Question

Background:

You are a CDC (Center for Disease Control) researcher studying malaria. Your goal is to understand the evolution of malaria in order to be able to possibly predict where a future outbreak may be. To do this you will be using information from the evolution of the pathogen, the host, and changes in the environment. Topics to think about include:

• What is the evolutionary origin of human malaria? (hint: think about host-pathogen cospeciation or host shift)
  • What could you learn about the adaptability of the malaria pathogen based on its reconstructed evolutionary history?
• How does the information on the origin of human malaria affect your prediction for possible outbreaks? (hint: think what are the reservoirs for malaria pathogens)
• What would happen if changes in climate affect the distribution of the malaria mosquito vector?
  • Think if and how patterns of malaria geographic distribution would change by considering also the effect that the presence/absence of the sickle cell anemia allele would play
  • How likely is it that human populations living in a geographic area without malaria will have a high frequency of the sickle cell anemia allele?

The information that you have is:

• Knowledge of the malaria cycle, which includes one host, primate (human, chimpanzee, or gorilla), and one vector, mosquito. Malaria is prevalent in areas where the mosquitoes thrive (hot and humid).
• Access to three different pathogens: Plasmodium falciparum (host: human), P. reichenowi (host: chimpanzee), Plasmodium sp. (host: gorilla).
• The knowledge that individuals that have one (heterozygotes) or two (homozygote recessive) alleles for sickle cell are partially protected from malaria (although they still suffer some negative effects from the sickle cell disease)

Think about what type of information you would need to approach the problem above and how you would interpret different patterns. Answer the questions with a series of “if-then” scenarios in which you explore a different types of information you may get from the data and how you would interpret it.

Malaria life cycle

Question:

How would you distinguish between cospeciation and host shift (draw phylogenetic trees with the malaria pathogens and their hosts to illustrate the two scenarios)? Briefly describe and explain your phylogenies.

Answer 1

Hosts and their symbionts are involved in intimate physiological and ecological interactions. The impact of these interactions on the evolution of each partner depends on the time‐scale considered. Short‐term dynamics can foster parasite specialization, but that these events can occur following host shifts and do not necessarily involve cospeciation. Coevolutionary dynamics of hosts and parasites do not favor long‐term cospeciation.

The systematic analysis of patterns of host-parasite co-evolution in the primate malaria system reveals that the phylogenetic history of primate-infecting plasmodia is constrained by the phylogenetic associations of their hosts. One of the most important factors that shape this pattern is hosts switching, and some parasites can preserve a great flexibility to infect hosts across a large phylogenetic distance resulting in broad host ranges that can be observed in nature. The emergence of new malaria disease in primates including humans cannot be fully predicted from the phylogeny of parasites.