BI2CV1 Lecture 3 - Vertebrate Origin PDF

What makes a vertebrate? BI2CV1 Comparative Vertebrate Biology Dr Manabu Sakamoto [email protected] Vertebrates within the animal tree of life Vertebrates are chordates along with urochordates (tunicates) and cephalochordates (lancelets). Chordates are deuter...

What makes a vertebrate? BI2CV1 Comparative Vertebrate Biology Dr Manabu Sakamoto [email protected] Vertebrates within the animal tree of life Vertebrates are chordates along with urochordates (tunicates) and cephalochordates (lancelets). Chordates are deuterostomes alongside hemichordates and echinoderms. Vertebrates within the animal tree of life Hemichordates were long thought to be the closest relative (sister group) of chordates. Both clades have a pharynx with pharyngeal slits that open to the outside. Pharynx with slits is now considered to be an ancestral feature of deuterostomes. Chordata Cephalochordata Lancelets Urochordata Tunicates Vertebrata Cyclostomata Gnathostomata Chordata All chordates are united by having four shared derived features (synapomorphies) Notochord Neural tube Postanal tail Endostyle  By Jon Houseman - Jon Houseman and Matthew Ford, CC BY-SA 3.0 Lancelet Chordate features Notochord Stiffening rod, attachment for A: Buccal Cirri, B: Gill bars, C: Gonads, D: Antiopore, E: Ventral fin, F: Anus , G: Intestine, H: Dorsal fin, segmental muscles (myomeres). I: Tail, J: Notochord, K: Nerve cord. Neural tube (dorsal hollow nerve cord) Coordinates muscle activities. Postanal tail Myomeres provide powerful propulsion. Endostyle Ciliated, glandular groove on the floor of the pharynx. Assists in filter feeding and takes up iodine. A: Buccal cirri, B: Wheel organ, C: Velum, D: Rostrum, A: Fin box, B: Nerve cord, C: Notochord, D: Myomeres, E: Notochord extending beyond nerve cord, F: Nerve cord, E: Epibranchial groove, F: Endostyle, G: Secondary gill bar, H: Primary gill bar, I: Pharyngeal cavity, J: Metapleural fold, G: Hatschek's pit, H: Fin rays, I: Gill bar, J: Buccal cavity (vestibule). K: Epidermis  By Jon Houseman - Jon Houseman and Matthew Ford, CC BY-SA 3.0 Lancelet Chordate features Masunaga et al. 2022. Marine Biology 169: 157 A: Buccal Cirri, B: Gill bars, C: Gonads, D: Antiopore, E: Ventral fin, F: Anus , G: Intestine, H: Dorsal fin, I: Tail, J: Notochord, K: Nerve cord. Free-swimming tunicate, Oikopleura A: Buccal cirri, B: Wheel organ, C: Velum, D: Rostrum, A: Fin box, B: Nerve cord, C: Notochord, D: Myomeres, E: Notochord extending beyond nerve cord, F: Nerve cord, E: Epibranchial groove, F: Endostyle, G: Secondary gill bar, H: Primary gill bar, I: Pharyngeal cavity, J: Metapleural fold, G: Hatschek's pit, H: Fin rays, I: Gill bar, J: Buccal cavity (vestibule). K: Epidermis Cephalochordata: lancelets Vertebrate-like anatomy, but simpler. Live on sandy seafloor of tropical and temperate seas. Motile larval and adult stages. May be very similar to the hypothetical chordate ancestor. Model organism for understanding chordate evolution! D’Aniello et al. 2023. eLife 12: e87028, CC BY 4. Urochordata: Tunicates Larval stage retains chordate synapomorphies. Appendicularian tunicates retain these into adulthood. Tunicates share features with vertebrates: Heart Cells that may be homologous with the vertebrate neural crest.  Hypothetical chordate ancestor Losses of features in adults Chordate ancestor was probably very lancet-like Notochord Nerve cord Postanal tail Endostyle Heart Neural crest-like cells By Peiyun Cong, CC BY 4.0 Fossil early chordates are lancelet-like! By Nobu Tamura, CC BY-SA 3.0 Yunnanozoon 525 Mya Haikouichthys 518 Mya  Hypothetical chordate ancestor Mussini et al. 2024. Curr Biol 34: 2980-2989.e2 Chordate ancestor was probably very lancet-like New interpretations of fossils provide insights Tunicates into chordate evolution. Lancelets Vertebrates By Peiyun Cong, CC BY 4.0 Pikaia Fossil early Yunnanozoon chordates are lancelet-like! Vetulicolians By Nobu Tamura, CC BY-SA 3.0 Yunnanozoon 525 Mya Haikouichthys 518 Mya Vertebrata Notochord Precursor to the vertebrae Neural tube Spinal cord and brain Postanal tail V-shaped myomeres  W- shaped myomeres Endostyle -- visceral Develop into thyroid gland. -- somatic Vertebrate features Vertebrae Cranium (braincase) Head, sense organs and brain Complex endocrine organs Muscularized gut tube Multichambered heart Mineralized tissues Gills derived from the endoderm Martinez-Morales 2016. Brief Funct Genomics 15: 315-321 Early vertebrate evolution Paired Paired pectoral pelvic fins fins Placoderms Ossified endoskeleto ns Donoghue & Keating 2014. Palaeontology 57: 879–893 The vertebrate body plan Basic Bauplan of vertebrate skeletons Basic Bauplan of vertebrate skeletons Splanchnocranium Primary palate and jaws, branchial elements Neurocranium Braincase Axial skeleton Backbone and ribs Appendicular skeleton Pectoral and pelvic fins or limbs and girdles Dermal skeleton External portions of the skull, teeth, armour plates, clavicle, patella Developmental origins Endoderm Splanchnocranium Mesoderm Splanchnocranium Neurocranium Axial skeleton Appendicular skeleton Ectoderm Splanchnocranium Dermal skeleton  Two types of vertebrate skeletons Cartilaginous skeleton Bony skeleton Evolution of ossification Evolution of the endoskeleton Cartilagino us Basal (non-osteichthyan) endoskelet on vertebrates all have cartilaginous endoskeletons. But could show some level of mineralization Cartilage calcification Accumulation of calcium salt in cartilage Perichondral ossification Bone forms on cartilage surface Osteichthyes have bony endoskeletons. Formed through endochondral Bony ossification endoskeleton Bone replaces cartilage Hirasawa and Kuratani 2015. Zoological Letters Evolution of the exoskeleton Bony exoskeletons are widespread across both cartilaginous and bony vertebrates, first appearing in Galeaspida. Exoskeletons form through cartilage calcification, intramembranous ossification or perichondral ossification Hirasawa and Kuratani 2015. Zoological Letters Basal vertebrates only possess cartilage Basal vertebrates only possessed cartilage Lack mineralised skeletons Calcified cartilage in galeaspids but no bones Living examples: Cyclostomes (Hagfish & lampreys) Extinct examples: Conodonts Galeaspids Hirasawa and Kuratani 2015. Zoological Letters Hagfish skeleton Alcian blue stain – stains polysaccharides in cartilage Hagfishes only have cartilaginous skulls (chondocranium) and no vertebral column around notochord But have arcualia (cartilaginous precursors to vertebrae) in the tail. Ota et al. 2011. Nat Comms 2: 373 Lamprey skeleton Lampreys have an internal skeleton consisting of: A notochord Vertebra-like structures An attached cartilaginous skull and gill arches Fin rays Early vertebrate skeletons Miyashita et al. 2019. PNAS 116: 2146-2151 Hagfishes Lampre ys Ossification in jawless fishes Ossified exoskeleto n Calcified cartilaginous Galeaspid endoskeleton a Ossification in jawless fishes Ossified Paired pectoral exoskeleto fins with ossified n Ossified endoskeleton exoskeleto n Calcified cartilaginous Galeaspida Osteostra endoskeleton ci Ossification in placoderms Placoderms have cartilaginous endoskeletons with ossification perichondral ossification Head and torso covered in extensive bony armour plates. Cartilaginou s First vertebrates to have endoskeleto n paired pelvic fins.  Ossification in chondrichthyans Endoskeleton of prismatic calcified cartilage Cartilaginous skull Bone present in scales (dermal denticles) and Dermal denticle teeth s Some sharks show Devonian shark with prismatic perichondral ossification calcified cartilage along vertebrae Ossification in chondrichthyans Dermal denticles Cartilage staining blue stained for bone (Alcian blue) Dermal denticles staining red for bone (Alizarin red) Perichondral ossification along outer layer of the vertebrae Stained blue Outer layer (black for cartilage arrow) stained red for bone Eames et al. 2007. J Anat 210: 542-554 Evolution of ossification A. In stem vertebrates, endoskeleton composed entirely of cartilage. B. Osteostracans and non- osteichthyes jawed vertebrates evolved ossified endoskeletons. Endo- and exoskeletons developed on the surface of cartilage (perichondral Evolution of ossification C. Osteichthyes acquired endochondral ossification Bony tissues are produced within (as well as on top of) cartilage. Bony tissues eventually replace cartilage.