Chordate and Vertebrate Origins

Protochordates Phylum Hemichordata: acorn worms (Balanoglossus) Phylum Chordata: Subphylum Urochordata: sea squirts (Ciona/Molgula) Subphylum Vertebrata: vertebrates Common Protochordates Characteristics Possess some or all chordate characteristics All are suspension feeding marine organisms cilia transports water, mucus collects food, cilia transports to GI tract Larvae are planktonic, adults are usually benthic all chordate characteristics may not appear at once Hemichordate larvae (tornaria) resemble echinoderm larva recently validated by molecular techniques

Subphylum Urochordata: Tunicates 3 groups: Ascidiaceans, Larvaceans, and Thaliaceans Sea squirts, appendicularia, & salps approximately 2000 species; most are sea squirts wide distribution: fouling organisms to deep sea forms Planktonic larvae; sessile adults Suspension feeder specialists Tunic: flexible outer layer, thick & can be brightly colored diagnostic for the group

Subphylum Cephalochordata: Branchiostoma/Amphioxus (lancelets) Adults exhibit all chordate characteristics Cosmopolitan distribution in tropical and warm temperate seas. 1-way water flow thru pharyngeal slits Cilia driven filter feeding apparatus Oral hood w/buccal cirri encloses pharynx Food & mucus:Hatschek’s & epibranchial groove thread to gut wheel organ Water flows to atrium & out atriopore Start to see characters of vertebrates chevron-shaped myomeres precursors to vertebrate organs -- endostyle: thyroid -- midgut cecum: liver & pancreas cardiovascular: -- dorsal & ventral aorta -- paired cardinal arteries; carotids -- overall circulation similar to vertebrates -- pharyngeal arches; aortic arches

Branchiostoma/Amphioxus Excretory/Osmoregulatory System: located in atria along pharyngeal slits parts: glomerulus, pedicel, solenocytes, nephridial tubule, atriopore -- single cell w/ pedicels surround the glomerulus vessels * bridge from glomerulus to nephridial tubule Pedicels of the solenocytes differ from inverts similar to the processes of podocytes in vertebrate kidneys

Phylogenetic History of Protochordates Living protochordates: 500 million years of evolution independent of vertebrates Actual chordate ancestors are extinct; minimal fossil record ancestral features can help determine phylogeny Early theories: Arthropods & Annelids to Chordates All 3: segmented, similar brain organization, similar but inverted body plan Weaknesses: annelid segmentation differs from myomere segmentation mouth & anus positions differ; no evidence of migration during development overlooks protostome/deuterostome developmental patterns

Phylogenetic History of Protochordates Cephalochordates from Echinoderms: W. Garstang Echinoderms are deuterostomes; a more likely chordate ancestor phylogenetic key are echinoderm larvae According to Garstang “Chordates arose from larval echinoderms” Chordate characteristics 1st appeared in echinoderm larva tornaria larvae share traits with echinoderm larvae Bilateral symm. 1-way gut hemichordate larvae share traits with chordate larvae Body elongation creates tail that could undulate; post anal tail formed causes CCB to elongate, move dorsal & fuse; forms a proto-dorsal nerve cord causes CAB to elongate; forms a proto-endostyle Only needs pharyngeal slits & notochord to be considered a chordate Which selection pressures are involved?

Phylogenetic History of Protochordates SA L2 V  L3 SA  V2/3 Changes in body plan must have been advantageous As size increases SA covered w/cilia cannot keep up with increase in volume (Ch 4) individuals with alternate locomotion favored evolutionary solutions: notochord, segmented muscles, further elongation 24 cm2 48 cm2 Same geometric principle applies for ciliated feeding endostyle & pharyngeal slits increase water flow; swimming increases flow Garstang suggested adult form abandoned If larvae > successful then adults then sexual maturity at larval stage escapes an echinoderm adult life How was metamorphosis into an adult echinoderm terminated? new phylogenetic direction

Heterochrony Relative change in timing of developmental event change in ontogenetic onset, offset, or timing of a character appearance one process of evolutionary change Paedomorphosis (“child form”): juvenile or embryonic features are present in adults (axlotls:gill retention in adults) 3 Mechanisms: 1. progenesis: body growth ends earlier; sexual maturity achieved earlier than normal -- paedomorphosis ≠ neotony 3. postdisplacement: features appear late; features maintain juvenile characteristics

Heterochrony Peramorphosis (“beyond form”): appearance of ancestral features in adults; exaggerated features or characters 3 Mechanisms acceleration: rate of growth increases predisplacement: onset of growth is early; characters appear earlier than normal best example is Irish elk example of hypermorphosis

Phylogenetic History of Protochordates Garstang viewed protochordate evolution as a series of paedomorphic steps However! Urochordate larvae (ascidian) have decreased larval time Garstang’s theory requires a reversal of time spent as larvae -- unlikely Ascidian larval morphology divergent gut morphology is only analogous to Amphioxus true gut does not develop in larvae Similarities of larval form can be explained by convergence alone strong selection pressures in an aquatic environment

Phylogenetic History of Protochordates Malcolm Jollie’s Dipleuruloid Theory “Similarities of echinoderm & chordate deuterostome development too strong to ignore” proposes a theoretical ancestor: Dipleurula Dipleurula: small, bilateral, & ciliated larval characters found in both echinoderm and hemichordate larvae Pharyngeal slits arose among Hemichordates assists ciliary and mucus feeding system Notochord, tail, nerve cord, & myomeres develop to serve adult forms Chordates Split: one group secondarily moves back to filter feeding niche -- cephalochordates & urochordates Emphasizes trend toward predation Instead of filter feeding one group evolves as active predators

Chordate Clade Vertebrates arose within the deuterostome radiation includes echinoderms & hemichordates Chordate evolved from a common echinoderm/chordate ancestor chordates did not evolve from echinoderms (sensu Garstang) Chordate body plan established early in time, among invertebrates Basic chordate plan includes: pharyngeal slits, notochord, dorsal hollow nerve cord, and a post anal tail. Locomotion relied upon a notochord and serially segmented musculature