topic 19 biol 108.pdf

Topic 19: Introduction to animals Fig 28.2 BIOL 108 Winter 2024 © 2024 Neil Harris The hypothesis for phylogenetic relationships among eukaryotes suggests that Fungi and Animals belong to the Opisthokonts clade, which is a part of the Unikonta protist supergroup. − Biologists have classified >1.3 mi...

Topic 19: Introduction to animals Fig 28.2 BIOL 108 Winter 2024 © 2024 Neil Harris The hypothesis for phylogenetic relationships among eukaryotes suggests that Fungi and Animals belong to the Opisthokonts clade, which is a part of the Unikonta protist supergroup. − Biologists have classified >1.3 million animal species. But what exactly defines an animal? − Animals are multicellular, heterotrophic eukaryotes with tissues that originate from embryonic layers.  However, there are exceptions to nearly every criterion used to distinguish animals from other life forms. Highly diverse Complex, multicellular Photosynthetic Kingdom Animalia 1 Characteristics of animals 1. Cell structure and specialization: − Animals are multicellular eukaryotes, except for gametes. − Animals lack cell walls found in other multicellular eukaryotes (plants, algae, and fungi).  Cells and tissues are interconnected by extracellular structural proteins, with collagen being the most abundant in the human body.  Tissues consist of groups of cells with shared structure and/or function. BIOL 108 Winter 2024 © 2024 Neil Harris − Animal somatic (non-reproductive) cells differentiate into specialized types, such as those involved in digestion, secretion, protection, and transport.  The number of cell types varies widely among animals, ranging from around 4-5 in simple organisms like sponges to >100 in humans.  Includes specialized cells not found in other multicellular organisms: Neurons (nerve cells), which generate and conduct nerve impulses, are components of nervous tissues. Contractible muscle cells, which form different types of muscle tissues, are responsible for the body movement of animals. Nervous tissue and muscle tissue are defining characteristics of animals. 2 Characteristics of animals 2. Nutritional mode: − Animals are chemoheterotrophs, relying on preformed organic molecules for both carbon and energy (as are fungi).  Animals cannot construct all their organic molecules; they obtain these organic molecules by consuming other organisms. − Animals are phagotrophic heterotrophs.  Animals ingest and digest food particles internally.  cf. external digestion in fungi (absorptive heterotrophs). Fig 13.6 BIOL 108 Winter 2024 © 2024 Neil Harris 3. Reproduction: − Most animals reproduce sexually, with the diploid (2n) stage dominating the life cycle.  cf. haploid-dominated life cycle in fungi.  While animals may have multiple life stages, some of which reproduce asexually, all stages are diploid. − Sexual reproduction involves meiosis, producing haploid (1n) gametes that fuse during fertilization to form a diploid (2n) zygote.  Animals produce gametes of different sizes: female gametes (eggs) are large, non-motile cells; male gametes (sperm) are smaller, motile cells. 3 Characteristics of animals ©Tom Adams/Getty Fig 46.2 Fission of sea anemone Anthopleura elegantissima (phylum Cnidaria) 3. Reproduction: − Many animals also reproduce asexually, generating genetically identical offspring from a single parent without the fusion of gametes (no fertilization).  Asexual reproduction is found in nearly half of all animal phyla. − Mechanisms of asexual reproduction:  Fission or fragmentation occurs commonly in invertebrate animals. The animal splits into two or more parts that regenerate into complete organisms.  Budding, the formation of new individuals from outgrowths of existing ones, is found only among invertebrates. BIOL 108 Winter 2024 © 2024 Neil Harris Planarian fission (phylum Platyhelminthes) − Mechanisms of asexual reproduction:  Parthenogenesis is the development of an embryo from an unfertilized egg cell. Parthenogenesis is found among invertebrates and vertebrates. − Benefits of asexual reproduction.  Supports rapid population growth when conditions are favourable.  Provides an alternative to sexual reproduction when reproductive opportunities are limited.  Some species can alternate between sexual and asexual strategies. Aphid gives birth to live nymph following parthenogenetic embryo development (WC) Fig 13.2a Budding of Hydra (phylum Cnidaria) 4 Characteristics of animals 4. Development: − Development occurs at many points in the life cycle of animals. − Fertilization creates a spherical diploid zygote that undergoes embryonic development. BIOL 108 Winter 2024 © 2024 Neil Harris Stages in early embryonic development of animals: 1) Following fertilization, the diploid zygote undergoes a series of rapid mitotic cell divisions called cleavage. 2) Cleavage transforms the zygote into an eight-cell embryo. Fig 32.2 Early embryonic development in animals Fig 47.2 Developmental events in the life cycle of a frog 5 Characteristics of animals BIOL 108 Winter 2024 © 2024 Neil Harris 4. Development: Stages in early embryonic development of animals: 3) In most animals, continued cleavage produces a multicellular, hollow blastula.  The blastula is typically a hollow ball of cells surrounding a central cavity called the blastocoel (coel = opening).  The blastula stage of embryonic development is found only in animals. Fig 32.2 Early embryonic development in animals 6 Characteristics of animals 4. Development: Stages in early embryonic development of animals: 4) Most animals also undergo gastrulation (gaster = stomach), forming a gastrula with different layers of embryonic tissues. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.2 Early embryonic development in animals  Cells from one end of the blastula fold inwards, eventually filling the blastocoel, producing two layers of embryonic tissues: the ectoderm (outer layer) and the endoderm (inner layer) (derm = skin).  Gastrulation is unique to animals; it’s not found in fungi or other multicellular eukaryotes.  The pouch formed by gastrulation, called the archenteron, opens to the outside via the blastopore. 7 Characteristics of animals 4. Development: BIOL 108 Winter 2024 © 2024 Neil Harris Fig 47.2 Developmental events in the life cycle of a frog Animal embryonic development is regulated by conserved genes: − Most animals, and only animals, have Hox genes.  Hox genes are crucial in animal evolution because they play a fundamental role in determining the body plan and segment identity during embryonic development.  Hox proteins coordinate the development of various structures along the anterior-posterior axis, e.g. legs, antennae, and wings in fruit flies, or the different types of vertebrae in humans.  The Hox family of genes is highly conserved. The arrangement of Hox genes along chromosomes remains similar across animal phyla. However, the number of Hox copies varies among animal phyla, e.g. least in jellyfish, most in vertebrates.  Additionally, many other developmental genes are conserved among animals, contributing to the regulation of embryonic development. Fig 21.20 Conservation of homeotic genes in a fruit fly and a mouse 8 Characteristics of animals 4. Animals exhibit two primary modes of development: − Direct development is where the animal after birth or emergence from an egg is a smaller version of its adult form.  A juvenile resembles an adult but is not yet sexually mature, e.g. humans.  No larval stages or metamorphosis. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 13.6 − Indirect development has intervening stages (larvae) with morphological and behavioural differences from the sexually mature adult stage, e.g. caterpillar → butterfly.  Most animals have at least one larval stage.  A larva is sexually immature and morphologically distinct from the adult; it eventually undergoes metamorphosis to become a juvenile. Fig 47.2 Developmental events in the life cycle of a frog Direct development http://dx.doi.org/10.4236/psych.2011.29140 Indirect development 9 Characteristics of animals BIOL 108 Winter 2024 © 2024 Neil Harris 4. Motility in development: − Animals are motile: they are capable of moving their entire multicellular body using metabolic energy in at least one stage in their life cycle. − Many marine animals with sessile (immobile) adult forms have motile larval stages in their development. WC Walking/running WC Flying WC pbs.org Undulatory locomotion (slithering) Swimming WC Sessile adult stage of barnacle Elminius modestus (crustacean) WC Motile larva of barnacle Elminius modestus 10 Animals can be characterized by “body plans” BIOL 108 Winter 2024 © 2024 Neil Harris Animals can be characterized by their distinct body plans, a set of morphological and developmental traits. Some body plans have remained remarkably consistent throughout evolution, while others have undergone multiple changes over time. − e.g. the molecular regulation of gastrulation has remained unchanged for more than 500 million years. 11 Body plans Symmetry BIOL 108 Winter 2024 © 2024 Neil Harris Animals can be classified based on the symmetry of their bodies, or lack thereof (asymmetry). − Radial symmetry: Some animals exhibit radial symmetry, meaning they lack distinct front and back or left and right sides.  Radially symmetrical animals can be divided into numerous planes of symmetry, e.g. sea anemones.  Radially symmetrical animals are often sessile or planktonic (drifting or weakly swimming). Fig 32.8 Body symmetry − Bilateral symmetry: Most animals display bilateral symmetry, with a distinct left and right side and a single plane of symmetry, e.g. lobster.  Bilateral symmetry associated with: Cephalization, the development of a head region containing sensory organs. Specialized appendages for directional movement, grasping, or defence. Bilaterally symmetrical animals tend to be more active and possess a centralized nervous system. 12 Body plans Symmetry BIOL 108 Winter 2024 © 2024 Neil Harris Bilaterally symmetrical animals have a: − Right and left side. Dorsal (top) side Posterior (back) end Anterior (front) end Cephalization is anterior, i.e. a head region Ventral (bottom) side 13 Body plans Tissues Animal body plans also vary in the organization of animal tissues. − Most animals have tissues, specialized groups of cells with common structures and/or functions, which are isolated from other tissues by membranous layers.  Sponges and a few other groups lack true tissues and display atypical embryonic development. BIOL 108 Winter 2024 © 2024 Neil Harris − During development, distinct embryonic cell layers (germ layers) give rise to tissues and organs of animal embryos:  Ectoderm is the germ layer covering the embryo’s surface and gives rise to the skin and nervous system.  Endoderm is the innermost germ layer and lines the developing digestive tube, the archenteron. 14 Body plans Tissues Animal body plans also vary according to the organization of the animal’s tissues. − Diploblastic animals have two embryonic cell layers: ectoderm and endoderm.  Radially symmetrical animals are diploblastic; includes cnidarians and a few other groups. BIOL 108 Winter 2024 © 2024 Neil Harris − Triploblastic animals have an additional intervening mesoderm (meso = middle) layer that gives rise to muscles and other organs.  Bilaterally symmetrical animals are triploblastic: three embryonic cell layers (ecto-, endo-, and mesoderm).  Most animals are bilaterians (triploblastic), e.g. flatworms, arthropods, vertebrates, and others. 15 Body plans Body cavities Most triploblastic animals possess a fluid-filled body cavity. − A body cavity is a fluid-filled space or compartment in an animal body where internal organs develop. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.9 Body cavities of triploblastic animals Larger animals develop a coelom during embryonic development of the mesoderm.  The coelom is lined by mesodermal tissue, forming between the outer body wall (ectoderm) and the digestive tract (endoderm).  Coeloms contain coelomic fluid. − Coelom function varies between animals:  Inner and outer layers of mesoderm that surround the coelom connect and form structures that suspend the internal organs.  Allows internal organs to shift without deforming outside of the body, e.g. the digestive tract's movement and heart beating.  Cushions internal organs from external impacts.  A fluid-filled coelom often functions as a hydrostatic skeleton in softbodied animals by tensing muscles against the incompressible coelomic fluid, e.g. earthworms. Earthworm (WC) 16 Body plans Body cavities Many triploblastic animals have a hemocoel. − A hemocoel forms between the mesoderm and the endoderm. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.9 Body cavities of triploblastic animals  The hemocoel arises from the blastocoel, the embryonic cavity of the blastula/gastrula. Animals with hemocoels retain the blastocoel cavity during mesoderm development to form the hemocoel.  Because the hemocoel has a simple embryonic origin, hemocoels have evolved independently in many animal groups. − Hemocoels contain hemolymph (fluid) (hemo = blood).  Hemolymph is analogous to blood and is circulated throughout the body cavity in an open circulation system by the heart.  The hemocoel and hemolymph are involved with internal circulation, nutrient transport, and waste removal, but can also function as a hydrostatic skeleton in some animals. − Since coeloms and hemocoels have different embryonic origins, both can be found in some animal groups. Roundworms (Nematoda) possess hemocoels, e.g Caenorhabditis elegans (WC) 17 Body plans Body cavities BIOL 108 Winter 2024 © 2024 Neil Harris Some triploblastic animals are compact and lack a body cavity. − These are small, flat animals, e.g. flatworms (Platyhelminthes). − They do not require an internal transport and circulation system, instead relying on diffusion across the body surface. Fig 32.9 Body cavities of triploblastic animals Animals possessing coeloms are sometimes called coelomates. − But coeloms (and hemocoels) have been reduced or lost in several groups. − The presence or absence of coeloms and hemocoels is not a good indicator of phylogenetic relationships.  Animals with coeloms or hemocoels do not form clades. Marine flatworm (WC) 18 Body plans Embryonic development BIOL 108 Winter 2024 © 2024 Neil Harris Embryonic development of many bilateral (triploblastic) animals can be categorized as: − Protostome development or − Deuterostome development Protostome and deuterostome development differ in several key aspects: − Embryo cleavage − Coelom formation − Fate of the blastopore Fig 32.10 A comparison of protostome and deuterostome development 19 Body plan Embryonic development – Embryo cleavage In protostome development, cleavage is spiral and determinate. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.10 A comparison of protostome and deuterostome development − Each new row of cells is twisted slightly off-center (spiral). − Determinate cleavage means that each new cell is predetermined to form a specific part of the later embryo.  Removal of some cells results in an incomplete embryo, e.g. missing specific organs. 20 Body plans Embryonic development – Embryo cleavage In deuterostome development, cleavage is radial and indeterminate. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.10 A comparison of protostome and deuterostome development − Each cell division stacks the new cells directly above the previous ones. − Indeterminate: each cell in the early stages of cleavage retains the capacity to develop into a complete embryo. − Indeterminate cleavage allows each cell in the early stages of cleavage to retain the capacity to develop into a complete embryo.  Indeterminate cleavage enables the formation of identical twins and embryonic stem cells. 21 Body plans Embryonic development – Coelom formation In protostome development, the coelom forms through the splitting of solid masses of mesoderm. In deuterostome development, the mesoderm folds from the wall of the archenteron to form the coelom. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.10 A comparison of protostome and deuterostome development 22 Body plans Embryonic development – Fate of the blastopore The blastopore forms during gastrulation and connects the archenteron to the exterior of the gastrula. In protostome development, the blastopore becomes the mouth (proto = first, stoma = mouth). In deuterostome development, the blastopore becomes the anus, while a second invagination forms the mouth (deutero = second). BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.10 A comparison of protostome and deuterostome development 23 Animal phylogeny BIOL 108 Winter 2024 © 2024 Neil Harris Animal phylogenies integrate morphological, molecular, and fossil data to understand evolutionary relationships. − Traditional animal systematics relied solely on morphological and developmental traits. − Modern methods incorporate molecular and fossil evidence, resulting in multiple hypotheses for relationships among animal groups, one of which is shown in Fig 32.11. − Zoologists recognize ~35 animal phyla.  15 major phyla are shown in Fig 32.11. Fig 32.11 A phylogeny of living animals 24 Animal phylogeny True tissues − Kingdom Animalia constitutes clade Metazoa, multicellular animals. 2. Sponges are basal animals in the phylogeny. 3. Eumetazoa (“true animals”) is a clade of animals with true tissues. 4. Most animal phyla belong to the clade Bilateria, animals with bilateral symmetry (bilaterians). 5. There are three major clades of bilaterian animals, all of which are invertebrates, animals that lack a backbone, except most of Chordata, which are classified as vertebrates due to the presence of a backbone. Bilateral symmetry, triploblastic Radial symmetry, diploblastic Fig 32.11 A phylogeny of living animals Key features of the phylogeny of extant animals: 1. All animals share a single common ancestor, an ancestral colonial flagellated protist. BIOL 108 Winter 2024 © 2024 Neil Harris 25 Major clades of bilaterian animals The bilaterians are divided into three clades: 1. Deuterostomia includes hemichordates (acorn worms), echinoderms (starfish and relatives), and chordates. − Deuterostomia exhibits deuterostome embryonic development and includes both vertebrates and invertebrates. “Protostomia” Fig 32.11 A phylogeny of living animals. BIOL 108 Winter 2024 © 2024 Neil Harris 26 Fig 32.12 Ecdysis 2. Ecdysozoans are invertebrates that undergo ecdysis, the process of shedding (moulting) their exoskeletons. − e.g. Arthropoda, which includes insects, arachnids, crustaceans, and others. 3. Lophotrochozoans − Some have a feeding structure called a lophophore, a tentaclecovered feeding structure, while others exhibit a distinct larval stage called a trochophore larva. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 32.11 A phylogeny of living animals Major clades of bilaterian animals  e.g. molluscs and annelids − Some Lophotrochozoan phyla exhibit characteristics of both proto- and deuterostome development. Fig 32.13 Morphological characteristics of lophotrochozoans 27