Final - Pathogens

Question 1

Protein Mediated Resistance results in

Strategy for pathogen-derived resistance to viruses

Answer 1

Coat protein mediated resistance.
-interferes with virus reassembly via over expression of proteins
-
Results in moderate and highly variable levels of resistance (delay in symptoms, decreased virus concentrations)

Question 2

RNA Mediated Resistance results in

Strategy for pathogen-derived resistance to viruses

Answer 2

-RNA silencing (most common natural form of plant resistance to viruses)
Often results in complete immunity but displays a high level of sequence specificity

Question 3

Risks associated with transgenic virus resistance

Answer 3

• Pollen flow: genes moving to wild relatives
• Interfere with or stop the plant’s natural gene silencing mechanisms
• Non-target effects: Bt and monarch butterflies

Question 4

Transgenic strategies for fungal and bacterial disease resistance

Answer 4

• Intro of R-genes from unrelated plant species
• Detoxification of necrotrophic virulence factors (toxins, cell wall degrading enzymes)
• Over-expression of pathogensis related (PR) proteins
• Activation of signalling pathways

Question 5

R Gene Transfer- why it could work

Answer 5

• plants have 100s of R-gene sequences
• Thousands of non-ag plants are potential donors
• R-genes are efficient, only activate defenses when needed

Question 6

R Gene Transfer- potential problems

Answer 6

-Not very durable because R-genes lead to further selection within the pathogen. Often, by the time a new cultivar is released the pathogen already has resistance to that R gene

Question 7

For more durable resistance we need more R genes because

Answer 7

Once a plant is able to recognize an effector, the pathogen changes effectors and is virulent again. Need a new R gene to recognize the new effector

Question 8

Example of transfer broad spectrum resistance

Answer 8

Transferred a gene from Arabidopsis that keeps basal defense on at a higher level into susceptible tomato plant to make a resistant tomato plant.

Question 9

PR protein function

Answer 9

Commonly enzymes that attack fungal cell walls

Question 10

Potential issues with Over-expression of PR proteins as a defense strategy

Answer 10

Stressful on the plant and takes a lot of energy. You don’t get high levels of resistance either, so it’s not really worth the cost.

Question 11

How to exploit RNA silencing

Answer 11

1. Find the gene to be silenced
2. transform it in two orientations
3. It will be expressed as double stranded RNA (harpin shaped)
4. Signals enzymes in the cell to cut it up into siRNAs (small interfering RNAs)

Question 12

Two major Evolutionary forces driving coevolution in the gene for gene system

Answer 12

Directional selection: Favoring greater host resistance and pathogen virulence (arms race)
Balancing: Selection favoring rare alleles or different genotypes in different environments

Question 13

Polymorphisms can be maintained in a gene-for-gene system by:

Answer 13

1. cost of virulence (losing an effector)
2. Cost of resistance (plants cost)
3. Multiple alleles at a single locus keeps resistance broad bc only 2 R genes are passed at once
4. Limited spread between local populations = less mixing in the meta population
5. Multilocus inhertiance - if we have 20 genes involved in an effect, can have some variation and still get the same effect

Question 14

Evolutionary forces of pathogen resistance

Answer 14

1. Mutation
2. Genetic drift (a loss of genetic diversity)
3. Gene Flow
4. Reproductive systems
5. Selection

Question 15

Mutations

Answer 15

the ultimate source of genetic variation (new alleles)

A large population will generate more mutations than a small one

Question 16

Mutation- Selection Balance

Answer 16

Frequency of virulent allele at equilibrium

fe = u/s

Question 17

P

Answer 17

Frequency of dominant allele (virulence)

Question 18

q

Answer 18

Frequency of recessive allele

Question 19

Gene Flow
-what
how to capitalize on it

Answer 19

Exchange of alleles between geographically isolated populations of asexual reproducing pathogens. (between races, wheat stem rust from mexico could deploy multiple R genes south to north)

Question 20

Sexual reproducing pathogens have lots of diversity because of

Answer 20

recombination

Question 21

Pros/Cons of out-crossing (sexual reproduction)

Answer 21

• Potential for rapid adaptatation in changing environment

- bad- frequent recombination tends to break up co-adapted combination of alleles.

Question 22

Asexual/inbreeding pros/cons

Answer 22

• Potential to retain co-adapted combinations of alleles through time
• bad- limited adaptation to to new environment (or an R-gene)

Question 23

Mixed sexual reproduction def, advantage

Answer 23

• A sexual cycle, with lots of asexual cycles. Allows better retention of adapted genotypes with ability to bring in new alleles
• most pathogens have this

Question 24

Why does it take asexual pathogens longer to develop r virulence to multiple genes

Answer 24

It takes time for all the virulence genes to join together because it happens via mutations.

Question 25

Why is purely asexual reproduction a bad long term strategy

Answer 25

1. most mutations are bad | 2. Once fitness is lost, you can't get it back

Question 26

How do asexual pathogens overcome drawbacks of their asexual nature.

Answer 26

Para sexual cycle

Question 27

What is the parasexual cycle

Answer 27

Two hyphae join, and their respective nuclei fuse, mitosis crossing over occurs, and then the nuclei become haploid again with new alleles.

Question 28

Advantage of mitotic crossing over

Answer 28

Allows multiple virulence alleles to be put together from two asexual pathogens.

Question 29

Plasmid

Answer 29

All bacteria have plasmids which are an extra chromosonal section that will duplicate itself and occasionally be incorporated into the genome, or it can be exchanged with other bacteria via phages into their genomes.

Question 30

Genomic island

Answer 30

Occurs when genes get grouped together because of their effect on fitness. Can get virulence to as many R genes as you can imagine. How bacterial resistance works

Question 31

Durable resistance -def

Answer 31

is that which “remains effective during prolonged and widespread use in environments favorable to the pathogen or disease spread”

Question 32

Costs of resistance gene breakdown

Answer 32

– Cultivars need to be replaced – New R genes are then needed and the supply of R genes is not infinite – Cultivars take time and money to develop – Epidemics can occur after breakdown

Question 33

Why are R genes against viruses more durable than bacteria or fungus

Answer 33

Viruses have very some genomes and can't lose many effectors, so sometimes R-genes can be durable

Question 34

Why is PAMP triggered immunity durable, what are the draw backs

Answer 34

Broad spectrum defenses that are always on at a higher level. Weaker/less effective than ETI triggered immunity Comes with a fitness cost

Question 35

Gene Pyramiding | def

Answer 35

Simultaneous deployment of several R genes in the same cultivar. Ideally, R genes should be undefeated at the time of deployment. If a new gene is simply added to series of already defeated R genes, it is not really a pyramid.

Question 36

Gene Pyramiding | advantages

Answer 36

Pathogen has to mutate at multiple Avr loci simultaneously, which should be very rare if the Avr loci are independent. – Crops are highly resistant until breakdown. – Crop is as uniform as other pure-lines or hybrids.

Question 37

Gene Pyramiding | disadvantages

Answer 37

– If resistance is overcome, you may have lost all your best R genes.

Question 38

Genotype mixtures -strategy for R gene deployment

Answer 38

several R genes, but in different plants. – Reduces density of plants susceptible to any pathogen component. – Resistant plants intercept inoculum. – Resistant reactions may induce some systemic resistance.

Question 39

Genotype mixtures -strategy for R gene deployment | -pros/cons

Answer 39

pro=**Reducing selection pressure on the pathogen for virulence** concern= Concern is the development of complex races (bigger problem with pathogens that have a sexual cycle) Resistance to multiple R genes

Question 40

Rotations of R gene deployment in space and time - -strategy for R gene deployment

Answer 40

– Each R gene is deployed over a limited number of years or area and is withdrawn before the Avr gene becomes frequent. – Disrupts the directional selection for virulence on a single R gene. drawback= – Requires development and increase of many cultivars.

Question 41

Vertical vs horizontal resistance durability

Answer 41

Horizontal is durable, vertical is not. | Horizontal is non-race specific

Question 42

Steps in QTL analysis:

Answer 42

1) Make a large segregating population that is segregating for the trait of interest. 1) Evaluate and score the individuals (or lines) in the population for the trait. 1) Score the individuals for DNA markers spread throughout the genome. 1) Perform statistical tests for associations between the markers and the trait.

Question 43

Tolerance- strategy for durable resistance

Answer 43

tolerance doesn't exert selection pressure

Question 44

QTL mapping advantages over classic genetics

Answer 44

of genes involved in a trait, location, GxE effect