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