Developmental Biology Concepts

Early Developmental Stages

• Development progresses through distinct stages: zygote, morula, blastula, gastrula.
• Early development involves rapid cell division without growth, resulting in smaller cells.
• Cells form a hollow ball called a blastula.

Germ Layers and Cell Activities

• Three primary germ layers form: ectoderm, mesoderm, and endoderm.
• Ectoderm forms epidermis and nervous system.
• Mesoderm develops into muscle, blood vessels, blood, and bone.
• Endoderm forms the inner lining of the gut.
• Key cellular processes include division, migration, differentiation, signaling, gene transcription, and translation.

Morphogenesis and Signaling

• Morphogenesis generates body anatomy.
• Signaling proteins, like Noggin, guide development.
• Noggin signals surface cells, inducing Sox1 expression, activating genes for nervous tissue development.

Evolutionary Conservation

• Phylotypic stage demonstrates similar early developmental stages within a phylum.
• Developmental constraints create this conservation.
• Interactions between multiple genes limit changes due to common evolutionary ancestry.

Drosophila as Model Organism

• Drosophila development stages include egg, larva, pupa, and adult.
• Hox genes control body segment identity.
• Hox genes are clustered, arranged co-linearly with their expression pattern.
• They contain a conserved homeobox region.
• A protein region called the homeodomain binds to DNA for gene transcription control.

Conservation Across Species

• HOX genes are present in all animals.
• They are homologous to Drosophila homeotic genes.
• This demonstrates conservation of developmental control genes.
• Developmental patterns are similar within phyla.
• Gene duplication and polyploidization contribute to evolution.

Cell Signaling and Position

• Cells receive location information.
• Position determines cell fate and differentiation.
• Signaling pathways guide differentiation.
• Complex interaction networks regulate development.

Key Findings—Longevity Control

• MicroRNAs mir-84 and mir-241 regulate longevity through the gonadal pathway, targeting lin-14.
• lin-14 is regulated to control lifespan.
• Early developmental components are repurposed for adult lifespan regulation.

Molecular Mechanisms

• lin-14 RNAi restores lifespan in triple mutants.
• lin-14 loss extends lifespan via daf-16.
• lin-14 RNAi restores daf-16 target gene expression.
• miRNAs downregulate lin-14 through the 3'UTR.
• lin-14 is downregulated in intestinal nuclei without a germline.

Developmental Timing and Longevity

• Components of early developmental timing control adult lifespan (DAF-12, ligands, let-7 miRNAs, LIN-14).
• This switch involves regulating reproduction versus survival.
• When germ stem cells are prevented from proliferating, lifespan extends.

Proposed Model for Lifespan Regulation

• Reproduction and survival are regulated by a hormone-activated switch.
• Lifespan increases when germ stem cell proliferation is suppressed.
• This involves increased DA production, activation of DAF-12 and miRNA targets, downregulation of akt-1 and lin-14, leading to increased DAF-16/FOXO, and increased lifespan.
• Conversely, lifespan decreases if germ stem cells proliferate.

Digit Patterning and Evolution

Turing Model of Digit Patterning

• Turing mechanism, rather than morphogen gradients, shapes digit patterning.
• Hox genes modulate digit wavelength and spacing.
• Reduced Hox genes result in more, thinner, and densely packed digits.
• Mutant patterns resemble ancestral fin patterns: numerous, densely packed, iterative elements.

Evolutionary Implications

• An ancient Turing-like patterning mechanism is conserved.
• Hox genes established the pentadactyl limb pattern.
• Ancestral patterning adaptations are modified rather than replaced.

Significance

• Development timing mechanisms control longevity.
• Ancient patterning systems are conserved and modified.
• Gene regulation evolves complex traits in major transitions.

Species and Speciation

• Species definition is complex, needing multiple concepts.
• Key concepts include: phenetic, phylogenetic, and biological.

Isolating Mechanisms

• Isolating mechanisms prevent gene flow between species; prezygotic and postzygotic.
• Prezygotic mechanisms include temporal, behavioral, geographic, and mechanical barriers.
• Postzygotic mechanisms affect hybrid fitness.
• Haldane's rule: reduced hybrid fitness occurs in the heterogametic sex.

Modes of Speciation

1. Allopatric Speciation: Population split by physical barrier, leading to divergence (e.g., Haemulon species).
2. Sympatric Speciation: Speciation within the same geographic area, driven by differences in mating times, preferences, or resources (e.g., Hawthorne flies).
3. Parapatric Speciation: Speciation where populations don't fully overlap.

Evidence for Speciation

• Speciation is observable and involves various stages.
• Examples include allopolyploidy, selection experiments, and incipient speciation in Hawthorne flies.

Challenges in Species Classification

• Classifying closely related species is difficult.
• Different species concepts can lead to different conclusions (e.g., Drosophila melanogaster, Drosophila simulans).

Practical Applications

• Gene flow is crucial for biological species concept (BSC) definition. Multiple species concepts may be required.

Extinct & Evolutionary Traits

Background Extinctions

• Most species are extinct.
• Modern extinctions provide insights into causes: immigration, disease, competition, and climate change.
• Similar factors likely caused historical extinctions.

Mass Extinctions

• Mass extinctions involve large-scale species loss (global impact).

Late Permian Mass Extinction ("The Great Dying")

• Largest known extinction.
• ~90% of species lost.
• Cause: massive Siberian Traps volcanic eruptions (~10 million years).
• Effects: aerosol release, global cooling/warming, acid rain, reduced photosynthesis, oxygen levels.

Cretaceous-Tertiary (K-T) Mass Extinction

• 65 million years ago; ended dinosaur dominance (except birds).
• Cause: 10 km meteor impact in Central America, concurrent volcanic eruptions.

Evolutionary Trends

• Adaptive radiation follows mass extinctions as species fill ecological niches.
• Mammalian evolution and diversity was facilitated by the K-T extinction.

Rates of Evolutionary Change

• Two main theories: phyletic gradualism and punctuated equilibria.

Early Life Evolution

• Ediacaran Period: soft-bodied organisms.
• Cambrian Explosion: rapid diversification of life.
• Contributing factors: snowball earth end, increased oxygen, evolution of predation, and Hox genes.

The Evolution of Sex

• Various reproductive strategies across organisms.

• Pros and cons of asexual vs. sexual reproduction.

• Costs of sex: reduced genetic relation, mate search, courtship, predation susceptibility, disease.

• Benefits of sex: purging harmful mutations, faster beneficial gene combination discovery (Fisher-Muller hypothesis).

• Evolution of males and sexual dimorphism due to different gamete sizes.

• Sexual selection and its forms (intersexual and intrasexual).

Human Evolution

• Human unique characteristics and primate traits.

• Primate classification suborders (strepsirrhines, haplorhines) and family Hominidae (apes and humans).

• Hominina subtribe (hominins), erect walking hominids.

• Key similarities between humans and apes.

• Evolutionary timeline: late Cretaceous, Tertiary periods.

• Geographic spread and primate development.

Early Brain Evolution

• Gradual increase in hominin brain size.
• Encephalization quotient (brain size relative to body size) significant in primates.
• Social brain development influenced by factors: reciprocal altruism, tool use, and social learning.

The Genus Homo

• Homo habilis (2-1.5 million years ago): first members, brain size ~600cc, stone tool use.
• Homo erectus: variable cranial sizes (750-1200cc), expanded tools (hand axes), fire use, first hominin to leave Africa.
• Homo heidelbergensis: evolved from H. erectus, gave rise to Neanderthals and H. sapiens.
• Neanderthals (250,000-28,000 years ago): adapted to cold climates, heavy spears, burials, disappeared (possibly due to competition).

Origins of Modern Humans

• Competing theories: multiregional and Out of Africa.
• Out of Africa is currently accepted.
• Key evidence for Out of Africa: archaic H. sapiens fossils, greater genetic diversity in Africa.

Cultural Development

• Early symbolic thinking emerged in South Africa (~80,000 years ago).
• Advanced tool use, artistic expression, and Cro-Magnon culture (cave paintings, figurines).

Recent Discoveries

• Interbreeding evidence between Neanderthals and H. sapiens.
• Homo floresiensis ("Hobbits"): small-bodied species, coexisted with modern humans.
• Taxonomic revisions based on new finds.