Genetics And Evolution Lecture 11

Front 1

1) What would we expect to see with gradual evolution?

Back 1

1) If evolution is slow and steady, we'd expect tosee the entire transition, from ancestor todescendant in the fossil record. For example, the evolution of whales fromland-dwelling mammals, highlighting thetransition of the walking forelimb to theflipper.

Front 2

1) Examples of rapid contemporary evolution?

Back 2

1) Antibiotics create a selection pressure for antibiotic resistance which leads to an increase in the frequency of resistance alleles in a population. Since the invention of penicillin there has been a rapid rise of resistance due to antibiotics being widely used. The majority of antibiotics were discovered between 1930 and 1960, it is becoming more difficult to find antibiotics. Antibiotic resistance works via several mechanisms: increased efflux, decreased influx, target site alteration, target amplification, and antibiotic inactivation.

The rapid evolution of antibiotic resistance was sped up by the wide spread use in animal husbandry, plant production, aquaculture, hospitals, communities --> entered the lakes, rivers, and soils, by urine and faeces and by production plants --> resistance bacteria move back into communities/livestock/etc

Front 3

1) Alternatives to antibiotics? (4)

Back 3

1) Predatory bacteria? The study of Bdellovibrio bacteriovorus, a bacterial predator that is an efficient killer of other bacteria, such as the prevalent E. coli. It is present in soil and, just like E. coli, it can also be found in the human gut, where a complex ecosystem of bacterial inhabitants exists. “Predatory bacteria might be genetically modified to specifically target harmful bacteria”.

Phages? Phage therapy or viral phage therapy is the therapeutic use of bacteriophages to treat pathogenic bacterial infections. Phage therapy has many potential applications in human medicine as well as dentistry, veterinary science, and agriculture. If the target host of a phage therapy treatment is not an animal, the term "biocontrol" (as in phage-mediated biocontrol of bacteria) is usually employed, rather than "phage therapy".Bacteriophages are much more specific than antibiotics. They are typically harmless not only to the host organism, but also to other beneficial bacteria, such as the gut flora, reducing the chances of opportunistic infections. They have a high therapeutic index, that is, phage therapy would be expected to give rise to few side effects. Because phages replicate in vivo, a smaller effective dose can be used. On the other hand, this specificity is also a disadvantage: a phage will only kill a bacterium if it is a match to the specific strain.

Metals? Certain metals disrupt antibiotic-resistant biofilms, exert synergistic bactericidal activity with other biocides, inhibit metabolic pathways in a selective manner, and kill multidrug-resistant bacteria. Certain non-essential metals — such as silver (Ag), mercury (Hg) and tellurium (Te) — are extremely poisonous to most bacteria and have microbicidal activity at exceptionally low concentrations.

Antimicrobial peptides? AMPs target the lipopolysaccharide layer of cell membrane, which is ubiquitous in microorganisms. Having a high level of cholesterol and low anionic charge puts eukaryotic cells out of the target range of many AMPs. Another important feature of AMPs is their rapid killing effect. Some AMPs can kill in seconds after the initial contact with cell membrane. AMPs are also known to enhance the activities of antibiotics through synergistic effects. Because AMPs are made with amino acids, it is relatively easy to modify the structure (including library construction and screening) and immobilize AMPs on surfaces. It is possible to make fully synthetic peptides by chemical synthesis or by using recombinant expression systems.

Despite these advantageous features of AMPs, there are still some challenges to their applications, such as potential toxicity to humans, sensitivity to harsh environmental conditions (susceptibility to proteases and extreme pH), lack of selectivity against specific strains, high production costs, folding issues of some large AMPs, reduced activity when used for surface coating, and bacterial resistance to some AMPs.

Front 4

1) Studying evolution of muticellularity in real time?

Back 4

1) Will Ratcliff and Michael Travisano.

They did a study to try to observe the shift from single-cell to cluster-level selection and the evolution of among-cell division of labour.

They carried out experimental evolution, where there was a selection for cell aggregation with single-celled S.cerevisiae yeast.

1) Grow yeasts to high density

2) Let cells settle to the bottom

3) Move large cells to a new tube (selection for large cells that settle to the bottom)

4) Repeat selection (60 days)

They found that: single cells of snowflake phenotype yeast regenerate new snowflake-phenotype clusters. All cells are related- and adhere to each other post-division.

Multicellularity had a selective advantage when let to settle, and showed that multicellularity can be inheritable.

Assymmetric cell division emerged, there was the Evolution of division of labour: most cells remain viable and reproduce, but a minority of cells become apoptoticApoptotic cells act as break points allowing snowflake yeast to produce a greater number of propagulesThis is functionally analogous to germ-soma differentiation into reproductive and non-reproductive cells

Front 5

1) Rapid coevolution between a host and it's parasite?

- What is an evolutionary arms race?

- Example of bacteria and phages?

- Example of Daphnia magna (host) and pastouria ramosa (parasite).

Back 5

1) In evolutionary biology, an evolutionary arms race is a struggle between competing sets of coevolvinggenes, traits, or species, that develop adaptations and counter-adaptations against eachother, resembling an arms race.

- Bacteria vs phages?

Bacteria can evolve rapidly and become resistant to phages, which leads to phage selection. However, phages can also coevolve and become more infective and infect the bacteria. The bacteria evolve resistance to this phage and then this leads to phage selection and a more infective phage emerges etc.

Bacterial resistance and phage infectivity increase in time = coevolutionary arms race.

- Evidence for rapid coevolution in lakes:

P. ramosa has coevolved with its host Daphnia magna. The mode of coevolution in this system fits the model with negative frequency dependent selection where the rare genotype is favored since the more common host genotype is more likely to become the target of a specialized pathogen.

Increase in parasite reproductive success through evolutionary time, hosts are more resistant to parasites from the past.

Front 6

1) Repeated evolution of herbicide resistance with plants?

2) Two broad catergories of resistance?

Back 6

1) Herbicides are chemicals that disrupt major plant physiological processes: disrupt the production of amino acids and fatty acids or arrest microtubule formation or the electron transport chain of photosynthesis.

- Herbicide is sprayed

- Resistant plant survives and sets seed

- Herbicide is used on weeds with more resistant plants

- Eventually the majority of the plants are resistant.

2) Target site resistance (TSR): same genes are mutated with different plant species. e.g- reduced binding to the target protein.

Nontarget site resistance (NTSR): Different adaptations with same or different plant species. e.g- reduced penetration of herbicide, altered translocation of herbicide, enhanced metabolism, protection against oxidative damage.

The convergent evolution of resistance among species can occur via different paths. E.g- same physiological mechanism, same mutation in same locus. OR, same physiological mechanism and different mutation in same locus. OR, different physiological mechanism and different loci. These multiple paths to resistance lead to rapid emergence of various herbicide resistant plant species.