The Chordate
Chordate phylum:
6 vertebratePhylum
and 2 invertebrate
classes, all having a similar set of 25,000 genes
Chordates: 6 vertebrate classes
Class: Birds. Hummingbird
Class: Reptiles. Bardick snake
Class: Birds. Hornbill
Class: Mammals.
Monotremes; Echidna
Class: Mammals. Mouse
Class: Mammals. Mandrill
Phylum: Chordates. 6 vertebrate classes
Class: Jawless fish. Hagfish
Class: Cartilaginous fish.
Ratfish, Chimera
Class: Bony fish. Coelacanth
Class: Bony fish. Teleost.
Anemone fish
Class: Amphibia. Xenopus tadpoles
Invertebrate chordates: 2 classes
posterior
anterior
Phylum: Chordates. Class: Cephalochordates
Phylum: Chordates. Class:
Urochordates; Ascidians
Nearest phyla to chordates
Phylum: Echinoderms. Starfish
Phylum: Hemichordates; Class: Enteropneust
Chordates and Arthropods are descended from a pre-Cambrian
bilateral ancestor; as are the other 24 bilateral phyla
Xenopus laevis,
the South African three clawed frog
Oocytes surgically removed
Stages of Xenopus development
Oocytes in the ovary. Note blood vessels Unfertilized eggs, in jelly coats
Sperm
entry
point
Fertilized egg, 30 min.
Grey
crescent
4-cell stage, 2.5 hr
Stages of Xenopus development
Animal
pole
view
Blastopore
almost
closed
Late blastula, 9 hr. About 8000 cells
Late gastrula, 16 hr
Neural folds
Late neurula, 30 hr
Tadpoles, 2 weeks old, feeding
Phylotypic stage(pharylgula/tailbud stage): first
embryonic stage to possess the main chordate
traits, and an “adult”map of >200 gene expression
domains covering the 3 germ layers
(all preceding organs and cell types)
Gene expression domains at the midbrainhindbrain boundary (zebrafish example)