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Evolutionary Significance of Flatworms: Insights from Embryology and Genetic Studies

This article explores the importance of flatworm evolution in understanding animal evolution, focusing on embryological patterns and genetic sequencing studies. It discusses the evolution of the coelom, the nature of Urbilateria, and the planuloid-acoeloid hypothesis.

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Evolutionary Significance of Flatworms: Insights from Embryology and Genetic Studies

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  1. Urbilateria UrBilateria Planuloid-Acoeloid Theory “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

  2. acoelomorph polyclad • - • - Fee living flatworms have: • No coelom: acoelomates • No thru gut or vascular system: have a g-v cavity • Primitive kidneys: protonephridia • All 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

  3. Primitively simple? Secondarily simple? Why is flatworm evolution important to our understanding of animal evolution? Drawing by Charles Messing

  4. Polyclad flatworm Acoelomorph flatworm More recent evidence: Acoelomorph embryology Hox Genes Genetic sequencing studies

  5. 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

  6. Animal pole view of DUET SPIRAL CLEAVAGE in acoelomorphs Henry et al. 2000 for Neochildia fusca

  7. 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 Sponges – variable…. no typical type of cleavage Cnidaria Nemertodermatida Deuterostomia Acoela Protoostomia radial spiral Duet-like radial

  8. Mueller’s Larva of Many Polyclad Flatworms Acoel flatworms have direct development (( (no larva)

  9. 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”

  10. Hox and ParaHox gene-complex 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 Duplications

  11. 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

  12. Ruiz-Trillo et al. Science 1999 18S ribosomal DNA genes including analysis of acoela Other Protostomes Ecdysozoa Deutero- stomes Polycladia Acoela acoelomate coelom Diploblastica Triploblastic acoelomate

  13. 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)

  14. 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

  15. 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

  16. 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?

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