Download MICROBIOLOGY MIMM211 Lecture 3 Evolution of early schemes for

MICROBIOLOGY MIMM211
(Biology of Microorganisms)
Lecture 3
Dr. Benoit Cousineau
Department of Microbiology & Immunology
McGill University
Phylogeny
evolutionary relationship between organisms
• Before the concept of evolution, organisms were grouped
based on morphological similarities (taxonomy), no
relationships between the groups
• Many fossils of animals and plants were found and used
to suggest the appearance of the different groups, built
evolutionary family trees
• Microorganisms were left out until late 1960s
- No microbial fossils
- Very similar shapes
• Use ubiquitous gene sequence to compare
microorganisms and construct a universal phylogenetic
tree of life (include microorganisms)
Evolution of early schemes
for classifying microbes
• Traditional early schemes (before 1866)
- Plants and animals
• Ernest Haeckel’s proposal (1866)
- Plants, animals, and microorganisms
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Haeckel’s tree of life
Evolution of early schemes
for classifying microbes
• Traditional early schemes (before 1866)
- Plants and animals
• Ernest Haeckel’s proposal (1866)
- Plants, animals, and microorganisms
• Edouard Chatton (1937)
- Eucaryotes and procaryotes
• Roger Stainer & C.B. van Niel
- Supported Chatton’s proposal
Recent classifications of living
organisms
• The five-kingdom scheme of Robert Whittaker
(1959)
-Monera, Protista, Fungi, Plantae, and Animalia
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Whittaker’s five kingdom classification
Recent classifications of living
organisms
• The five-kingdom scheme of Robert Whittaker
(1959)
-Monera, Protista, Fungi, Plantae, and Animalia
• The three-kingdom scheme of Carl Woese and
colleagues (1977). They used a molecular approach
(16S ribosomal RNA gene)
- Bacteria, Archaea, and Eucaryotes
- These three groups are equally distant
Woese’s three kingdom scheme
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Evolution of classification schemes
Three kingdom universal tree of life
The endosymbiotic theory of evolution
• Procaryotes do not contain organelles (mitochondria,
chloroplasts)
• Almost all eucaryotes have organelles
- Plants have both mitochondria and chloroplasts
- All other eucaryotes only have mitochondria
• Mitochondria and chloroplasts resemble procaryotes
- Approximately the same size (1 µM)
- The only organelles containing DNA (an active genome)
- Mitochondrial and chloroplast genomes are circular
- They contain 70S ribosomes (80S in eucaryotes)
- Double membrane resemble cytoplasmic and outer
membranes of gram-negative bacteria, however they do
not have the cell envelope
- They multiply and divide by binary fission
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The endosymbiotic theory of evolution (2)
• Lynn Margulis proposed in 1981 the endosymbiotic theory:
A primitive eucaryotic cell engulfed an ancient procaryote to
create the first eucaryotic cell
- First procaryote appeared 3.5 billion years ago
- First eucaryote appeared only 1.0 billion years ago
- The two cells continued to evolve in a mutually beneficial
symbiotic relationship for 1.0 billion year
- The procaryotic cell benefited from having a sheltered
environment rich in nutrients
- The eucaryotic cell benefited from containing an organism that
produced energy (ATP) by respiration (mitochondria) or by
photosynthesis (chloroplast)
- Mitochondria are the descendants of an alpha proteobacteria
(gram-negative, Rickettsia prowazekii)
- Interestingly R. prowazekii is an obligate intracellular parasite
- Chloroplasts are the descendants of a cyanobacteria (Prochloron)
The endosymbiotic theory of evolution
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Woese’s three kingdom scheme
The endosymbiotic theory of evolution (3)
• More recent endosymbiosis (an evolutionary snap shot)
- Eucaryotes constantly phagocytoze procaryotic cells
- Need time to co-evolve and create a stable new organism
• Aphids and related insects: a recent endosymbiotic
relationship (200 million years)
- The endosymbiont still has its gram-negative double
membrane and cell envelope, phylogenetically closely
related to E. coli
- Stable endosymbiotic event
- The endosymbiont can no longer grow outside its host
- Aphids are also dependant of their endosymbiont,
they die if the endosymbiont is killed (antibiotic)
- Supports the endosymbiotic theory of evolution
Building a phylogenetic tree
• Get the sequence of an ubiquitous gene (phylogenetic
marker, e.g., 16S ribosomal RNA gene, 1600 nucleotides)
for all the organisms to be included in the phylogenetic tree
• Align the gene sequences using a sequence alignment
program
• Feed the aligned sequences to a phylogenetic algorithm
- Find which sequences share the highest level of
homology and link closest neighbors
- Create phylogenetic tree, branching the organisms
related to the similarity of their gene sequence
- The length of the branches linking the different
organisms are proportional to their evolutionary distance
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The bacterial kingdom
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