PDF lecture 7 notes- The origin and evolution of bilateral symmetry PDF

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Lecture 7: The origin and evolution of bilateral symmetry Bilaterally symmetrical animals Terrestrial groups- bilateral

- Anterior- head - Posterior- tail - Gut runs along this anterior posterior axis - Front back axis • Back –dorsal • Front- ventral - This gives Left right symmetry -

Traditional view is that bilateral body with 2 axis evolved once Give rise to majority of animal species Speculation as to why this was advantageous When you have 1 axis- cells in development can receive more information when developing about where it is Bilateral symmetry gives increase potential to complexity in animal body plan More positions in relation to 2 axis than 1 in patterning Do more with 2 axis of information

- Rotational symmetry - Sea anemone • Take any plane and rotate it you see same thing - Considered outside of the group- radial symmetry A traditional view of animal/metazoan phylogeny Old tree • Cnidarians and ctenophore are grouped closely together due to radial symmetry • Traditional grouping- radiata • Sponges- basal group • First to branch off • Don’t have any obvious axis along body • Random arrangement of body • They are animals • Common ancestor- urmetazoa Out of date

Non-bilaterally symmetrical animal phyla Branched off early All are diploblasts, with ectoderm and endoderm, but lacking an ordered, complete mesoderm germ-layer (but some have mesoderm-type cells) 1. Sponges (Porifera) and their choanocyte cells

- Sponges - chimney-like structures through which water flows. - Not many cell types - The inside of the body is lined with choanocyte cells, sometimes in chambers - Only primitively multicellular - no clear axes of symmetry and very little cell type diversification in the multicellular form

2. Ctenophora (comb jellies) ‣ ‣ ‣ ‣ ‣

These are complex predators Move around and capture prey by sticking to it using tentacles Look similar but look closer- differences to traditional relatives cnidarians ≈ 200 species Gelatinous marine animals

Features - Biradial symmetry • Mirror image down 1 plane - Hermaphrodites • Self-fertilize - Direct developers (free living cydippid stage) • Embryo to adult - Eight longitudinal rows of ciliated comb plates along oral-aboral axis • Use to move • Beats of cilia will move through water - Smooth muscle cells • Jellyfish- don’t have smooth muscle- have striated- overlooked when put together into radiata

- Apical sensory organ • Primitive brain - Nerve net • Nervous not centralised- nerve nut runs through tissues - Mesoglea nerve fibres • Gap between ectoderm and endoderm- nerve fibres run through

- Tenticular nerves • Run along tentacle- involved in moving, detecting stimuli - Colloblast cells for catching prey • Stick to prey 3. Cnidaria ‣ Made up of two monophyletic groups: Anthozoa & Medusozoa Anthozoa:# E.g. corals and sea anemones Features: - Ectodermal and endodermal nerve nets - Nematocysts - Striated muscle - Radially asymmetrical body Medusozoa E.g. Jellyfish, marine hydroids & freshwater Hydra Comes from sexual; reproductive phase=- jellyfish part of life cycle Features: Nerve nets Nomatasists= specialised cells for stinging (Different approach to hunting and feeding and protecting) Striated muscle 4. Placozoa - a less well known non-bilaterian clade Discovered on the walls of saltwater aquariums (1970s) Nearshore habitats (mangrove communities) Morphologically indistinguishable E.g. Trichoplax adhaerans (only named species) Features: Two epithelial layers with layer of multinucleate fibre cells in between 6 cell types Feeds by crawling on top of prey - digestion extracellular and phagocytic Move using cilia of lower epithelium and fibre cell layer Reproduce by fission. Sexual reproduction likely - embryos in f, g 6 cell types: ✦ Ventral epithelium cell (VEC)(72%) – multiple microvilli & single cilium. ✦ Lipophil

cell (11%) – secretes digestive enzymes, cell body in interior, only on ventral surface.

✦ Gland

cell (3%) – ventral, near periphery, half of them in ring around rim of body, neurosecretary cells involved in feeding/movement?

✦ Fiber

cells (4.4%) – internal, evenly spaced in single layer, contact all other cells, equivalent to nerves?

✦ Crystal ✦

cells (...