Chapter 32 Introduction to Animal Evolution p

Chapter 32 Introduction to Animal Evolution p. 633

What is an animal? p.633

  1. Multicellular, heterotrophic eukaryotes that must consume preformed organic molecules (preformed by autotrophic organisms). Ingestion- eating other organisms or organic material that is decomposing.
  2. Lack cells walls. Replaced by structural proteins, especially collagen. Also the plasmodesmata in plants are replaced by more specialized gaps…i.e. tight junctions, desmosomes, and gap junctions.
  3. Contain two types of tissues: Nervous tissue and muscle tissues.
  4. Reproduce sexually, with the diploid stage being the dominant form. A small motile (flagellated) sperm fertilizes a larger nonmotile egg forming a diploid zygote.
    1. Zygote undergoes Cleavage a succession of mitotic cell divisions p. 634 Figure 32.1. Sea Urchin 1-4 Cell Cleavage

·         Read p. 403 From a single cell to multicellular organisms 

 

 

·        Embryonic development involves cell division, cell differentiation, and morphogenesis  Note the terms differentiation and morphogenesis.

 

 

·        Read p. 1002 Cleavage partitions the zygote into many smaller cells (see summary of this section below)  See website

 

 

 

    1. This division leads to the formation of a multicellular hollow ball called the Blastula

 

    1.  The Blastocoel is the name given to the cavity inside the Blastula.

 

 

    1. Next Gastrulation  takes place. During this process the embryonic tissues are produced. These 3 layers, called germ layers will differentiate into different parts of the adult form.  The vegetal poles buckles inward ( a process called invagination) forming a pouch called the Archenteron, the primitive gut. The opening of archenteron is called blastopore, which later becomes the anus. See p. 1005 Figure 47.9

·        Note that some animals do not go from egg to adult form. A larval stage is separate stage with specific requirements not associated with adult.          

·        The three-layered embryo is called the gastrula

 

 

 

 

3 layers

(see table 47.1 p. 1007 for more info)

1. Ectoderm (outer layer)

                        becomes epidermis of skin, lining of mouth and rectum, eyes, nervous system, various glands

 

 

2. Mesoderm (middle layer)

                        becomes notochord, and the following systems: skeletal, muscular, circulatory, lymphatic and excretory.

 

 

3. Endoderm (inner layer)

                        becomes liver, pancreas, urinary bladder, innermost lining of digestive tract and associated organs, and reproductive organs

 

 

 

 

Check out the following:      Amphibian Stages of Development

                                           Mammalian Cleavage and Morula

                                           Stages of Human Development:   Weeks 3-8

                                                                                              Human Development Animation

Also Read

    • Each cell of a multicellular eukaryote express only a small fraction of its genes p. 362  Chapter 19
    • Different cell types make different proteins usually as result of transcriptional regulation p. 410...you may stop reading after the example of lens production. 
    • Gastrulation rearranges the blastula to form a three-layered embryo with a primitive gut See figure 47.9 p. 1005

           

 

 

 

 

  • Transformation of zygote to adult form is controlled by regulatory genes called Homeobox genes (Hox gene)

o       Read Homeotic genes direct the identity of body parts p. 417, first paragraph only and 

o       Homeobox genes have been highly conserved in evolution p. 417 First paragraph only

 

 

 

 

Summary: Cleavage partitions the zygote into many smaller cells  p. 1002 See Figure 47.6

3 stages of development

1. Cleavage- cell division

·        Cell division creates a ball of closely packed smaller cells called blastomere

·        In most animals cell division occurs in a specific pattern. This pattern is determined by the location of the yolk (stored nutrients). Yolk is most concentrated at one pole of the egg, called the vegetal pole. The animal pole has the lowest concentration of the yolk. See Figure 47.8 p. 1004

·        At the end of this process a solid ball of cells will be produced called the morula.

 

 

 

 

2. Gastrulation- formation of 3-layers of embryonic tissues

 

 

 

 
  1. Organogenesis- formation of rudimentary organs which grow adult structures

 

 

 

 

 

 

The traditional phylogenetic tree of animals based mainly on grades in body plans. P. 635

 

Grade- refers to body plans shared by animals belonging to that branch...see Figure 32.4 p.636

 

 

4 main dichotomes (branches)

1. True tissues vs. no true tissues

            parazoans: lack true tissues

            eumetazoan: true tissues

 

 

2. Radial-Bilateral symmetry see Figure 32.5 p. 637

            Radial: has top and bottom but no head or tail (no left or right)

                        Diploblastic: contain only 2 germ layers (ectoderm and endoderm)

 

 

            Bilateral: has Dorsal (top), ventral (bottom), anterior (head) and posterior (tail).

                        Also show cephalization, the concentration of sensory equipment on the

                        anterior end.

                        Triploblastic: contain 3 germ layers (endo,vecto and meso)

 

 

 

3. Cavity arrangement see Figure 32.6 p. 638

            Acoelomates- solid bodies; no cavity between the digestive tract and outer body wall i.e. the flatworms (Phylum Platyhelminthes)

 

 

            Pseudocoelom- contain a body cavity ( a fluid filled space separating the digestive tract from the outer body wall). The cavity of the pseudocoelom is not lined with tissues derived from the mesoderm i.e.  roundworms (Phylum Nematoda)

 

 

            Coelomates- animals that contain a true coelom, a fluid filled body cavity lined with tissues derived from mesoderm.

            Function of body cavity:

                        The fluid cushions the organs from injury

                        In worms it acts a hydrostatic skeletal system which allows muscle to work

 

 

 

           

4. Protostome-Deuterostome

            1. This difference is basically in how the body cavity forms in Coelomates. If the body cavity is made from a mass of cells breaking away from the mesoderm the animals are called protostomes. The body cavity of deuterostome is formed when the mesoderm buds from the wall of archenteron. See figure 32.7b p. 639

 

 

            2.  Differences in cleavage. Protostomes show a radial cleavage and the deuterostomes show spiral cleavage. See Figure 32.7a p. 639 See web Spiral vs Radial Cleavage

 

 

            3. Blastopore Fate. In protostomes the blastopore becomes the mouth; in deuterostomes the anus. See Figure 32.7c p. 639

 

 

 

 

 

3 Main hypotheses for what caused the diversification of animals

 

  1. Ecological Causes: The emergence of the predator-prey relationships during the Cambrian period. The evolutionary advantage of developing protective shells and diverse modes of locomotion led to diversity

 

 

  1. Geologic Causes: Example: atmospheric oxygen levels reached a point where life with high metabolism could obtain the energy needed to thrive. 

 

 

  1. Genetic Causes: Evolution of the Homeobox (Hox gene). These regulatory genes led to variations in morphology.

 

 

 

           

           

 

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