Host-Parasite Coevolution - Queen Mary University of London

Host-Parasite Coevolution - Queen Mary University of London

Host-Parasite Coevolution Rob Rob Knell Knell Aims What is is aa parasite? parasite? What Parasite diversity diversity Parasite Parasite-host coevolution coevolution Parasite-host Parasites and and the the evolution evolution of of Parasites sexual sexual reproduction reproduction Parasites in

in control control -Parasites manipulation manipulation of of host host behaviour behaviour 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 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

worms from inhabitants of a single village (mainly children) Ascaris arge Roundwormlumbricoides ffects 1.5billion people worldwide ggs ingested, worms hatch, migrate to lungs, then back to intestines Plasmodium falciparum Malaria kills more people than any other infectious disease (1000-2000 per day) rotozoan (single-celled eukaryote, not a bacterium Aphids Venturia canescens arasitic wasps typically lay eggs into larvae of other ecies (here flour moth) which the parasitic larvae en gradually consume from inside Not all animals that live in close association with a host are parasites...

Demodex yelash mite folliculorum commensal ives with about 1/3 of humans) Even bacteria have parasites Bacteriophage Herpes simplex Plants can parasitise other plants Rafflesia arnoldii ameter 1m, has no leaves, stems or even roots the base is completely embedded in host plant. ower smells of rotten flesh. Host-parasite coevolution Parasitism Parasitism is is an an extremely extremely popular

popular lifestyle lifestyle The The majority majority of of organisms organisms are are either either infected infected by by parasites parasites or or at at risk risk from from infection infection Parasites, Parasites, by by definition, definition, harm harm their their hosts hosts

Hosts Hosts are are therefore therefore under under selection selection pressure pressure to to avoid avoid parasitism parasitism Parasites Parasites are are under under selection selection pressure pressure to to evade evade host host defences defences Evolution of a parasite Influenza Influenza virus:

virus: Immunity Immunity is is determined determined by by two two antigens, antigens, HA HA and and NA. NA. Antigenic Antigenic drift drift leads leads to to new new strains strains with with different different HA HA or or NA NA antigens, antigens, that that are

are able able to to infect infect people people who who are are resistant resistant to to other other strains strains This This is is why why we we see see periodic periodic epidemics epidemics of of influenza influenza when when new new strains strains emerge emerge and

and are are strongly strongly selected selected for. for. Phylogeny of influenza This is is aa phylogeny phylogeny This of influenza A virus of influenza A virus over over an an extended extended period period Note that that there there is is

Note continual continual replacement replacement of of one one strain strain by by another, another, and and that that old old strains strains go go extinct extinct Host-parasite coevolution Parasites Parasites will will be be selected selected to

to exploit exploit common common host host genotypes genotypes Hosts Hosts carrying carrying rare rare resistance resistance alleles alleles will will gain gain aa selective selective advantage advantage because because they they are are not not parasitised parasitised as as much much Rare Rare host

host genotypes genotypes will will become become common common The The parasites parasites will will then then be be selected selected to to exploit exploit hosts hosts carrying carrying these these resistance resistance alleles. alleles. Thus Thus the the fitness fitness of of aa genotype genotype will will depend depend on

on its its frequency frequency in in the the population population Host evolution In In the the same same way way that that parasites parasites are are constantly constantly evolving evolving to to overcome overcome host host defences, defences, Host Host organisms

organisms will will be be constantly constantly evolving evolving to to resist resist parasitic parasitic infection infection This This will will lead lead to to Frequency Frequency dependent dependent selection, selection, locking locking hosts hosts and and parasites parasites into into endless endless coevolutionary coevolutionary cycles cycles

This This is is what what is is often often called called the the Red Red Queen Queen Effect Effect 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 from Through the Looking Glass, least twice

as fast by Lewis Carroll as that Parasitism and sex The The evolution evolution of of sexual sexual reproduction reproduction is is aa big big puzzle puzzle in in biology biology One One possibility possibility is is that that sexual sexual reproduction reproduction benefits benefits an an organism organism by

by increasing increasing the the variability variability of of the the organisms organisms offspring offspring This This only only gives gives aa big big fitness fitness advantage advantage when when the the environment environment changes changes very very rapidly rapidly One One aspect aspect of of the the environment

environment that that does does change change fast fast enough enough is is the the parasites parasites that that an an organism organism is is exposed exposed to. to. Evidence? Very Very hard hard to to test test experimentally experimentally One One noteworthy noteworthy study

study by by Curtis Curtis Lively Lively and and coworkers coworkers Potomopyrgus Potomopyrgus antipodiarum antipodiarum -- freshwater freshwater snail snail with with both both sexually sexually and and asexually asexually reproducing reproducing individuals individuals The The proportion proportion of of asexually asexually reproducing reproducing individuals

individuals is is related related to to the the amount amount of of parasitism parasitism aa population population experiences experiences More More parasitism parasitism leads leads to to more more sexually sexually reproducing reproducing snails snails Parasites and host behaviour Parasites Parasites dont dont just just face

face selection selection pressure pressure to to overcome overcome host host defences defences Transmission Transmission between between hosts hosts is is crucial crucial to to aa parasites parasites fitness fitness Some Some parasites parasites have have complex complex life-cycles, life-cycles, infecting infecting more more than than one

one host host before before reaching reaching adulthood adulthood In In these these cases, cases, some some parasites parasites seem seem to to change change host host behaviour behaviour to to enhance enhance their their transmission transmission rate rate Dicrocoelium dendriticum the lancet liver fluke

Dicrocoelium dendriticum Ants are accidentally eaten by grazing sheep. Parasites grow to adulthood Slime balls eaten by ants. The larvae encyst Eggs passed in faeces, eaten by snail Snails produce slime balls containing parasite larvae Eggs hatch, parasite reproduces asexually

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