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Chapter 33

BIO SCI 94 Chapter Notes - Chapter 33: Comparative Genomics, Animal Locomotion, Meiosis

Biological Sciences
Course Code
Robin Bush

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FChapter 33 An Introduction to Animals
33.1 What is an Animal?
Ancestors to animals were single-celled protists
Occur in linage called Opisthokonta
Animals are eukaryotes that share key traits:
All animals are multicellular, with cells that lack cell walls but have an
extensive extracellular matrix, with proteins specialized for cell-cell
adhesion and communication
All animals are heterotrophs, most ingesting than absorbing.
All animals move under their own power at some point in life cycle
All animals other than sponges have neurons and muscle cells
Muscles are adaptations for large organism to move efficiently
Animals are the only multicellular heterotrophs on the tree of life that usually
ingest their food first, before they digest it.
The combination of multicellularity, heterotrophy, and efficient movement makes
animals the largest predators, herbivores, and detritivores on Earth.
33.2 What Key Innovations Occurred during the Evolution of Animals
Biologists study evolution of animals with three types of data
1. Fossils - important bc provide the only direct evidence of what ancient
animals looked like when they existed and where they lived
2. Comparative morphology - provide information about which embryonic,
larval, or adult morphological characteristics are common among groups
of animals and which are unique to individual lineages
Data can be used to define body plan of each animal lineage
3. Comparative genomics - provides information about the relative
similarity of genes or whole genomes of diverse organisms
Origin of Multicellularity
First important insight is that animals are monophyletic group
Sponges include two most ancient lineages of animals
Multicellularity appears to have originated in sponge-like animal
Sponges are earliest animals to appear in fossil record 600 mya
Sponges share several key characteristics with choanoflagellate
Both are sessile, meaning adults live permanently attached
to a substrate
Both feed using cells with nearly identical morphology
Choanocytes - sponge feeding cells
Some sponges have true epithelium - a layer of tightly joined cells
that covers the interior and/or exterior surface of the animal
Essential to animal form and function
Sponges distinguished in part by type of spicules they produce
Spicules - stiff spikes of silica or CaCO3 that with collagen
fibers provide structural supports to the ECM
Sponges have complex developmental tool kit of genes that contains
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Genes necessary for specialization of cell types
Genes necessary for regulation of cell cycling and growth
Adhesion among cells and b/w cells and ECM
Recognition of self and nonself, thus innate immunity
Developmental signaling and gene regulation
Programmed cell death
Series of important genetic innovations appears to have occurred
at root of animal tree along with multicellularity. Subsequent
duplication and diversification of these genes contributed to
diversification of animals
Origin of Embryonic Tissue Layers
Sponges don’t have complex tissues - groups of similar cells that are
organized into tightly integrated structural and functional units
Animals other than sponges typically divided into two major
groups based on number of embryonic tissue layers
Embryonic tissues organized in layers called germ layers
Diploblasts - animals w/ embryos of 2 types of tissues
Ectoderm and endoderm
Same pattern as in triploblasts, but muscle is
simpler in organization and derived from ectoderm
and reproductive tissues derived from endoderm
Triploblasts - animals w/ embryos of 3 types of tissues
Mesoderm in b/w ectoderm and endoderm
Mesoderm important bc gave rise to first
complex muscle tissue for movement
In general, ectoderm produces covering of animal
and endoderm generates digestive tract. Mesoderm
gives rise to the tissues in between.
Origin of Bilateral Symmetry, Cephalization, and the Nervous System
Body symmetry is a key morphological aspect of an animal’s body plan
Ctenophores, many cnidarians, and some sponges have radial symmetry
meaning they have at least two plants of symmetry
Radial symmetry evolved independently in phylum Echinodermata
(sea stars, sea urchins, feather stars, and brittle stars)
Organisms with bilateral symmetry have on plane of symmetry and tend
to have a long narrow body
Almost all cnidarians appear radially symmetric, but internal morphology
reveals bilateral symmetry in some species
In bilaterians, symmetry achieved by anterior-posterior axis
formation and dorsal-ventral axis formation
Hox genes and dpp gene expression pattern parallel those
observed in bilaterians, supporting hypothesis that bilateral
symmetry in Nematostella is homologous to bilateral
symmetry in triploblastic animals
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Genetic tool kit that determine bilateral symmetry
arose before the evolutionary split of cnidarian and
bilaterian lineages.
One hypothesis is that evolution of nervous system and head are tightly
linked to bilateral symmetry and these characteristics contributed to
radiation of bilaterians.
Sponge lack both nerve cells and symmetry
Ctenophores and cnidarians have nerve cells that are organized
into a diffuse arrangement called a nerve net
All other animals have CNS with some neurons clustered into one
or more large tracts or cords that project throughout the body with
others clustered into masses called ganglia.
Evolution of CNS coincide with cephalization: evolution of
head/anterior region, where structures for feeding, sensing
the environment, and processing info are concentrated
To explain the pervasiveness of bilateral symmetry, biologists
point out that locating and capturing food is particularly efficient
when movement is directed by a distinctive head region and
powered by the rest of the body.
Lineages with triploblastic, bilaterally symmetric,
cephalized body had the potential to diversify into an array
of formidable eating and moving machines
Acoels’, tiny worms, current position supports hypothesis that
CNS and cephalization were key to subsequent radiation of
Origin of the Coelom
The basic bilaterian body shape is a tube within a tube.
Inner tube = individual’s gut with mouth and anus at the ends
Outer tube forms nervous system and skin
“Tube-within-a-tube” body plan’s potential biomechanical and
physiological challenge if inner tube attached to outer tube via mesoderm
solved by fluid-filled cavity b/w the inner and outer tubes called coelom
Coelom provide space for circulation of oxygen and nutrients and
internal organs to move independently of each other.
Phylogenetic evidence suggests coelom arose in common ancestor of
protostomes and deuterostomes
True coelomates - bilaterians that posses coelom completely lined
with mesoderm
Acoelomates - bilaterians that subsequently lost their coelom
Pseudocoelomates - bilaterians that retained coelom but lost the
mesodermal lining in parts of their coelom
Coelom was a critically important innovation during animal evolution in
part bc an enclosed, fluid-filled chamber can act as an efficient
hydrostatic skeleton.
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