ANIMAL PHYLOGENY AND DEVELOPMENT
ANIMAL PHYLA
Key Taxa Overview
Choanoflagellata
Bilateria
Protostomia
Deuterostomia
Porifera
Cnidaria
Mollusca
Lophotrochozoa
Annelida
Platyhelminthes
Ecdysozoa
Nematoda
Arthropoda
Echinodermata
Chordata
ASSESSMENT INFORMATION
Assessment Format
Type: 1 hour MCQ exam covering TB2 content
Timing: During the summer exam period
Mode: In person, on computers
Topics Covered
Animals
Plants
Phylogeny
Evolution
Comparative characteristics
OVERVIEW OF ANIMALS
Key Questions Addressed
Structural Variety: What are animal structures like?
Functional Aspects: How do they function, including eating, moving, reproducing?
Development: How do they develop?
Relationship Among Groups
Non-bilaterians:
Sponges
(Ctenophores)
Cnidarians
Deuterostomes:
Echinoderms
(Hemichordates)
Chordates
Protostomes:
(Arrow worms)
Platyhelminths
Annelids
Molluscs
Nematodes
(Tardigrades)
(Onychophorans)
Arthropodes
STRUCTURE YOUR REVISION
Key Areas for Each Major Group
Phylogenetic Relationships:
Relationships between phyla
Subdivisions within phyla
Basic Developmental Biology:
Reproduction and typical larvae
Life history
Basic Body Plan and Function:
Phylum-typical features
Coelom: presence/absence/type
Digestive system: complete or incomplete?
Circulation
Excretion
Nervous system: present/absent/type
Motility: does it move?
Emphasis on Vertebrates
More detailed study of vertebrates compared to non-chordate phyla
Key evolutionary innovations and their timings
Differences and characteristics among vertebrate groups
Phylogenetic relationships within vertebrates
LECTURE PLAN
Animals Lectures Schedule
Introduction and development
Non-bilaterians: Ctenophores, Sponges, Cnidarians
Deuterostomes 1: Generalities, Echinoderms (Hemichordates)
Deuterostomes 2: Chordates and vertebrate evolution
Deuterostomes 3: Chordates: Fishes and jaw evolution
Deuterostomes 4: Chordates: Amphibians and reptiles; transition to land
Deuterostomes 5: Chordates: Birds and mammals
Practical: Comparative skeletal anatomy
Protostomes 1: Generalities; Lophotrochozoa; Platyhelminthes
Protostomes 2: Molluscs
Protostomes 3: Annelids
Protostomes 4: Ecdysozoa; Nematodes (Dr. Lena Grinsted)
Protostomes 5: Arthropods (Dr. Lena Grinsted)
Plants Lectures Schedule
Introduction to Plants
Early Vascular Plants
Seed plants: Gymnosperms
Seed plants: Angiosperms
Angiosperms 2
Angiosperms 3
Plants practical
LEARNING OUTCOMES
Expected Competencies Upon Lecture Completion
Understand phylogeny and taxonomic classification in the animal kingdom
Distinguish between Protostomia and Deuterostomia
Describe the general animal life cycle
List and describe stages of embryogenesis
Understand Hox genes and their functions
PHYLA SPECIFIC TO ANIMALS
Major Groups of Animal Phyla
Onychophorans
Nematomorphs
Kinorhynchs
Priapulids
Platyhelminthes
Nemerteans
Echiurans
Sipunculans
Brachiopods
Phoronids
Bryozoans
Hemichordates
Ctenophores
Placozoa
Tardigrades
Nematodes
Arthropods
Molluscs
Annelids
Echinoderms
Chordates
Cnidarians
Porifera
Recent Recognitions
Over 30 recently recognized phyla, including Xenacoelomorpha in 2011.
This recognition introduces a new body plan.
Example: Dendrogramma - A recent discovery, noted for raising new questions in animal evolution.
PHYLA AND ORGANIZATION
Definition of a Phylum
A phylum is a taxonomic level distinguished by major differences in body plans that may be apparent only at certain life stages.
Molecular evidence has reorganized several phyla definitions.
Xenacoelomorpha recognized in 2011, contributing to complexity in body plan discussions.
CLUES TO EVOLUTIONARY RELATIONSHIPS AMONG ANIMAL GROUPS
Sources of Evidence
Fossils
Morphology and physiology
Patterns of embryonic development
Protein structure
Gene sequences
EVOLUTIONARY CONTEXT
Genealogy
Relationship between choanoflagellates, animals, and fungi as part of the Opisthokonts group.
Ranking of Animals Closely Related to Humans
Mouse
Squid
Fruit fly (Drosophila melanogaster)
Sea squirt
(C. elegans)
Starfish
CLASSIFICATION IN ANIMALS
Hierarchical Structure
Deuterostomia
Cnidaria
Porifera
Protostomia
Echinodermata
Chordata
Arthropoda
Nematoda
Mollusca
Annelida
Platyhelminthes
Distinct Groups in Bilateria
Ecdysozoa
Lophotrochozoa
AMBIGUITIES IN CLASSIFICATION
Current Debates
Are sponges monophyletic or paraphyletic?
Which phyla emerged first: sponges, placozoans, or ctenophores?
Identifying sister groups to bilaterians raises ongoing questions.
EARLY STEPS IN EVOLUTION
Controversies in Path to Bilaterians
Assessing whether Porifera and Placozoa are relics representing early evolutionary stages
Examining the relationship among Diploblasts (Cnidaria and Ctenophora) and their two or three germ layers.
Bilateria is considered to have three germ layers and exhibit bilateral symmetry.
Ongoing debates regarding the evolutionary precursors: ctenophores vs. sponges.
KEY INNOVATIONS IN DEVELOPMENT
Mesoderm in Bilaterians
The mesoderm allows for the development of specialized internal structures.
This innovation is significant for the evolutionary trajectory during embryonic development.
ANIMAL LIFE CYCLE
Stages Involved
All bilaterian embryos undergo the following stages:
Fertilization
Cleavage
Gastrulation
Organogenesis
These processes are also present in Cnidarians and Ctenophores, indicating shared evolutionary traits among eumetazoans.
Fertilization and Cleavage
Fertilization: Fusion of haploid gametes (egg and sperm) results in a diploid zygote.
Cleavage: A series of mitotic divisions occur, forming a morula and then the blastula stage.
Gastrulation
Gastrulation is pivotal in embryogenesis, leading to the formation of three germ layers:
Ectoderm
Mesoderm (unique to Bilateria)
Endoderm
The fate of the blastopore differentiates bilaterians: mouth formation in Protostomes and anus formation in Deuterostomes.
Organogenesis
Organogenesis involves the development of organs from germ layers:
Ectoderm: forms skin and nervous/sensory systems
Mesoderm: develops muscles, circulatory system, blood, and various organs
Endoderm: forms the gut and associated organs
HUMAN EMBRYOGENESIS
Key Periods
Cleavage occurs before implantation in the first week
Gastrulation and neurulation occur between the second and third weeks
Organogenesis continues for several weeks thereafter
LARVAL STAGES AND METAMORPHOSIS
Development into Adult Forms
The end of embryogenesis often coincides with the hatching of a larval stage.
Typical Larvae Forms:
Trochophore (molluscs, annelids)
Nauplius (crustaceans)
Pluteus (echinoderms)
Caterpillar (lepidoptera)
Tadpole (amphibia, urochordates)
Larvae may have different forms compared to adults and undergo significant changes (metamorphosis) to reach adulthood.
COMPARATIVE EMBRYOLOGY AND SYSTEMATICS
Historical Discoveries
Comparative embryology contributed to recognizing fundamental embryogenesis features:
Germ layers and gastrulation processes
Notochord as a common feature of chordates
Influence on systematics has corrected classifications based on evolutionary hierarchies:
Barnacles misclassified initially as molluscs due to larval similarities with molluscs instead of crustaceans.
Sea squirts (urochordates) share larval traits with chordates, reclassifying them as sister groups to vertebrates.
GENES IN DEVELOPMENT
Genetic Blueprint Control
Embryogenesis is controlled by specific genes.
Advances since the 1970s:
Accessibility to developmental genes
Visualization of gene activity
Gene function manipulation
Targeted mutagenesis in embryos
Insight gained into evolutionary events
DEVELOPMENT AND EVOLUTION
Conserved Genetic Toolkit
The genetic toolkit governing developmental pathways is highly conserved across animal phyla.
Key Examples
Conserved genetic programming controls eye development (e.g., Pax6).
Changes in genetic toolkit lead to observable changes in body plans;
E.g., beak size and shape in Galápagos finches influenced by proteins from Bmp4 and Calmodulin.
HOX GENES AND DEVELOPMENTAL BIOLOGY
Homeobox Gene Importance
The Homeobox gene, a 180 bp fragment that encodes a DNA-binding homeodomain, was discovered through Drosophila analysis and underlies many developmental genes, referred to as Hox genes.
Hox Genes Function
Hox genes are responsible for determining positional identity along the body axis.
Mutations result in homeotic transformations, where body parts may be ectopically replaced, such as legs growing instead of antennae in the Antennapedia mutant.
Patterning Defined by Hox Genes
Combinatorial expression of Hox genes delineates body region identities, serving a conserved role in all bilaterians.
Cluster sizes correlate with complexity, showing increased number in bilaterians and duplicative events in vertebrates.
KEY POINTS
Summary of Evolution of Metazoa
Animals (Metazoa) are monophyletic.
Non-bilaterians branched off early, with Porifera and Cnidaria as foundational groups.
Most metazoan phyla belong to Bilateria, characterized by bilateral symmetry and three germ layers.
Bilateria further divides into Protostomia (where the blastopore forms the mouth) and Deuterostomia (where it forms the anus).
Protostomia separates into Ecdysozoa (characterized by exoskeleton and moulting) and Lophotrochozoa (identified by trochophore larvae and lophophore).
Embryological Development Characteristics
Shared embryonic features such as fertilization, cleavage, gastrulation, and organogenesis underpin evolution in body plans, driven by developmental control genes like Hox genes.
READINGS FOR FURTHER RESEARCH
Suggested Literature
Sadava, Life, The Science of Biology
Chapters 30, 31 & 32: Animal Origins and the Evolution of Body Plans
Overview of embryonic development in animals
Neil Shubin (2009), Your Inner Fish, Penguin
Raff, RA (2007), Written in Stone: Fossils, Genes and Evo–Devo. Nature Rev. Genet. 8, 911-920.