Download Flatworms and Evolution

Planuloid-Acoeloid
Theory
Urbilateria
UrBilateria
“primitively simple”
Archecoelomate
Theory
“secondarily simple”
UrBilateria
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
acoelomorph
- Fee living flatworms have:
•
•
•
All
polyclad
No coelom: acoelomates
No thru gut or vascular system: have a g-v cavity
Primitive kidneys: protonephridia
characters that are considered primitive
-- BUT They also have:
• Quartet spiral determinate cleavage
• Mouth forms before the anus in embryology
Embryological characters of advanced protostomes
However the embryological evidence is from studies of
the large POLYCLAD WORMS
Primitively simple?
Secondarily simple?
Why is flatworm
evolution important
to our understanding
of animal evolution?
Drawing by Charles Messing
Polyclad flatworm
More recent evidence:
Acoelomorph
flatworm
Acoelomorph embryology
Hox Genes
Genetic sequencing studies
Embryological Patterns:
• Animal pole views of the typical quartet spiral cleavage
that is displayed by most POLYCLAD turbellarians...this
is a typical spiral pattern
Boyer et al. 1998 for Holoplana inquilina, a polyclad
•Animal pole view of DUET SPIRAL CLEAVAGE in
acoelomorphs
Henry et al. 2000 for Neochildia fusca
Acoels show an unique form of development involving DUET SPIRAL
CLEAVAGE, in CONTRAST to the QUARTET SPIRAL PATTERN of
Spiralia, including most turbellarians
The cleavage pattern of acoels appears to be COMPLETELY DIFFERENT
from any other phylum specific program.
Cnidaria --- radial
Acoel flatworms --- duet spiral
Deuterostomes--- Radial
Protostomes --- spiral’
Polyclad flatworms --- spiral
Ctenophores ---asymmetric
radia
l
Duet
-like
Sponges – variable….
no typical type of cleavage
radia
spiral
Mueller’s Larva of
Many Polyclad
Flatworms
Acoel flatworms
have direct
development ((
(no larva)
Evolution of Hox Cluster: A New
Approach to Animal Phylogeny
• 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.
Orthologs: homologous sequences separated by “speciation event”
Paralogs: homologous sequences separated by a “duplication event”
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
Duplications
Proposed pattern of Hox
and ParaHox gene-complex
evolution
Acoels: basic Hox cluster with 4
Hox and 3 ParaHox genes
Triclad, polyclad flatworms are
like the coelomates
?
Inferred to be
present ancestrally
but not yet
documented
Ruiz-Trillo et al. Science 1999
18S ribosomal DNA genes including analysis of acoela
Other Protostomes
Ecdysozoa
DeuteroPolycladia
Acoela
stomes
acoelomate
coelom
Triploblastic
acoelomate
Diploblastica
So 16S rDNA sequence analyses
Indicate the Platyhelminthes
Phylum is polyphyletic
(myosin gene sequences, more 18S-rDNA genes,
mt DNA gene sequences , embryology, HOX gene
analysis)
LCB =
UrBilateria
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.
What can we infer from this evidence about:
1. The evolution of the coelom
2. The nature of the Urbilateria
3. The planuloid-aceoloid hypothesis
Rudolph Zallinger’s March of Progress
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?