09.9.7 1 CHAPTER 2, LIFE CYCLE AND THE EVOLUTION OF DEVELOPMENTAL PATTERNS ZOO3603C Satomi Kohno • The frog life cycle The Circle of Life: the Stages of Animal development The Cycle of Life  Fertilization  Embryogenesis  Cleavage  MorulaBlastula  Gastrulation  Gastrula  Organogenesis  Neurula  Metamorphosis  Gametogenesis  Germ cell ↔ Somatic cell  Germ cell: sperm & egg  Somatic cell: other cells of body Developmental history of the leopard frog, Rana pipiens 2.2 Early development of the frog Xenopus laevis (Part 1) Vegetal Hemisphere: More yolk Animal Hemisphere: Less yolk Oocyte Mating Laid eggs Brown side is animal cap. 2.2 Early development of the frog Xenopus laevis (Fertilization - Cleavage) Fertilization Cleavage Cleavage 2 cell 4 cell Morula Blastula Blastula Blastocoel: A fluid filled cavity 09.9.7 2 2.3 Continued development of Xenopus laevis (Gastrulation - Organogenesis) Gastrulation Blastopore Ventral Dorsal Neural folds Neural groove Open neural tube Neurula Organogenesis Gastrula 2.3 Continued development of Xenopus laevis (Part 2) Organogenesis Neurula Somites Tailbud Brain Gill Stomodeum Forebrain; Eyes Larval stage • Somites: future dermis, skeletal muscle or vertebrae. • Stomodeum: future mouth or anterior lobe of pituitary. 2.4 Metamorphosis of the frog Metamorphosis Initiated by thyroid hormone Premetamorphic tadpole Prometamorphic tadpole Onset of metamorphic climax Climax Stages Climax Stages Detail mechanism in chapter 18 As metamorphosis ends, the development of the first germ cells begins. 2.5 Gametogenesis 1. Chromosome replication 2. Holding together by kinetochores (centromeres) The nuclear envelope breaks down and homologous chromosomes align in pairs. Chromatin: complex of DNA and protein Chromosomal rearrangements 2.5 Gametogenesis 3. The 1st meiotic division separates the pair together. The two original homologous chromosomes are segregated into different cells. 2.5 Gametogenesis 4. The 2nd meiotic division splits the kinetochore. 5. Each cell has a haploid nucleus. Chromosome align on the middle 09.9.7 3 The Diploblasts, Protostomes and Deuterostomes Developmental Patterns among the Metazoa 2.16 Major evolutionary divergences in extant animals Metazoans: multicellular animals Diploblastic animals: Ectoderm & endoderm, but no mesoderm Triploblastic animals: Three germ layers, ectoderm, endoderm & mesoderm Deuterostomes: Mouse second Protostomes: Mouse first Ecdysozoa: Exterior skeletons Lophotrochozoa 2.16 Major evolutionary divergences in extant animals (Part 3) Diploblastic animals: Ectoderm & endoderm, but no mesoderm 2.16 Protostomes in Triploblastic animals Protostomes: Mouse first; Schizocoelous, body cavity formed by a solid block of mesoderm Ecdysozoa: Exterior skeletons; Molting Lophotrochozoa: Spiral cleavage 2.16 Deuterostomes in Triploblastic animals Deuterostomes: Mouse second; Enterocoelous, mesodermal pouches extending from the gut. Protostomes vs Deuterostomes Phyla Characteristic Body Cavity Formation Body Cavity Characteristic Protostomes Mouse first Schizocoelous a solid block of mesoderm Deuterostomes Mouse second Enterocoelous mesodermal pouches extending from the gut VS Protostomes Deuterostomes 09.9.7 4 2.19 Amniote egg of the chick, showing membranes the enfolding 7-day chick embryo  Amniote egg  Occurred among the deuterostomes.  Carrying own water and food supplies---no more pond!!!  Containing 4 sacs: 2.19 Amniote egg of the chick, showing membranes the enfolding 7-day chick embryo  Amniote egg  Containing 4 sacs:  Yolk sac: storing nutritive proteins.  Amnion: containing the fluid.  Allantois: waste materials.  Chorion: interacting with the outside environment. Summary  Whole basic life cycle from fertilization to gametogenesis is the development.  Depending on the development of blastopore, triploblastic animals are categorized into two group, protostomes and deuterostomes.  Amniote egg is one of the greatest evolutionary advances. Next class  Chapter-5: The paradigm of differential gene expression