Transcript ctenophores
Phylum Ctenophora Comb jellies ~100 sp., all marine - Similar to cnidarians: radial symmetry, simple nerve net, jelly-like mesenchyme (middle layer) - Monomorphic (one form throughout life), never colonial - Adhesive colloblasts (similar to cnidae) - Unique cydippid larva - ciliated tracts called ctenes (comb plates) used in movement - the first animals?? or the first predators?? Most ctenophores are pelagic predators Ecologically, can be the dominant predators in the plankton Colloblasts Colloblast cells develop into cluster of granuoles packed with strong adhesive, at the end of a spiral filament Spiral filament coils around the cell nucleus On discharge, spiral filament wraps around prey; adhesive released Ctenophore body plan Apical sense organ Aboral canal Ctene, or comb plate - have 8 evenly spaced rows of cilia; each one is a ctene Pharynx Mouth Tentacle sheath Tentacle Ctenophore body plan ctenes, the locomotory apparatus - linear arrays of cilia - 8 rows along oral-aboral axis Apical sense organ contains a gravity- sensing statolith, supported by 4 tufts of cilia called balancers - ciliated grooves connect sense organ to each ctene, direct beating and hence movement of animal Digestive system: GVC + tiny, partially functional anal pores - GVC is highly differentiated into system of canals thru body Controversial whether their embryos have mesoderm or not Ecological disaster: Mnemiopsis In 1982, the Atlantic ctenophore Mnemiopsis was accidentally introduced into the Black Sea (Europe), likely from U.S. ship’s ballast water Underwent explosive population growth, consuming most fish larvae and causing collapse of fisheries in 8 countries - cost: >$350 million Mnemiopsis leidyi In 1997 it was nearly eliminated itself, by accidental introduction of its major predator-- ctenophore Beröe ovata Beröe Benthic ctenophores deformable amoeba-like bodies; common on tropical green algae bottom surface is pharynx, swallows prey caught by 2 branched tentacles; body filled with branches of 8 digestive canals ctenophores & animal phylogeny Given their similarities, ctenophores and cnidarians were long thought to be sister to each other, forming a clade that was itself sister to the bilaterian animals symmetry + tissues evolved bilateral symmetry evolved ctenophores & animal phylogeny However, analyses of partial or whole genomes suggest that ctenophores are basal animals (more primitive than sponges..?!) implies the ancestor of all animals had symmetry, tissues, and a GVC Ryan et al. 2013, Science Moroz et al. 2014, Nature Whelan et al. 2015 PNAS ctenophores & animal phylogeny symmetry, tissues, GVC lost? This would mean: - sponges lost tissues, GVC, symmetry - the similarity of choanoflaggellate protists and sponge choanocyte cells is coincidence, not co-inherited - the 1st animals were predators, not bacteria-filterers Gene presence/absence data indicate ctenophores are more similar genetically to sponges than to other animals, but does not answer who is sister to the rest of animals genes a b c d e f x x x x + + x x x x + + + + + + + + + + + + + + General agreement: ctenophores are simpler than cnidarians, which share many more genes with Bilateria genes a b c d e f x x x x + + x x x x + + + + + + + + symmetry + tissues + + + + + + new genes evolved I said 2 years ago: I’ll be shocked if this doesn’t turn out to be the correct phylogeny of animals (see above!) How have the last two years of research changed our view...? Re-analysis of the same datasets using other evolutionary models of sequence evolution support sponges as basal, argue that artifacts of computer analysis are to blame Pisani et al. 2015, PNAS One artifact is long-branch attraction: most dissimilar sequences tend to find each other and clump together on a tree fungi choanoflaggelates sponges long branches find each other ctenophores Cnidaria bilaterian animals Say this is the true tree, but both fungi and ctenophores happen to have had an unusually high number of amino acid changes in their proteins over the past billion years long branches One artifact is long-branch attraction: most dissimilar sequences tend to find each other and clump together on a tree fungi ctenophores choanoflaggelates sponges Cnidaria bilaterian animals when more distantly related outgroups are included in a tree, it tends to artifically pull ctenophores to the base of the tree Analyses of 3 different datasets (different genes, different species) suggest that both long-branch attraction and poor models of amino acid change are driving the placement of ctenophores probability ctenophores are most basal fungi probability sponges are most basal Pisani et al. 2015, PNAS