Three Domains of Life

Three Domains of Life

Protists

Three Domains of Life

Changes in Classification• The ‘old school’ method of classification

included 5 Kingdoms (what I learned in school)– Monera– Protista– Fungi– Plantae– Animalia

• Today, advances in molecular technology expanded our understanding (and interpretation) of systematics

Modern Systematics

• Three Domain classification of life

• Numerous, virtually countless Kingdoms

• Bacteria and Archaea are now 2 distinct Domains (once included together in Kingdom Monera)

• Protista, Fungi, Plantae, and Animalia remain classified as distinct Kingdoms, although classification of the kingdom Protista has been met with complications

Prokaryotes

• Includes the kingdoms Archaea & Bacteria • Oldest, structurally-simplest, and most

abundant forms of life• Photosynthesis Bacterial and

Eukaryotic Diversity• Important decomposers and symbionts

Prokaryotes

• Unicellular

• Typically 1μm or less (1000 μm = 1mm; 1000mm = 1 meter)

• No membrane-bound nucleus; instead a single circular chromosome made of DNA

• Asexual reproduction by binary fission

• Photosynthetic bacteria utilize oxygen or chemical compounds, such as sulfur

Prokaryotic Cell Structure• Three basic forms:

– Bacillus – rod-shaped– Coccus - sphercal or ovoid-shaped – Spirillum – spiral or helical

Prokaryotic Cell Structure

• Prokaryotes have a tough cell wall and other external structures

• Cell wall consists of peptidoglycan; a rigid network of polysaccharide strands cross-linked by peptide side chains; unique to Bacteria

• Maintains the shape of the cell and protects it from swelling and rupturing

• Prokaryotes can have 1 or more flagella (much less complex than in Eukaryotes)

• Some Prokaryotes possess pilli, which helps fasten cell to host membrane

Domain Archaea

• Once considered a subdivision of the Kingdom Monera, now its own domain

• Like all prokaryotes, Archaea are single-celled microorganisms that lack a nucleus and membrane-bound organelles

• Best known for the “extremophiles” – Archaea which thrive in extremely harsh environments

Archea - Extremophiles

• Thermophiles – thrive at 60-80°C (>176°F!)

• Acidophiles – thrive at pH at or below pH 3

• Xerophiles – grow in extremely dry conditions

• Halophiles – require extremely high concentrations of salt

http://www.dpchallenge.com/image.php?IMAGE_ID=448561

Archaea - Extremophiles

• Evidence for evolution of life on Earth?

• Many of the harsh conditions which extremophiles require to survive were characteristic of our early Earth

• Likely that extremophiles evolved to dwell in such conditions billions of years ago and retained ability to survive today in specific environments

• Archaea differ from Bacteria in numerous ways– Plasma membranes are made of different

kinds of lipids– RNA and ribosomal proteins more like those

of Eukaryotes– Mostly anaerobic

Photosynthetic

Domain Bacteria

• Two types:– Gram-positive– Gram negative

• Refers to the Gram Stain (purple dye)

• Gram-positive bacteria – possess a thicker peptidoglycan cell wall; retain stain

• Gram-negative bacteria – contain less peptidoglycan; do not retain stain

Gram-positive and negative

Bacterial Conjugation• Transfer of genetic material

• Horizontal gene transfer

• NOT sexually (no gametes)

Eukaryotic origin

• The nucleus and endoplasmic reticulum arose from infolding of the prokaryotic cell membrane

Eukaryotic origin

• Eukaryotic organelles arose from a consortium of symbiotic prokaryotes– Mitochondria were aerobic heterotrophic

prokaryotes– Chloroplasts (for photosynthesis) were

photosynthetic prokaryotes

Endosymbiotic theory

• Evidence?– Mitochondria have their own independent

DNA, and a double membrane– Chloroplasts resemble cyanobacteria; also

have their own independent DNA and a double membrane

Kingdom Protista (the trouble-maker)

• Kingdom Protista is NOT monophyletic

Paraphyletic – includes common ancestor but not all descendents

Kingdom Protista

• Eukaryotic (must be! Domain Eukarya)

• Largely unicellular with some multi-cellular ‘exceptions’ (e.g., kelps, seaweed)

• May be autotrophic or heterotrophic

• Debate over classification – – Are some protists members of other kingdoms?– Would protists best be considered as several

different kingdoms?

Kingdom Protista• Characterized by:

– Mode of locomotion (e.g., flagella, cilia)– Mode of nutrition (e.g., autotrophic,

heterotrophic)– Body form (unicellular, multicellular)– Pigmentation (e.g., Red, Green, Brown alga)– Reproduction (asexual, sexual)

• Multicellular protists are distinguished from other Kingdoms by their lack of specialized tissues

Kingdom Protista

Kingdom Protista

• Have you ever eaten a protist?, or should I ask, have you ever eaten seaweed???

• Just to complicate matters, green algae is categorized as a plant in Kingdom Plantae…

Green Plants evolved from Green Algae

• We’ll come back to this…

Kingdom Fungi

Kingdom Fungi• Unicellular and multi-cellular

• ~1.5 million species

• Important decomposers

• Includes many disease-causing organisms

• Others are important symbionts and fermenting organisms

Kingdom Fungi• Mycology – the study of fungi

• All fungi are heterotrophic– Obtain their food by secreting digestive

enzymes and absorbing the nutrients released by the enzymes

• Unicellular fungi may have flagella; multicellular fungi are primarily filamentous in form

• Cell walls composed of chitin

Kingdom Fungi

• Six phyla– Cytrids (flagellated), Zygomycetes (inc. bread

molds), Glomeromycetes (mycorrhizae), Ascomycetes (inc. yeast), Bascidiomycetes (mushrooms), and Deuteromycetes (not pictured)

Kingdom Fungi

• Phylogeny based on the 5 major Phyla (based on mode of sexual reproduction)

Kingdom Fungi

• Multicellular fungi consist of long, slender filaments called hyphae

• Some hyphae are continuous; others are divided by septa

• Mycelium – a mass of connected hyphae

Kingdom Fungi• Mycelium grows through and digests its

substrate

• Fungi live in their food!

Kingdom Fungi• Hyphae (mycelium) form complex

structures

• A mushroom is a spore-bearing body of a fungus; composed of hyphae

• A puffball is a spore-bearing body of certain species of fungi, including the deadly Death Cap mushroom; composed of hyphae

Kingdom Fungi

• Fungi can also be monokaryotic or dikaryotic– Monokaryotic – one nucleus per cell– Dikaryotic – two nucleii per cell

• Fungi reproduce sexually and asexually– During sexual reproduction in some fungi, 2

haploid nuclei fuse creating a dikaryotic (dikaryon) stage, which precedes the normal diploid nucleus

Kingdom Fungi

• Some fungi produce specialized mycelial structures to house spores (e.g., mushroom, puffballs, ‘shelf’ mcycelium on dead trees)

• Spores can form as a result of sexual or asexual reproduction

• Spores can withstand degradation and survive for long periods of time; because of their size, they can travel long distances

Kingdom Fungi• Chestnut Blight – a fungal disease which

has virtually eliminated the American chestnut

• Accidentally introduced into the U.S. on imported lumber from Asia

• The roots of the tree are fairly resistant to the fungus, but the tree succumbs once it grows enough shoots to reproduce

• Unknown spreading agent (the spores are everywhere!)

Kingdom Fungi

http://www.kychestnut.org/images/openGrownTree.jpg

…Jack Frost nippin’ at your nose…

• The American chestnut once covered large tracts of forest in the U.S.

• The chestnut was a very important source of food for wildlife (and the inspiration for at least 1 Christmas song…)

• At the turn of the twentieth century, one quarter of all trees in the eastern United States were chestnut!

The Chestnut Blight

• Only a few mature survivors remain of the American Chestnut, which once consisted of 4 billion trees (that’s over 99.99% gone)

• If you have ever eaten a chestnut, you had a European import; only our grandparents may have ever tasted an American chestnut

• The American Chestnut Foundation seeks to restore the great chestnut, but how?

The Chestnut Blight

• Development of blight-resistant American chestnuts is accomplished through a process known as “backcross breeding”

• Hybrids between American and Chinese chestnuts are repeatedly crossed back onto purely American specimens, yielding offspring which are blight- resistant

• The resulting offspring are ~94% American (6% Chinese) and disease-resistant

Kingdom Fungi• Spores are frequently dispersed by wind,

but may also be spread by insects and small animals

• Chytrids are an ancestral group and retain flagella; have motile zoospores

Why did the mushroom go to the party?

• Many fungi live underground, and can reach great sizes

• One of the largest living organisms in the world is a fungus!

• The largest known specimen covers more than 3.4 square miles and is thousands of years old

• And some species of fungi are bioluminescent!

Armillaria fungus

• Connected underground by hyphae!

Fungal Ecology

• Fungi often have interactions or symbioses with other organisms

• Obligate symbiosis – essential for survival; fungus cannot survive without symbiont

• Facultative symbiosis – fungus can survive without symbiont

• Mutualistic relationships – both partners benefit

• Commensal relationships – one partner benefits, but the other is unaffected

Fungal Ecology• A lichen is a symbiotic association

between a fungus and a photosynthetic partner (usually green algae or cyanobacteria)

Fungal Ecology

• Mycorrhizae – association between a fungus and the root of a tree

• Mycorrhizae function as an extension of the plant root system; the fungus increases surface area for absorption and aids in transfer of nutrients

• The plant, in return, supplies organic carbon to the fungus

Mycorrhizae

• Very important!

• Mycorrhizal plants are more resistant to drought and even microbrial soil-borne pathogens

• Two types– Arbuscular mycorrhizae– Ectomycorrhizae

Mycorrhizae

Fungal Ecology• Leaf-cutter ants – an animal

symbiont with fungi!• The ants feed on special

structures produced by a fungus that they have domesticated

• The ants feed the fungus leaves and protect it from pests and molds

• In return, the ants eat the fungus and feed it to their young

Just in case you didn’t believe me…

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