Major Steps in Early Animal Evolution (Review Nielsen, 2008)

4,5,6 3 2 1 numbers in green represent Neilsen’s (2008) 6 major steps Tubular Gut 6 4,5,6 Triploblastica: 3 germ lines 5 Bilateria (Cnidaria included or excluded?) 4 3 Eumetazoa: organs (include organized nervous system) 3 2 3. Basal lamina, true sealed 2 epithelium (include Homoscleromorpha) 2. Desmosomes (sealed epithelia) 1. Metazoa (multicellularity) 1 1

Rudolph Zallinger’s March of Progress There is support, from both 18S rDNA and the Hox gene sequences that most flatworms are derived coelomates. What is the (mis)perception depicted in Zallinger’s “March of Progress” ? How is the evolution of flatworms relevant to the “March of Progress”?

Major Steps in Early Animal Evolution (Review Nielsen, 2008)

Outline 1. Multicellularity Self non-self recognition, immunity 2. True Sealed Epithelium 3. An Organized Nervous System 4. Third Germ Line Bilateral Symmetry Tubular (Through) gut Why were these steps important? In what groups were these characters first evident?

Multicellularity The ancestral metazoa or Urmetazoa was a highly specialized pelagic choanoflagellate B. nutrients were shared between cells, and integrity is maintained by cadherins etc. Differentiation of roles charac-terizes the more advanced choanoblastea. Differentiation followed by transition to bottom dwelling adult phase (pelago-benthic) and life cycle with drifting larvae

Multicellularity (cntd) E. Evolution of multicellularity perhaps accompanied/or closely followed by individuation, including self recognition, histoincompatibility Immune system, and apoptosis (Mueller 2003) The Urmetazoa possessed these characters, which were passed on to PORIFERA (but dedicated sensory cells and nerve cells are not present)

2. Origin of sealed epithelia and extracellular digestion scattered cadherin molecules become organized near apical pole of cells, where they form occluding and adherens juctions and seal the epithelia B. Sealed epithelia allow for extracellular digestion; digestive processes can only function in an enclosed space; invagination of the epithelium could lead to origin of archenteron. C. Digestive epithelium becomes endoderm Cadherins or calcium adherent proteins are a type of transmembrane protein involved in cell adhesion

2. Origin of sealed epithelia and extracellular digestion D. Homoscleromorphs larvae and not adults show some features of metazoa...i.e. Ciliated epithelia with metachronal waves E. Eumetazoa more likely to have evolved from Homoscleromorph larva that became sexually mature (dissogony: reproductive structures. This phenomenon is generally known as neoteny and the end result is a juvenalization of the organism…. E.g. humans

2. Origin of sealed epithelia and extracellular digestion D. Homoscleromorphs larvae and not adults show some features of metazoa...i.e. Ciliated epithelia with metachronal waves E. Eumetazoa more likely to have evolved from Homoscleromorph larva that became sexually mature (dissogony: reproductive structures. This phenomenon is generally known as neoteny and the end result is a juvenalization of the organism…. E.g. humans

Origin of a Nervous System Absent in Sponges and Placozoa Electrical and Chemical Synapses present in Cnidaria Animals with a nervous system except Ctenophora form a monophyletic unit Apical organs found in all Eumetazoa may be first nervous system Precursors of many of the genes involved in nervous system function are present in sponges (e.g. post-synaptic scaffold) Neurogastrea develops ciliated band to resemble trochaea, which is a larva found in most Eumetazoa.

Origin of Mesoderm (Triploblastica) A. Exact phylogenetic origin uncertain (Ctenophore mesoderm has independent origin) B. Mesoderm certainly derived from Endoderm; cells involved in specification and differentiation of mesodermal cell types are found in the the starlet anemone Nematostella

Radial bilateral 5. Evolution of Bilateral Symmetry Symmetry along A-P axis and an anterior brain Body Plan Radial Bilateral Asymmetry Symmetry Symmetry Porifera Cnidaria all other animals Placozoa Ctenophora (~ 32 phyla) Echinodermata 32 Phyla of Bilateria share: Triploblasty Coeloms Through-guts Central nervous system oral dorsal anterior ventral Radial Why the great success? posterior aboral bilateral

5. Evolution of Bilateral Symmetry Question hinges on whether bilateral symmetry in sea anemones is homologous to that of bilateria Position of the syphonoglyph and retractor muscles impose bilateral symmetry. But these characters are unique to anemones and are not the characters that determine bilateral symmetry in bilateria – it is likely that they evolved independently.

At what point did Bilateral Symmetry Evolve? Ancestor of Cnidaria Bilateria? (homologous; lost in most Cnidaria) Independently in Anthozoa and Bilateria

5. Establishment of a Tubular Gut (Eubilateria) More efficient digestion, food absorption Enabled evolution of larger organisms with more complicated behaviors; exact point of origin unknown Acoels are most primitive bilateria ; lack a thru gut while Platyhelminthes lost a thru gut and Ctenophores evolved thru gut independently Origin of tubular gut not resolved phylogenetically