PROTOCHORDATES and other interesting critters…

1. PROTOCHORDATES and other interesting critters… Mrs. Ofelia Solano Saludar Department of Natural Sciences University of St. La Salle Bacolod City

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10. PROTOCHORDATES

11. CHORDATE CHARACTERISTICS • Phylogeny (based primarily on embryonic criteria) • Coelomate (enterocoelous) • Deuterostome • Radial cleavage (generally) • Diagnostic characteristics (present at some stage of life cycle): • Notochord • Pharyngeal slits, often with an endostyle • Dorsal, hollow nerve cord • Postanal tail • Other characteristics shared with some non-chordates: • Bilateral symmetry • Segmentation or metamerism

13. The NOTOCHORD is a dorsal, flexible, hydrostatic organ that resists compression Derived from mesoderm, it is the first part of the endoskeleton to appear in an embryo Place for muscle attachment In vertebrates, the notochord is replaced by the vertebrae Remains of the notochord may persist between the vertebrae.

15. In chordates, the NERVE CORD is a tube dorsal to the alimentary canal The anterior end becomes enlarged to form the brain The hollow cord is produced by the infolding of ectodermal cells that are in contact with the mesoderm in the embryo Protected by the vertebral column in vertebrates.

16. PHARYNGEAL SLITSare openings that lead from the pharyngeal cavity to the outside. Originally functioned for filter feeding, but were later modified into internal gillsused for respiration. Embryonic, but may persist in some adult chordates In tetrapods, the pharyngeal pouches give rise to the Eustachian tube, middle ear cavity, tonsils, and parathyroid glands

17. The endostyle in the pharyngeal floor, secretes mucus that traps food particles Found in protochordates and lamprey larvae Secretes iodinated proteins and thus homologous to the iodinated-hormone-secreting thyroid glandin adult lampreys and other vertebrates.

18. The POSTANAL TAIL, along with somatic musculature and the stiffening notochord, provides motility in larval tunicates and Amphioxus Evolved for propulsion in water Reduced to the coccyx (tail bone) in humans.

19. HEMICHORDATES (Enteropneusts) • Chordate body plan: • Pharyngeal gill slits open in lateral walls of foregut • Dorsal nerve cord does not constitute CNS, and has an invertebrate ventral nerve strand • Usually solid, but may be tubular in some species • The stomochord- a short, hollow diverticulum of the foregut, which lacks the fibrous sheath of the chordate notochord • Incomplete notochord gives the name "hemichordate"

20. Similarities to echinoderms: • Deuterostome • Tornaria larvae resemble the auricularia larvae of echinoderms • Indicates possibility that they represent an evolutionary route taken by early chordates • Exact ancestral affinities remain debatable • Hemichordates are placed in a separate phylum with 2 major groups with 100 species, all marine • Acorn worms - distinct proboscis, collar, trunk • Pterobranchs- "feathery wing" feeding structure; sessile, colonially dwell in connected tubes

21. PHYLOGENY OF HEMICHORDATA

22. Subphylum UROCHORDATa • Has all the chordate characteristics except that its notochord is confined to larval tail (tail-chordates) • The free-swimming larvae don't feed, but metamorphose into usually sessile adult stage with: • No notochord or dorsal, hollow nerve cord • Well-developed pharyngeal slits.

23. Composed of 3 major groups: • Thaliaceanshave no larval stage, no notochord, no tail, and resemble ascidian pharynx

24. Larvaceans are paedomorphic, retain tail and notochord, reproduce in larval-like stage

25. Ascidians undergo complete metamorphosis before becoming sexually mature and encase its body in a tunic (tunicate); about 1,250 species 15.5 Its closed, U-shaped circulatory system periodically reverses flow, hence the name, “sea squirt”

26. (b) In the adult, pharyngeal slits function in suspension feeding, but other chordate characters are not obvious. (c) A tunicate larva is free-swimming, but nonfeeding“tadpole” in which all 4 chief characters of chordates are evident. (a) An adult tunicate, or sea squirt, is a sessile animal

27. Cionaintestinalis(a solitary sea squirt) Synoicumpulmonaria  a colonial sea squirt

28. A 530 million-year-old (although perhaps as old as 560 million years) creature, Cheungkongellaancestralis, probably a tunicate, found in the Chengjiang fauna in China's southwest Yunnan Province, might be the earliest known fossil evidence of primitive chordates

29. Subphylum Cephalochordata • Possess all chordate characteristics • Amphioxus (sharp at both ends), lancelet (little spear) or Branchiostomalanceolatus; 25 marine species; small (4-5 cm) suspension feeders, • Notochord extends from tip of rostrum to tip of tail as an adaptation to burrowing (head-chordate).

30. Muscular discs surrounding notochord are used for flexing in swimming movements, and for stiffening the rod • Pharyngeal slits open into atrium; muscles expel water from atriopore; posterior atrial opening for excurrent water flow • Sexes are separate, fertilization is external

31. The Hatschek’s pit or groove in the roof the oral cavity resembles the vertebrate pituitary gland and may have endocrine function. • Hepatic cecum may be forerunner of liver and pancreas • The small brain size is correlated with paucity of sense organs, although chemoreceptors and tactile receptors are abundant. • Sense organs are light-sensitive, pigmented cells, ocelli, embedded within ventrolateral walls of the spinal cord.

33. Amphioxus vs. Craniates • Has no cephalization and paired sense organs • Has a notochord but has no vertebral column • Has pharyngeal gill slits but in large numbers, emptying into an atrium • Has a dorsal hollow CNS, but the brain lacks craniate divisions and a skull • Has segmented musculature, but the segments extend to anterior tip of the head • Has a 2-layered skin, but epidermis is only 1-cell thick • Has arterial and venous channels similar to basic craniate channels, but has no muscular heart • Is coelomate, but is restricted only to adults • Lacks a muscular gut and pharynx (food moved through gut by ciliary action not peristalsis)

34. Lacks neural crest (ectodermal cells that are found on the embryonic neural tube and are engaged in the formation of the cranium, tooth dentine, some endocrine glands and Schwann cells, which provide myelin insulation to nerve cells) • There are 39 or more pairs of cranial nerves vs. 10-18 of craniates; no semicircular ducts, eyes, lateral-line system, foramen magnum (no skull) • Spinal nerves consist solely of dorsal roots (sensory fibers from and motor fibers to the visceral organs) • Lacks a kidney, the excretory system consisting of invertebrate solenocytes, but pedicels that embrace glomerular blood vessels are similar to foot processes of the vertebrate podocytes.

35. Amphioxus vs. Craniates • Because craniates are large they cannot depend on slow processes such as diffusion and ciliary action to support them. • Increased size and activity of craniates require a suite of traits to support it. • sensory systems so they can seek things out (predation vs. suspension-feeding) • complex nervous system to coordinate activity • more efficient circulatory, respiratory and digestive systems to fuel the activity • muscles and skeleton to facilitate movement.

36. CHORDATE ORIGINS • Annelid/Arthropod Hypothesis • Evidence: • Segmentation • Brain regionalization • Similar (but inverted) body plans. • Problems: • Similarities due to analogy, not homology • Exoskeletons, jointed appendages vs. endoskeletons, myotomes • Solid nerve cords vs. hollow nerve cords • Inversion of annelid/arthropod body plan would require migration of mouth and anus ventrally or formation of new mouth and anus ventrally • Different coelom formation • Protostomes vs. deuterostomes

37. Echinoderm Hypothesis (W. Garstang) • Evidence: (seen primarily in larval echinoderms) • Deuterostomes w/ similar cleavage and coelom formation • Bilaterally symmetrical larvae similar to hemichordate larvae • How to get from larval echinoderm to chordate? • Increase size of larvae • Development of alternate locomotor system: segmental musculature, elongated body, stiffened support rod (notochord) • Increase in volume would also necessitate feeding change from ciliary to pharynx w/slits • Larval echinoderm --> chordate tadpole: paedomorphosis

38. Problems: • Existing chordates should not be viewed literally as examples of ancestral forms • Urochordate (ascidian) tadpoles were specialized for relatively short (safer) planktonic life; it is doubtful that reverse selection would prolong this stage • Structural problem: intestine of ascidian tadpole opens into atrium; not homologous to intestine of other chordates such as Amphioxus. • Ascidian tail musculature is not segmental • Ascidians seemed to be evolving away from acquisition of chordate features • Similarity of echinoderm and protochordate larval types may be example of convergence to a similar life style.

39. Dipleuruloid Hypothesis (Malcolm Jolly) • Embryonic similarities too close to be accidental, may result from retention of characteristics derived from ancestor common to both groups. • Dipleurula is a hypothetical ancestor; small, ciliated bilateral, benthic or pelagic. • Primitive features of dipleurula were incorporated into larvae of echinoderms & hemichordates • Pharyngeal slits arose later in each group as aids in ciliary or filter feeding • Notochord, tail, nerve tube arose to serve better locomotion; undulatory movement accompanied by muscle segmentation • Chordates evolved along two pathways: • Toward more specialized filter feeding - urochordates & cephalochordates • Enhanced active predatory lifestyle - vertebrates.

40. Where do new genes that give rise to new life forms come from? 1. Errors in Meiosis

41. 2. Transposons in DNA New copy of transposon Transposon DNA of genome Transposon is copied Insertion Mobile transposon (a) Transposon movement (“copy-and-paste” mechanism)

43. WITH THESE MECHANISMS FOR CHANGES, EVOLUTION MARCHES ON… UP NEXT, THE CRANIATES

44. QUESTIONS?