HostParasite Coevolution

1
Host-Parasite Coevolution

• Rob Knell

2
Aims

• What is a parasite?
• Parasite diversity
• Parasite-host coevolution
• Parasites and the evolution of sexual
  reproduction
• Parasites in control - manipulation of host
  behaviour

3
Definitions of parasite
Websters International Dictionary An organism
living in or on another organism, obtaining from
it part or all of its organic nutriment, commonly
exhibiting some degree of adaptive structural
modifications and causing some real damage to its
host
4
Hookworm Ancylostoma duodenale
-infects 1billion people worldwide -feeds on
blood in intestinal mucosa -penetrate from soil
into skin -often without symptoms but can cause
anaemia, and mental retardation in children
5
Ascaris lumbricoides
worms from inhabitants of a single village
(mainly children)

• Large Roundworm
• affects 1.5billion people worldwide
• -eggs ingested, worms hatch, migrate to lungs,
  then
• back to intestines

6
Plasmodium falciparum
- Malaria kills more people than any other
infectious disease (1000-2000 per day) -
protozoan (single-celled eukaryote, not a
bacterium)
7
Aphids
8
Venturia canescens
parasitic wasps typically lay eggs into larvae of
other species (here flour moth) which the
parasitic larvae then gradually consume from
inside
9
Demodex folliculorum
Not all animals that live in close association
with a host are parasites...
eyelash mite commensal (lives with about 1/3
of humans)
10
Bacteriophage
Even bacteria have parasites
11
Herpes simplex virus
12
Rafflesia arnoldii
Plants can parasitise other plants
Diameter 1m, has no leaves, stems or even roots
the base is completely embedded in host plant.
Flower smells of rotten flesh.
13
Host-parasite coevolution

• Parasitism is an extremely popular lifestyle
• The majority of organisms are either infected by
  parasites or at risk from infection
• Parasites, by definition, harm their hosts
• Hosts are therefore under selection pressure to
  avoid parasitism
• Parasites are under selection pressure to evade
  host defences

14
Evolution of a parasite

• Influenza virus
• Immunity is determined by two antigens, HA and
  NA.
• Antigenic drift leads to new strains with
  different HA or NA antigens, that are able to
  infect people who are resistant to other strains
• This is why we see periodic epidemics of
  influenza when new strains emerge and are
  strongly selected for.

15
Phylogeny of influenza

• This is a phylogeny of influenza A virus over an
  extended period
• Note that there is continual replacement of one
  strain by another, and that old strains go extinct

16
Host-parasite coevolution

• Parasites will be selected to exploit common host
  genotypes
• Hosts carrying rare resistance alleles will gain
  a selective advantage because they are not
  parasitised as much
• Rare host genotypes will become common
• The parasites will then be selected to exploit
  hosts carrying these resistance alleles.
• Thus the fitness of a genotype will depend on its
  frequency in the population

17
Host evolution

• In the same way that parasites are constantly
  evolving to overcome host defences,
• Host organisms will be constantly evolving to
  resist parasitic infection
• This will lead to Frequency dependent selection,
  locking hosts and parasites into endless
  coevolutionary cycles
• This is what is often called the Red Queen
  Effect

18
The Red Queen
Now here, you see, it takes all the running you
can do, to keep in the same place. If you want to
get somewhere else, you must run at least twice
as fast as that
from Through the Looking Glass, by Lewis Carroll
19
Parasitism and sex

• The evolution of sexual reproduction is a big
  puzzle in biology
• One possibility is that sexual reproduction
  benefits an organism by increasing the
  variability of the organisms offspring
• This only gives a big fitness advantage when the
  environment changes very rapidly
• One aspect of the environment that does change
  fast enough is the parasites that an organism is
  exposed to.

20
Evidence?

• Very hard to test experimentally
• One noteworthy study by Curtis Lively and
  coworkers
• Potomopyrgus antipodiarum - freshwater snail with
  both sexually and asexually reproducing
  individuals
• The proportion of asexually reproducing
  individuals is related to the amount of
  parasitism a population experiences
• More parasitism leads to more sexually
  reproducing snails

21
Parasites and host behaviour

• Parasites dont just face selection pressure to
  overcome host defences
• Transmission between hosts is crucial to a
  parasites fitness
• Some parasites have complex life-cycles,
  infecting more than one host before reaching
  adulthood
• In these cases, some parasites seem to change
  host behaviour to enhance their transmission rate

22
Dicrocoelium dendriticum
the lancet liver fluke
23
Dicrocoelium dendriticum
Eggs hatch, parasite reproduces asexually
Snails produce slime balls containing parasite
larvae