726 (2010) doi:10.1038/nature09201
Reproduction
• Asexual
–
–
–
–
Budding
Fragmentation
Binary fission
Regeneration
• Sexual?
– Each individual probably produces 1 egg
– Forms a blastula embryo
– Not many other details are known
Placozoan body plan and reproduction.
M Srivastava et al. Nature 454, 955-960 (2008) doi:10.1038/nature07191
Trichoplax adhaerens
• It can eat through any part of its body surface,
• It can change shape and squish around,
• If you chop it in half, you get two smaller
Trichoplaxes,
• Even if it gets all mashed up, it re-forms,
• It’s very hard to see – light shines straight through
it,
• It’s unlike any other animal in the animal kingdom.
Developing egg?
Other Mesozoans
• Phylum Monoblastozoa
– Salinella salve
• May not even exist!
• Phylum Rhombozoa
– Class Dicyemida
– Class Heterocyemida
• Phylum Orthonectida
• Will study these later in chapter
Metazoan Multicellularity
and Cancer
• The emergence of multicellular animals from singlecelled ancestors over 600 million years ago required
the evolution of mechanisms for coordinating cell
division, growth, specialization, adhesion and death.
• Dysfunction of these mechanisms drives diseases such
as cancers, in which social controls on multicellularity
fail, and autoimmune disorders, in which distinctions
between self and non-self are disrupted.
• The hallmarks of metazoan multicellularity are
therefore intimately related to those of cancer and
immunity.
Origins of complexes and pathways of bilaterian cell types.
M Srivastava et al. Nature 466, 720-726 (2010) doi:10.1038/nature09201