Final- Entomology Flashcards
Transgenic arthropod resistance - how does it make the plant function
Basically makes an insecticidal plant that produces a toxin to kill the insect (bt corn or cotton)
Mode of action of Bt
on the villa of the insect gut
within an hour of introduction, the insect gut is getting holes and disintegration
Bt Resistance (virulence)
Mutation in the receptor on the membrane (brush border)
For Cry
Cry toxin mode of action (product of Bt)
- Cadherin is principal receptor making this happen*
1. Protoxin binding to cadherin receptors on the brush border
2. Cell death from toxin olgomers
3. Toxin olgomers bind to enzymes, and insert into basement membrane
4. Pore forms, gut bacteria enter the body cavity, gut cell dies
Inheritance of Bt resistance (virulence)
The insect gut is able to mutate rapidly and change the hertibility mechanism
Inherited as nonrecessive, incomplete dominant, recessive in different insects
Transgene Bt Release Strategies:
Single Bt release / Moderate Bt toxin Expression
What, why, Pro/Con
Allows some susceptible insects to survive and dilute expression of virulence through intermating
Allows survival of benefical insects
bad = variable efficacy
Transgene Bt Release Strategies:
Mixed release / Mixture of different toxins Expression
What, why, Pro/Con
Kill different susceptible homozygotes for each toxin to slow virulence
good = assumed no cross-resistance, no linked virulence
bad = with cross resistance and linkage the mixture would last just as long as a single toxin release
Durability of Bt relies on
Refuge seed
Idea behind refuge seed
Get a higher biomass of susceptible survivors from the non bt plants, than resistant survivors from the bt plants.
They intermate, and the level of virulence will be diluted and remain controlled.
Refuge % vary by
crop and pest
Where does the Bt gene come from?
Soil bacterium Bacillus thuringiensis (Bt) that produces endotoxins
Advantages of molecular vs phenotypic markers
- Unaffected by environment
- More economical
- Track multiple genes
Markers tell us
The location on the chromosome is linked in some way to the resistance gene
For Molecular Assisted Selection for insect resistance to work the resistance gene locus must be ___
Linked in close proximity during crossover and co-segregate.
Linkage possibilities between R genes and markers
could get complete linkage. Marker and gene always linked from one gen to another
incomplete- crossing over occurs differently for marker and allele,
none when the marker and allele are on different chromosomes or very far apart
cM
- what
- how to interpret
Centi-morgans.
A statistical estimate of probability of linkage. Smaller number, tighter the potential linkage
Recombination frequency of 0% and linkage
0% = completely linked
50% - entirely unlinked
What types of R genes have successfully been identified with markers
-what crops
Dominant, single genes for resistance to insects
rice, soybean, sugar beet, wheat
Major gene segregation
Discrete classes, easy to tease apart
Minor gene segregation
A continuum, harder to tease apart
Genotyping by Sequence and Association mapping benefits
GBS- good for plants where we don’t know the entire genome
AM- Use thousands of markers to screen for R-genes, lots of work not widely used
For insect plant co-evolution this must happen
First- Need insect movement to the plant
Second-insect selects nutritive host or oviposition host based on volatiles, visual or contact ques.
If this happens enough, we get co-evolution. If it dead ends enough, insect selects another host, no co-evolution
Bound to be found - hypothesis of plant cost/benefit of defense
- Perennials are bound to be found by insects due to longer life cycles, so have more broad defenses.
- Put more protection on their seed to protect future progeny
- Put more protection in areas that are susceptible to feeding
Carbon/Nitrogen ratios influencing plant defensives
Plants living in high carbon environements have carbon based defenses (high silica & lignin)
Plants living in high nitrogen low carbon environments have nitrogen (toxin) based defenses
Resource availability influencing plant defenses
Available resources determine plant growth, defense production increases when growth decreases
Growth-Differentiation Balance influencing plant defenses
tradeoffs between growth & defense over a resource availability gradient
too Cloudy, partly cloudy (ideal), too sunny
maysin (glycoside) QTL
Corn – corn earworm
Why do some seed companies now breed for only drought tolerance.
-how does this relate to co-evolution
because an insect sucking the nutrients out the plant requires a similar resistance as drought tolerance. so they just breed for drought tolerance
-Drought stress accelerates coevolution by selecting of insect defenses with lower metabolic costs because of resource costs related to drought
stress.
How do temp flucations influence co-evolution
Increased temperature fluctuations eliminate insect populations adapted
to heat or cold, reducing selection pressure for insect resistance & decelerating coevolution.
How do increased CO2 levels influence co-evolution
Increased CO2 concentrations shift C/N ratios to C-based defenses, force
directional selection & accelerate co-evolution.
Why are some insects more prone to developing biotypes
High number of biotypes because they move large distances and can reproduce rapidly
-There are major genes (sometimes many) exerting antibiosis effects on them. So survivors are forming new biotypes
Factors Affecting Biotype Occurrence
Genetic diversity of pest, adaptive ability on different hosts and to
different ecological conditions
• Number of resistance genes
• Resistance category
• Percent of crop in resistant cultivars
• Integrated pest management program efficacy
Biotype avoidance via combined antibiosis and antixenosis
combining antibiosi and antixenosis extend the time you can produce a crop without virulence take you from 2 to 8. Keeps virulence frequency low, so it develops slowly over time. Will be a yield hit
Year 1 Refuge - 1:1 ratio (Bt cotton currently the recommendation), Year two- use back selection and plant only susceptible
