developmental basis of evolutionneural crest groups • cephalic, leave before neural folds close...
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Developmental Basis of Evolution
Evolution• Building blocks present early
• -genes gave rise to position and axes
• HOX (A-P), SOG/Chordin (DV), Distalless (P-D), PAX (eye)- insect/chordate
• Urbilateria- cartoon on following slides
• evolution by tinkering rather than developing new genes-
• most established before Cambrian explosion when 37 phyla were evident
• modularity- building blocks available
Major events in evolution-mitochondria/chloroplasts (O2)-meiosis-multicellularity, embryos-axes
Modules
• Genes–Change in reading frame–Change in upstream region
• Inductive interactions–Paracrines and receptors–Juxtacrines
• Tissues–Specified, determined, stem cells–Migration
Gene Families
• Gene duplication followed by change of function
• Change in pattern of expression• Change in role of transcription factors
• Globin, Hox genes
Heterotopy- Change in location
Mammal
Hox genes
• Proximal-distal pattern set by hox A
• Anterior-posterior pattern set by hox D
• Examples of mutations -synpolydactyly – -hoxD13 mutated
Early Vertebrate Evolution
• Dorsal nerve cord/notochord• Myomeres-locomotion• Filter feeders to carnivores
–Cephalization- brain and sensory structures–Jaws–Skull
• Limbs- tetrapods
Vertebrate origins
• Basics in Amphioxus–simple brain with
retina, notochord
–neural crest equivalent
• change from filter feeding to raptorial
• mouth bones, teeth
• head development strongest
Heterochrony- urochordata metamorphosis -tunicates -larvaceansTail remains in adult(handout)
Neural Crest Groups
• Cephalic, leave before neural folds close– form pharyngeal pouches, craniofacial, cranial nerves
• Trunk, – dorsal path- melanocytes
– ventral path (through ant sclerotome)- dorsal root ganglion, sympathetic ganglion, adrenal medulla
• Vagal (somites 1-7)
• Cardiac (myel, somites 1-3)– travel via blood vessels, form septa between aorta and
pulmonary artery
• Cervical (somite 28)
New cell types (ectomes) arise in vertebrates
• Origin of neural crest cells at boundary of epidermis and neural plate
• Cell types present in neochordates but they do not migrate
• Transcription factors AP2 and distalless present in cephalochordates in non-neural ectoderm
• These genes are transcribed in vertebrates and leads to migration of neural crest cells
• Pleuripotent-specified-determined-differentiated
Limb Formation• Limb field established
through HoxC-6 gene near first thoracic vertebra, near pronephros
• -may be initiated by RA from Hensen’s node which would activate Hox genes
• Axes established early:
• A-P (shh)- by neurula• D-V (wnt-7)
• P-D (Hox, FGF, distalless)
Limb field
Humerus rad/ulna carpals/phalanges
femur tibia/fibula tarsals/phalanges
Limb Bud
• Hox 4,5 (forelimb)
• Hox 9-11 (hindlimb)
• Tbx (transcription factor)
• FGF
• distalless establishes evaginations
• Apical Epidermal Ridge stimulated by mesoderm (and vice versa)
• -cells enlarge and secrete FGF8,4
FGF
shh
Progress zone-controlled by FGF
Heterometry-change in amount-allometry
Pharyngula- most vulnerable stage-great deal of regulation before this stage-many inductive events but they are modular
Formation of the jaw
Neural crest cells as modules
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