Ch 2 Intro to Chordates PDF

Ch 2 – Origin of Chordates Bilateria – bilateral, symmetrical body plan Bilateria  Schizocoelic - splitting Deuterostome coelom forms by enterocoely – buds in mesoderm form to become coelomic cavities. Deuterostome coelomates include chordates (vertebrates tunicates and lancelets), echinoder...

Ch 2 – Origin of Chordates Bilateria – bilateral, symmetrical body plan Bilateria  Schizocoelic - splitting Deuterostome coelom forms by enterocoely – buds in mesoderm form to become coelomic cavities. Deuterostome coelomates include chordates (vertebrates tunicates and lancelets), echinoderms (starfish, sea urchins, sea cucumbers), and hemichordates (acorn worms)  Intro to embryonic development Bilateria Fig. 2.3 are divided into two major groups on the basis of embryonic characters “First mouth” mouth develops from blastopore “Second mouth” anus develops from blastopore Simple cladogram Chordate Characteristics: 5 diagnostic characters = synapomorphies Notochord Pharyngeal slits Endostyle/thyroid gland Dorsal hollow nerve cord Postanal tail Fig. 2.7 Fig. 2.33 Chordates – 3 groups 1. Cephalochordata 2. Urochordata 3. Vertebrata  Chordata - differences 1. Cephalochordata 2. Urochordata 3. Chordata Invertebrates Vertebrates Marine Marine, terrestrial, Support system: rods freshwater of collagenous Support system: material skeleton Suspension feeders Jaws for feeding  Chordata - similarities Notochord Pharyngeal slits Endostyle or thyroid Dorsohollow nerve cord Postanal tail Endostyle  Notochord Derived from mesoderm Elastic rod that can be flexed back and forth (c) In bony fish and vertebrates serves as a scaffold during embryonic development replaced by vertebral column In mammals, it is reduced to a remnant nucleus pulposus – intervertebral disks! Fig. 2.5 Endostyle or thyroid Endostyle - glandular groove in simple chordates, on floor of pharynx. Secretes mucus to trap food particles. Iodine metabolism. Indicates homologous with thyroid. Thyroid – produce hormones involved in metabolism, cardiovascular health, development, and iodine metabolism Trachea  Dorsal hollow nerve cord Derived from ectoderm Formed by invagination during embryogenesis Solid vs. hollow? Advantage unclear  Postanal tail Derived from segmented musculature and notochord (chordate locomotor apparatus). Anus is not terminal as in non-chordates Fig. 2.4 Pharyngeal slits Sac-like projections found between mouth and digestive tract. Formed by pharynx pushing outward Ancient inverts used for filter feeding In fish, become gills In humans, present only during embryonic development – become jaw and inner ear Synapomor- phies Missing pharyngeal slits Origins - Protochordates Hemichordates – possess some of the 5 traits but same embryonic development Cephalochordates, Urochordates – possess all 5 traits and the same embryonic development Important to note: these traits are present at one stage or another. All marine Adults benthic, larvae planktonic (different lifestyles and different morphology. Food- suspended particles extract from water propelled by cilia. Food collected on mucus and directed to the gut. Locomotion – Notochord (if present) and tail muscles provide more mobility than cilia alone  But how did we make the jump from protochordates to chordates which include Vertebrata? Pg 74 Hemichordata – acorn worms Cephalochordata - lancelets Urochordata - tunicates Chordates  Few invertebrate fossils (soft-bodied) Living vertebrates are highly derived (not ancestral traits) Invertebrates split 500 million years ago and didn’t form directly to chordates. Intermediate forms but no fossil record Using extant groups for ancestral traits, surmised ancestors to chordates are extinct Competing hypotheses of origins  Chordates from annelids and arthropods Segmented body forms Presence of brain regions – forebrain, hindbrain Upside down body plan Fig. 2.29 Chordates from annelids and arthropods - debunked Similarities are homoplasy, not homology (protostome vs. deuterostome) Segmentation and jointed appendages of arthropods are quite different from the chordate segmentation Mouth and anus – ventral vs. dorsal Embryogenesis Derivation of mesoderm Basic pattern of cleavage  Chordates from echinoderms Based on embryology Both are deuterostomes Bilaterally symmetrical Cilia bands and nerve tracts formed dorsal nerve cord Adoral cilia gave rise to endostyle Auricularian hypothesis – based on sea Paedomorphosis cucumber larva Fig. 2.30  Fig. 2.31 Garstang proposed a series of literal evolutionary steps through the larval stages that involved paedomorphosis (*) and eventually produced chordates. paedomorphosis Fig. 2.31 Chordates from echinoderms - debunked Simple appearance of structures Can’t reconcile gene expression and phylogenies with the auricularian (sea cucumber larvae) scenario Embryogenesis – gene expression mismatch in chordates and the dipleurula-type larvae. Chordates – expression along length of nerve cord In dipleurula-type larvae – expression in head only; implying no body From Hemichordates - Dorsoventral Inversion - key occurrence in chordate history Expression of signaling proteins, BMP and chordin are dorsal and ventral, respectively this is reversed in chordates – chordin = dorsal vs. BMP = ventral Only possible if the ancestral ventral side became the chordate’s dorsal side BASICALLY – signaling proteins turn everything upside down. Evidence of inversion Hemichordates – pharyngeal slits open dorsally but ventrally in chordates Hemichordates – pharynx particles trapped in mucus are transported dorsal to ventral then to the gut. In chordates, transported ventral to dorsal pharynx, then to the gut. Hemichordates - blood flows forward in the dorsal vessel, backward in the ventral vessel. In chordates, blood flows backward in dorsal vessel, forward in the main ventral vessel. Hemichordates – musculature developed dorsally. In chordates, ventrally Exception – mouth opens ventrally under both hemichordates and chordates. Fig. 2.33