Phylum Platyhelminthes Class Turbellaria
Various levels of complexity reflected
in the type of digestive system
What were the first bilateria?
Are the acoelomate flatworms a
good MODEL for the earliest
Bilateria
Implications
Nutritive cells Simple pharynx
Simple gut
Bulbous
Pharynx
-- Size
--Phylogeny
Order Acoela
3-lobed gut
tubular pharynx
Rhabdocoels
many-lobed gut
Polycladia
Tricladia
Two perspectives on the position of flatworms in
the evolutionary history of animals
Cnidaria
Planuloid-Acoeloid
Theory
UrBilateria
Small, planktonic,
reproductive “larva”
w/ nervous system
Septate junctions,
descended from
Homoscleromorph
Cnidaria planula
•  Simplicity of flatworm body plan is retained from a simple
bilaterally symmetrical ancestor- they are primitively simple
Flatworms
and
Bilateria
*lack of a coelom, of complete digestive & circ. system
Planula of Cnidaria and adult Acoels considered almost
unchanged descendants of a similar common ancestor =
the Neurogastrea according to Nielsen (2008)
Two perspectives on the position of flatworms in
the evolutionary history of animals
Archecoelomate
Theory
“Acoelomorph-like” Worm
Planuloid-Acoeloid
Theory
UrBilateria
UrBilateria
•  Ancestor of all bilateria was a coelomate organism with a
complete gut - therefore flatworms are secondarily simple
*loss of a coelom, complete digestive/circ. system is a
secondary condition derived from coelomate ancestor.
Archecoelomate
Theory
UrBilateria
“We are asked to believe that the Turbellaria have lost their coelom,
their anus, lost their nephrostomes. This is asking too much of one’s
credulity.” Libbie H. Hyman 1959
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Are Flatworms Derived
Protostomes?
--some species have spiral
determinate cleavage
Ruiz-Trillo et al. Science 1999
18S ribosomal DNA genes including analysis of acoela
Other Protostomes
Ecdysozoa
DeuteroPolycladia
Acoela
stomes
acoelomate
--mouth forms before the anus,
and it forms from the blastopore
-- no coelom...so we cannot say if
they undergo enterocoely or
schizocoely
-- Fee Living Flatworms Have:
•  No coelom: acoelomates
•  No thru gut or vascular system: gastrovascular cavity
•  Primitive kidneys: protonephridia
All characters that are considered primitive
-- They also have:
•  Quartet spiral determinate cleavage
•  Mouth forms before the anus in embryology
All are characters of the derived protostome lineage
BUT ALL BASED PRIMARILY ON STUDIES OF THE
COMMON, LARGER POLYCLAD WORMS
Embryological Patterns:
•  Animal pole views of the typical quartet spiral cleavage
coelom
Diploblastica
Triploblastic
acoelomate
So molecular sequence analyses
Do not yet resolve the questions!!
Are Platyhelminthes polyphyletic
or monophyletic?
Are acoel flatworms derived
protostomes or stem bilateria?
What other evidence can we
explore to solve this debate?
• Animal pole view of DUET SPIRAL CLEAVAGE in
acoelomorphs
that is displayed by most POLYCLAD turbellarians...this
is a typical spiral pattern
Henry et al. 2000 for Neochildia fusca
Boyer et al. 1998 for Holoplana inquilina, a polyclad
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Protoostomia
Acoel flatworms --- duet spiral
Deuterostomia
Cnidaria --- radial
Nemertodermatida
The cleavage pattern of acoels appears to be COMPLETELY DIFFERENT
from any other phylum specific program.
Acoela
CLEAVAGE; it is an unique cleavage pattern seen in
ACOELS.
Acoels show an unique form of development involving DUET SPIRAL
CLEAVAGE, in CONTRAST to the QUARTET SPIRAL PATTERN of
Spiralia, including most turbellarians
Cnidaria
•  This is a 9 cell stage that illustrates DUET SPIRAL
radial
Deuterostomes--- Radial
Duetlike
Protostomes --- spiral’
Polyclad flatworms --- spiral
spiral
radial
radial
Mueller’s Larva of
Many Polyclad
Flatworms
Acoel flatworms
have direct
development ((
(no larva)
Polyclad flatworm
Acoel flatworm
Developmental diversity in free-living flatworms
Martín-Durán and Egger
Martín-Durán and Egger EvoDevo 2012, 3:7
http://www.evodevojournal.com/content/3/1/7 (19 March 2012)
•  Complex of genes coding for DNA-binding transcriptional
regulators.
•  First discovered in fruit fly in which they regulate
segment identify during early development
•  Mutations cause phenotypes of homeosis (segments are
transformed into the likeness of a different segment)
•  Striking property of this complex is colinearity
• A-P body plan in other bilaterally symmetrical animals,
(including mice) is determined by Orthologs of
the fruit fly genes.
Hox and ParaHox genecomplex evolution
HOX genes proper (HOX and
Parahox) appear to be restricted
to bilaterians and diploblasts
Non HOX homeotic genes
probably arose in Protistans and
are certainly present in sponges
Cnidaria have a restricted Hox
gene set but they include
orthologs of protostome/
deuterostome Hox genes
Most bilaterans possess at least
8 distinct HOX clusters
hypothetical
Evolution of Hox Cluster: A New
Approach to Animal Phylogeny
Duplications
Orthologs: homologous sequences separated by “speciation event”
Paralogs: homologous sequences separated by a “duplication event”
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Proposed pattern of Hox
and ParaHox gene-complex
evolution
There is support, from both 18S rDNA, embryology and
the Hox gene sequences that:
1.
Acoels: basic Hox cluster with 4
Hox and 3 ParaHox genes
Most flatworms are aligned with the protostome groups
2. Acoelomorph
flatworms (acoels and a few other types)
are “stem” bilateria, the most “primitive” bilaterians in
existence (myosin gene sequences, more 18S-rDNA genes, mt DNA gene
Triclad, polyclad flatworms are
like the coelomates
sequences , embryology, HOX gene analysis)
?
Inferred to be
present ancestrally
but not yet
documented
There is support, from both 18S rDNA and the Hox gene
sequences that most flatworms are derived coelomates.
What can we infer from this evidence about:
LCB =
UrBilateria
1. The evolution of the coelom
2. The nature of the Urbilateria
3. The planuloid-aceoloid hypothesis
5A: basic Hox cluster with 4 Hox genes and 3 Parahox genes
5B: Expanded Hox cluster
7- through gut, 10- coelomic cavity, 11 segmentation
c: duet spiral cleavage
There is support, from both 18S rDNA and the Hox gene
sequences that most flatworms are derived coelomates.
How is the perception depicted in Zallinger’s “March of
Progress” relevant to the case of flatworm evolution?
Rudolph Zallinger’s March of Progress
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