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Lecture 17

BIO153 Lecture 17.pdf

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Christoph Richter

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2009 BIO153: Lecture 17 Deuterosrdmes and the Origin of Chordates Mar 23 , 2009 The deuterostomes are a superphylum within the Bilateria (bilaterally symmetrical animals). They are evolutionarily distinct from the Protostomes, and are defined by events that occur early in embryonic development: 1. Cleavage is radial rather than spiral; 2. During gastrulation, the blastopore becomes the anus rather than the mouth; 3. The coelom is formed when pockets of mesoderm pinch off from the embryonic gut. There are 4 phyla within the Deuterostomes, but we will focus on the 2 major phyla: Echinodermata and Chordata. 1. Echinodermata: starfish, sea urchins, brittle stars, sea cucumbers, etc. Characteristics of Echinoderms: ▯ bilaterally symmetrical as larvae; radially symmetrical (often 5-sided) as adults ▯ water vascular system with podia – functions as a hydrostatic system for locomotion; for feeding and gas exchange (replaces coelom) ▯ hard endoskeleton for support (derived from mesoderm) 2. Chordata: contains 3 major subphyla: a. Tunicata or Urochordata b. Cephalochordata c. Vertebrata (within the clade Craniata, which contains Vertebrata + Myxinoidea (hagfish)) All chordates have the following 4 characters at some point in their life cycle: 1. notochord: ▯ stiff rod derived from mesoderm notochord present in all embryos ▯ supports the main body axis ▯ may persists through adult stages; in vertebrates, replaced by vertebral column 1 without notochord: with notochord: contracting muscles of contracting muscles body wall just shorten pull on main body the animal along main axis; produces lateral body axis movement 2. pharyngeal gill slits ▯ used in filter feeding in early chordates ▯ slits persist in aquatic vertebrates – part of gill apparatus in fishes, etc. ▯ become parts of jaw, inner ear in terrestrial vertebrates 3. dorsal hollow nerve cord ▯ derived from ectoderm ▯ “flipped” from annelid, arthropod arrangement: in annelids & arthropods, the nerve cord is ventral and the gut is dorsal; in chordates the nerve cord is dorsal and the gut is ventral (controlled by HOX genes!) 4. post-anal tail ▯ extension of notochord, musculature ▯ important in locomotion 3 Subphyla: A. Subphylum: Urochordata (Tunicata) ▯ Urochordates (sea squirts, tunicates) ▯ have all 4 chordate characters as larvae ▯ only gill slits retained in adult – sessile lifestyle ▯ are thought to be similar to the organism that gave rise to the vertebrates (in the vertebrate lineage, there must have been a heterochronic (neotenous) mutation: in vertebrates, the larval form exhibited by tunicates is retained in the adult. B. Subphylum: Cephalochordata ▯ lancelets (Amphioxus) ▯ retains all 4 characters throughout life cycle 2 C*. Craniata (includes Subphylum: Vertebrata) ▯ clade (vertebrates + hagfish) ▯ chordates with heads (i.e., skulls, brains, sense organs) ▯ faster locomotion; more active than cephalochordates & urochordates ▯ gill slits pump water (feeding & gas exchange) ▯ muscles line gut for fast throughput ▯ cardiovascular systems: heart + vessels; blood with hemoglobin (efficient O 2 Steps in vertebrate evolution: 1. Pre-vertebrate – suspension feeder (similar to amphioxus) 2. Agnathan – no jaws; muscular pump to move food current (similar to hagfish; lampreys) 3. Gnathostome – vertebrate with jaws Agnathans: e.g. hagfish, lampreys ▯ no b
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