10. Parasites evolving resistance Flashcards
What causes sickle cell anaemia?
A mutation in the haemoglobin Beta subunit
What kind of mutation gives rise to sickle cell anaemia?
Mis-sense mutation
In the mRNA of the gene coding the haemoglobin Beta subunit, what is the normal protein?
GAG gives glutamic acid
In the mRNA of the haemoglobin Beta subunit, what is the sickle cell protein?
GUG gives VALINE protein leading to deoxygenation in tissues
What are the biological effects of the HBS gene if you have one copy?
- Decreased growth
- Delayed sexual maturity
- Impaired mental function
- Impaired infection resistance
- Increased chance of stroke, kidney failure, etc
What happens if you have two copies of the HBS gene?
You usually die at birth
What is the fitness of HBS/HBA babies in areas without malaria?
HBS/HBA babies will also survive and reproduce less so HBS allele will become less common even though HBS/HBA babies tend to survive malaria
What happens in an area with malaria but no parents have the sickle cell gene?
All offspring that are HBA/HBA are likely to die
What happens in an area with malaria and both parents are HBS heterozygote?
Only heterozygote offspring with survive, homozygote HBA and homozygote HBS will not survive
What happens to the allele frequency in areas with severe malaria?
HBS/HBA heterozygote babies would survive and reproduce more when they become parents so HBS allele will become more common
What is polygenic variation?
Many genes combine to determine a characteristic or trait
What is the hypothesis for evolution due to natural selection?
IF individuals vary in now well they cope with their environment and the relevant characteristics are heritable THEN those that cope well produce more offspring that survive to reproduce than those that do not cope
- The proportion in the population with these characteristics increases
Before a pesticide is used, who are the R mosquitos mating with?
- Very few mozzies will have the mutant resistant allele so every resistant will likely mate with normals to produce heterozygote resistants
Why do doses decay over time?
- The pesticide sprayed is always concentrated
- It lands on surfaces, mixes with dust, water, etc so it becomes diluted over time
- The dose a mosquito gets depends on when it lands on a sprayed surface
When will the adult mosquitoes die after initial dosage?
Adult mosquitoes in the sprayed area die
What happens to large and eggs after initial dosage is sprayed?
Larvae in ponds or eggs will hatch out later. All of these die if they become adults too some
When does selection happen after initial dosage?
- Other adults will fly in from the edges of the area
- Some by chance may have a resistant gene
- If they arrive after the dose has decayed enough then Resistants survive while Normals die
When is there no selection after initial dosage?
If they arrive after the dose has decade to very low levels then both normals and the few resistants survive so there is no selection
What happens once resistants are common?
- Heterozygote resistants may mate with other heterozygote resistants
- They produce homozygous RR resistants
- These have a higher resistance as most genes have incomplete dominance
What is incomplete dominance of alleles?
- If an allele is dominant it is fully expressed in heterozygotes
- But most gene alleles have incomplete dominance where neither of the alleles is dominant and the heterozygote shows intermediate characteristics
What is codominance?
Where both of the allele characteristics are evident in the individual
What happens to selection time and zone of selection once resistants are common?
Selection time increases and the zone of selection widens
What are some strategies to minimise selection for resistance?
- Repeat regularly to keep dosage high
- Don’t delay until the pest/disease returns so repeat doses at short intervals
- Use chemicals with rapid decay to minimise selection time during decay
- Prevent unnecessary use
- Prevent dispersion of resistant pests
- Minimise edge effects for pesticides
What are edge effects?
- For large areas if you spray at once then there is only one time when the dose is in the selection zone
- Small areas when sprayed one by one result in pests moving from other areas while the dose decays. Then selected resistant pests can move to areas sprayed later as doses decay there
Why do new mutations always have bad side effects?
- Otherwise the R alleles would be common already and another toxin would have to be used
- Therefore after the dose is gone they are selected against
How does polygenic variation result in the stabilisation of resistance?
- Genes that reduce the side effects of a resistance mutation may be combines with the R allele
- If the R allele is selected for then these genes that negate the bad side effects will also be selected for
- Now if you stop using the pesticide the resistance will stay in the mosquito
What are the effects of cross resistance when you use similar toxins?
If similar toxins are used then pests are already partly resistant and resistance build rapidly (polygenic process)
- So you must switch to an unrelated chemical
