The Chordate Phylum Chordate 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. MouseClass: Mammals. Mandrill Class: 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 tadpoles Class: 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 hr Fertilized egg, 30 min. Unfertilized eggs, in jelly coats Oocytes in the ovary. Note blood vessels Sperm entry point Grey crescent

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 Blastopore almost closed Animal pole 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)