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DiseaseEcology
I. Howdocoevolutionary armsraceswork?
Malariaasamodel(parasitismby4speciesofPlasmodium)
Kills~1millionpeople/year
Sickens~300millionpeople/year
Plasmodium cellsarepassedfrominfectedmosquitoes
o umans,an rom n ec e umans omosqu oes
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A.ArmsraceI:Malariaandpublichealthofficials
UseofDDTtokillmosquitoes:
Currentemphasis:useofinsecticideimpregnatedbed
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B.ArmsraceII:Evolutionaryresponsesinhumans
Question:In
areas
where
malaria
is
common,
does
naturalselectionfavorindividualswithresistantalleles?
Hemoglobin(Hb)
is
aprotein
that
carries
oxygen
in
red
bloodcells;thenormalalleleisHbA
whenitisheterozygous;causessicklecelldiseasewhen
homozygous
AA AS SS
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Recentdata:studyHb allelesinhealthyandPlasmodium
ObserverelativelyhighfrequencyofHbC allele
1.Inhealthy
people,
HbC genotypes
are
in
HW
proportions
2.AmongPlasmodiuminfectedpeople,observeno
HbCC genotypes.
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Estimate:Riskofinfectionreducedby29%inHbAC
n v ua s; y n n v ua s
Note:AfollowupstudyinMalishowednoprotection
against
severe
malaria
in
HbAC
heterozygotes.
HowisselectionactingonHbC alleles?
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II.Canparasitesmanipulatetheirhosts?
Doesnatural
selection
favor
alleles
that
allow
parasites
to
manipulatehostbehaviorinawaythatincreasestheir
transmissiontonew osts
HostmanipulationbyPlasmodium?
19humanvolunteers(Tanzania);eachindividualhas
Sleepinthesamehouse
Collectmosquitoes
the
following
day
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Underthehostmanipulationhypothesis:
1. Whatpercentage
of
uninfected
mosquitoes
bit
more
thanoneperson?
2.Whatpercentageofinfectedmosquitoesbitmorethan
oneperson?
Claim:Plasmodium ismanipulatingmosquitoesinaway
thatmakes
them
more
likely
to
bite
multiple
individuals
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Q:Whenyousneeze,isyourbodyriddingitselfofcold
viruses oriscoldvirusmani ulatin outoincrease
transmissionto
new
hosts?
ww u y u y u y
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.
Underwhatconditionsareparasitesexpectedtoevolveto
be
extremely
virulent?
=
Observation
2:
Virulence
is
a
function
of
parasite
w u .
Observation3:Theprobabilityofparasitetransmission
isafunction
of
the
number
of
parasites
present.
virulence
(it
can
respond
to
selection).
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Question:Give
two
reasons
why
condom
use
and
sexual/maritalfidelityshouldbeeffectiveinhaltingthe
.
Circumcisionas
aprophylaxis?
Pros:
Cons:
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A. SurveyincidenceofcholerabacteriuminwatersoffPeru
a
s
ere a ons p
e ween
seawa er
empera ure
andVibrio cholerae concentration?
ans
CooloceanwaterSe
in
hum
wa
ter
nrate
temp
nfectio
ratue
I
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B.Surveyincidenceofmicethatcarryhantavirus,asa
rodents,speciesthatpreyonmice)
FiveparksinPortland,Oregon