BIOL 4010 Lecture Notes - Lecture 14: Adaptation, Mutation Rate, Mitochondrion
2 May 2018
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Symbiosis
Symbiosis - interactions between two or more organisms in which all partners benefit
• Metabolic symbiosis - metabolic benefits to all partners
Why do animals engage in metabolic symbiotic associations?
• Animal genomes lack certain metabolic capacities that may limit their ability to access nutrients
leading to symbiotic associations with organisms that can provide these capacities
• For example…
o Inability to synthesize essential amino acids
o Inability to synthesize essential fatty acids
o Inability to capture energy in photons
o Inability to access certain types of chemical energy
o Inability to access low levels of environmental nutrients
Location of metabolic symbioses:
A. Extracellular
o Cultures of microorganisms in various parts of the digestive system
• For the digestion of cellulose in mammals, birds, insects and molluscs
B. Intracellular
o Entire ecosystems are based on symbiosis:
• Coral reefs (light energy)
• Hydrothermal vents (chemical energy)
• Cold seeps (chemical energy)
Types of symbionts:
A. Algal
B. Bacterial
Serial endosymbiont theory:
• Hypothesis proposes that mitochondria formed after a prokaryote that had evolved into an early
eukaryote engulfed and kept one or more alpha-proteobacterial cells
• Eventually, the bacterium gave up its ability to live on its own and transferred some of its genes to
the nucleus of the host, becoming a mitochondrion
• Later, some mitochondrion-bearing eukaryote ingested a cyanobacterium which became the
chloroplast
• *see slide
• Note: despite having their own genome, most mitochondrial proteins are encoded in the nucleus,
made in the cytosol and imported into the mitochondria
• Why weren't all the genes transferred? --> Possible Explanations:
o Protein hydrophobicity is a barrier to import and therefore transfer genes (but only for some)
o Need for organellar gene expression must be under local redox control
o Advantages in adaptation/speciation
• Mitochondrial evolution:
o Rapid mutation rates of mtDNA are due to…
• Poor repair system
• Proximity to sites of ROS
o Until recently, changes in mtDNA were assumed to be neutral with negligible contributions
to adaption to the environment
o Mito-nuclear interactions:
• Mitochondria gene sequences changes 10-30x faster than nuclear genes
find more resources at oneclass.com
find more resources at oneclass.com
• Mitochondrial DNA's high mutation rate may help some organisms adapt to changing
environments
• Genes in the nucleus must adapt quickly to partners in the mitochondria
o mtDNA evolution underlies small physiological adjustments to thermal environments and
altitude, as well as major adaptive radiations
• Ex. Adaptive evolution and functional redesign of core metabolic proteins in snakes
▪ Digestion in snakes involves a massive increase in metabolic rate for digestion
▪ Evolutionary pressure on proteins can be quantified by the ratio of substitution
rates at non-synonymous (N) and synonymous (S) sites
• Signature of positive selection over divergent lines is dN/dS >1
• **all mitochondrial proteins have dN/dS ~3
• Ex. Adaptive evolution of energy metabolism genes and the origin of flight in bats
▪ Genes involved in energy metabolism were targets of natural selection, allowing
the huge change in energy demand associated with flight
▪ 23.08% of genes showing evidence of positive selection in bats were 13
mitochondrial genes
• Mitonuclear coevolution in isolated populations leads to speciation because population-
specific mitonuclear coadaptations create between-population mitonuclear
incompatibilities and hence barriers to gene flow between populations
▪ In addition, selection for adaptive divergence of products of mitochondrial genes
(particularly in response to climate or altitude) can lead to rapid fixation of novel
mitochondrial genotypes between populations and consequently to disruption in
gene flow between populations as the initiating step in animal speciation
▪ By this model, the defining characteristic of a metazoan species is coadapted
mitonuclear genotype that is incompatible with the coadapted mitochondrial and
nuclear genotype of any other population
• *note: in tetrapods, mitochondrial mutation rate is very low
o COX1 is a mitochondrial gene associated with the ETC
Reef Forming Corals
• Environmental conditions:
o Warm
o Low nutrient levels (P and N)
• *note: nitrate levels are lowest nearest equator and highest near south pole
o High light levels (clear water)
o High UV levels
• Adaptations:
o Use light as energy source via plant symbiosis
o Recycle extensively
o Defenses against UV
• Trophic status: heterotrophy to autotrophy
o Although endosymbionts provide for most of the food (utilization of radiation energy to
convert H2O and CO2 into photosynthate) corals also rely on heterotrophic pathways
o There is no strict separation but rather a continuum between autotrophic and heterotrophic
nutritional pathways
• Symbiodinium (algae) is symbiotic in some anemones, scleractinian corals, zoanthids,
corallimorphs, blue corals, alcyonacean corals, sea fans and several representatives from the classes
Scyphozoa (with whizostome and coronate jellyfish) and Hydrozoa (with milleporine fire corals)
o They have also been identified from some non-cnidarians, including some gastropod/bivalve
molluscs, foraminiferans, sponges, and a giant heterotrich ciliate
o More than 80 strains of dinoflagellates related to Symbiodinium have been isolated
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Symbiosis - interactions between two or more organisms in which all partners benefit: metabolic symbiosis - metabolic benefits to all partners. Why do animals engage in metabolic symbiotic associations: animal genomes lack certain metabolic capacities that may limit their ability to access nutrients leading to symbiotic associations with organisms that can provide these capacities, for example . Inability to access certain types of chemical energy. Inability to access low levels of environmental nutrients. Establishment of symbiosis: phases of symbiosis establishment and persistence in cnidarian-algal symbiosis : If coral symbiodinium disappear coral becomes bleached and dies: constant degradation by wave action and predation require high rates of replacement of coral hard parts and tissue. Coral bleaching: co2 limitation is a unifying mechanism for bleaching sequence of photoinhibition, oxidative damage and host cell disruption, *see slide, with increasing severity of environmental stress .
