The Chordate PhylumChordate phylum: 6 vertebrate and 2 invertebrate classes, all having a similar set of 25,000 genes

Chordates: 6 vertebrate classes

Class: Birds. Hornbill

Class: Mammals. Mouse Class: Mammals. MandrillClass: Mammals.Monotremes; Echidna

Class: Reptiles. Bardick snake Class: Birds. Hummingbird

Phylum: Chordates. 6 vertebrate classes

Class: Jawless fish. Hagfish Class: Bony fish. Teleost. Anemone fish

Class: Amphibia. Xenopus tadpolesClass: Bony fish. Coelacanth

Class: Cartilaginous fish. Ratfish, Chimera

Invertebrate chordates: 2 classes

Phylum: Echinoderms. Starfish

Phylum: Chordates. Class: Urochordates; Ascidians

Phylum: Hemichordates; Class: Enteropneust

Phylum: Chordates. Class: Cephalochordates

posterior

anterior

Nearest phyla to chordates

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

4-cell stage, 2.5 hrFertilized egg, 30 min.

Unfertilized eggs, in jelly coatsOocytes in the ovary. Note blood vessels

Spermentrypoint

Greycrescent

Stages of Xenopus development

Tadpoles, 2 weeks old, feeding

Late gastrula, 16 hr

Late neurula, 30 hr

Late blastula, 9 hr. About 8000 cells

Neural folds

Blastoporealmostclosed

Animalpole view

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 midbrain-hindbrain boundary

(zebrafish example)