Week 2 - Evolution of resistance

Question 1

What are the two forms of resistance?

Answer 1

There is evolved and non-host resistance. Evolved resistance is that which is evolved during the challenge of host population with a parasite, its adapted and acquired. Non-host resistance is where the hosts are not considered as part of the normal host range of the parasite and doesn’t provide evidence for the evolution of the host against the parasite. An example of evolved resistance in hosts is sickle cell trait in malarious regions with high burden of P.falciparum or the lack of expression of DARC on erythrocytes in SSA which provides innate resistance to P.vivax. For non-host resistance, experimental inoculations with fungal pathogens done on tropical plants found that the novel host species was resistant.

Question 2

What is the rationale behind non-host resistance?

Answer 2

Most hosts are resistant to most pathogens and parasites. This is because the parasite has adapted to different, select, hosts. Adaption to one host has often been linked to reduced ability to infect hosts from other taxa. Non-host resistance is therefore a consequence of pathogen specialisation to a different host.

Question 3

What is artificial selection?

Answer 3

Artifical selection is a form of select breeding where traits are specifically selected. This often results in fast changes in the mean trait value and may have costs in terms of reduced performance of other traits.

Question 4

Why are not all hosts resistant?

Answer 4

Not all hosts are resistant because there is an associated cost. There are genetic and physiological costs to evolving and maintaining a resistance mechanism as well as using it which can include self-damage linked with self-reactivity. These different costs can be observed through experiemental evolution and artificial selection with hamiltosporidum in daphnia magna.

Question 5

What are the steps to defense in a host-parasite interaction?

Answer 5

1. The first step is host behaviour resistance which has been observed in Daphnia spp, genetic variation leads to the organisms being more or less phototactic which alters exposure to parasite transmission stages in the water
2. When there is parasite activiation and the parasite attached to the host, there is host barrier penetration. In the case of Daphnia and Pasteruria the attachment is highly specific and depends on the host and parasite clones rather than the environment
3. Lastly, there is also within host immune response, with Daphnia spp, it was observed that some maternal protection could be conferred to offpsring

Question 6

What are the evolutionary consequences of defense?

Answer 6

Evolutionary responses can only occur at steps where there is genetic variation. Changes at one step to make it more specific will make the entire process more specific, with multiple steps increasing variation. One infection blocking step is enough to determine the host range. Where one step blocks infection, evolution at the other steps will be less strong

Question 7

What is parasite mediated resistance?

Answer 7

This is where host resistance may be influenced by the presence of other symbiots. An example of this is Wolbachia and dengue; certain strains of wolbachia bacteria can protect host mosquitos against dengue virus. While some strains confer more protection, they can also be harmful to the host.

Question 8

What is cytoplasmic incompability?

Answer 8

CI is a phenomenon that results in sperm and eggs being unable to form viable offsprings which is an effect that arises from changes in the gamete cells caused by intracellular parasites like wolbachia. This can be exploited for biological vector control