Additional factors influence temperature-dependent sex determination in Leopard Geckos

2004·2.6 MB·English

by  Janes, Daniel E

ADDITIONALFACTORSINFLUENCETEMPERATURE-DEPENDENTSEX
DETERMINATIONINLEOPARDGECKOS
By
DANIELE.JANES
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2004
Idedicatethisworktomyparents. Theyhavealwaysencouragedmetofollowmyown
pathandwelcomedmebackhometorecoverfrommydefeatsandcelebratemyvictories.
Also,Idedicatethisworktomyteachersandstudents. Theyhavesharedtheirthoughts
andambitionswithme. Everyonehasshownmeaslightlydifferentwaytoseethe
worldandeveryonehasallowedmetosharemyownperspectivewiththem. Weshould
allbesolucky.
ACKNOWLEDGMENTS
Iextendmydeepestgratitudetomyco-advisors,Drs.MartaL.WayneandF.
WayneKing,fortheirsupport,encouragement,andwisdom. Myothercommittee
members,Drs.KarenBjomdal,LouisGuillette,andMaxNickerson,havemaintainedan
opendoorpoHcyofwhichIhavetakenfulladvantage. Conversationswithmyadvisors
havetaughtmethevalueofdiscussionatallstagesofresearchdesignandexecutionand
thenecessityofcollaboration. IwishtothankallofmyfriendsintheDepartmentsof
Zoology,WildlifeEcologyandConservation,andtheFloridaMuseumofNatural
History. TheirfriendshipandlaughterdailyreinforcedoneofthemainreasonsIama
scientistandteacher. Itisfrin. ThisresearchhasbeenfundedbytheFloridaMuseumof
NaturalHistory,SigmaXi,andWilliamandMarciaBrantofTheGourmetRodent.
ConversationswithBenBolkerhelpedshapemyanalysesofdata. Datacollection
assistancewasprovidedbyJasonPhillips,JenniferComiskey,SaraReyes,Jennifer
Mobberley,MargaretO'Brien,EricTimauer,CassandraPedrosa,AlanaSchoenberg,
JuliaHuang,CraigAjmo,JohnBowden,ErinTaylor,andKristinBams. Workwas
conductedinaccordancewithUniversityofFloridalACUCprotocolZOIO.
iii
TABLEOFCONTENTS
page
ACKNOWLEDGMENTS
iii
ABSTRACT vi
CHAPTER
1 GENERALINTRODUCTION 1
2 ABIOTICEFFECTSONSEXDETERMINATIONINLEOPARDGECKOS 6
Introduction 6
Methods 10
Animals 10
SexingProcedure 11
Analysis 12
Results 12
Discussion 13
3 QUANTITATIVEGENETICVARL\TIONINSEX-DETERMINING
RESPONSETOINCUBATIONTEMPERATUREINLEOPARDGECKOS 25
Introduction 25
Methods 28
Animals 28
SexingProcedure 29
Analysis 30
Results 30
Discussion 31
4 ESTROGENANDESTROGENMIMICINCREASEPRODUCTIONOFMALES
INATEMPERATURE-DEPENDENTSEX-DETERMININGSPECIES 38
Introduction 38
Methods 40
Animals 40
SexingProcedure 41
Analysis 42
iv
Results 42
Discussion 44
5 GENERALDISCUSSION 55
LISTOFREFERENCES 61
BIOGRAPHICALSKETCH 70
V
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
ADDITIONALFACTORSINFLUENCETEMPERATURE-DEPENDENTSEX
DETERMINATIONINLEOPARDGECKOS
By
DanielE.Janes
May2004
Chair:F.WayneKing
Cochair:MartaL.Wayne
MajorDepartment:Zoology
Hatchlingsexratiosofreptileswithenvironmentalsexdetermination(ESD)are
influencedbyincubationtemperature. LeopardGeckos,Eublepharismacularius,nestin
aregionwheretheyareexposedtoawiderangeoftemperatures. Iftemperatureisthe
soledeterminantofsexinthisspecies,thenhatchlingsexratiosshouldbehighly
correlatedwiththethermalgradientindifferentpartsofthespecies'range. Inthis
scenario,abroadthermalgradientwouldpromoteregionalvariationinhatchlingsex
ratiosof£.macularius. However,reportsofregionalvariationinhatchlingsexratiosare
rareamongESDreptiles. Ifasecondaryfactor(orfactors)playsaroleinsex
determinationofthisspecies,theirhatchlingsexratiosinnaturewouldbeexpectedto
varyseasonallyandregionallywithlesscorrelationwiththermalgradient. Inthisstudy,
E.maculariuseggswereincubatedatthreetemperaturesintwoconsecutiveyears. The
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8
rangeoftemperaturesstudiedintheseexperimentsincludesthespecies-specific
temperaturesatwhichthelowestproportionofmales(0.0)andthehighestproportionof
males(-0.7)areproducedpertreatmentgroup. Eachtreatmentgroupconsistedof1
maculariuseggs. Ateachtemperature,theeffectsofhumidity,quantitativegenetic
variation,andenvirorunentalendocrinedisruptersonhatchlingsexratiosoftreatment
groupsweremeasured. Humiditydidnotsignificantlyaffecthatchlingsexratios. Also,
resultsofthequantitativegeneticsexperimentsuggestthatESDinE.maculariusisa
genotypexenviroimientinteraction. Bothnaturalandsyntheticestrogensaffected
hatchlingsexratiosatbothmale-andfemale-producingtemperatures. Iconcludethat
ESDisamulti-dimensionaltraitinE.macularius. Theinfluencesofabioticfactorslike
localprofileofenvironmentalhormoneandhormonemimicconcentrationsandbiotic
factorslikequantitativegeneticvariationmayamelioratetheeffectsofclimatechangeon
ESD. EstimatesofextinctionratesofESDspeciesasaresultofchangingclimates
shouldbereconsideredinlightofthesedata. Asotherresearchershaveconcludedfor
otherESDreptiles,IconcludethatthepartialgeneticcontrolofsexdeterminationinE.
maculariusisapolygenictraitandrespondstotemperaturefluctuationasagenotypex
environmentinteraction. Inlightoftheseconclusions,sex-determiningmechanismsof
reptilesshouldbereconsideredasabroadspectrumofmulti-dimensionalmechanisms
insteadofasimpledichotomyofESDandgeneticsexdetermination.
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CHAPTER
1
GENERALINTRODUCTION
Theoptimalsexratioofapopulationmaychangedependingonfluctuating
environmentalstressesandsex-differentialvulnerabilitytoenvironmentalstresses.
AccordingtotheTrivers-Willardhypothesis,parentsmayadjusttheiroffspringsexratio
toaddresssex-differentialfitnessandgamerthegreatestreproductiveadvantage(Trivers
andWillard1973). TheTrivers-WillardhypothesisisbasedonFisher'stheoryofequal
parentalinvestmentintherearingofsonsanddaughters(Fisher1930). Ifsonsand
daughtersareunequallyaffectedbyenvironmentalstress,thenoffspringsexratiosshould
beskewedtowardthesexthatislessnegativelyaffected. AccordingtoFisher,thesex
thatismostnegativelyaffectedbyenvironmentalstresswillbeproducedlessfrequently
andwillgamermoreparentalinput. Thecostpaidbytheparentsfortherarer,more
energeticallyexpensiveoffspringsexshouldbalancewiththecosttheypayforthemore
common,lessenergeticallyexpensiveoffspringsex. Forexample,sex-differential
growthrateswouldcauseanimmediateincreaseinparentalinvestmentforonesexover
theother. Iftheimmediateinvestmentrequiredbythefastergrowingoffspringcannot
bemadebecauseofresourcescarcityorpoormaternalcondition,thentheslowergrowing
sexwillbeproducedinexcessofthefastergrowingsex. Theslowergrowingsexcan
surviveanimmediatebutshort-liveddearthofresourcesmuchbetterthanthefaster
growingsex. Environmentalstressesthatdifferentiallyaffectmalesandfemalesshould
causesexratiostodeviatefrom0.5. TheTrivers-Willardhypothesisismost
1
2
appropriatelytestedusingaspecieswithenvironmentalsexdetermination. Thistypeof
organismwillproduceoffspringsexratiosthatcanbedirectlylinkedtoexperimentally-
inducedenvironmentalconditionswithouttheconfoundingfactorofpredominantgenetic
controlofsexdetermination.
Thegonadalsexofallorganismshasbeenattributedtoeithergeneticsex
determination(GSD)orenvironmentalsexdetermination(ESD). hiGSD,thesexofthe
organismdependsonthesexchromosomecontributionsfromtheorganism'sparent(s).
hiESD,thesexoftheorganismdependsonthepost-fertilizationenvironment
experiencedbytheorganismasadevelopingembryo. Typically,ESDreferstothe
effectsofincubationtemperature,althoughHeiligenberg(1965)reportedasexratio
skewingeffectofpHincichlids. Amongvertebrates,theinitiationofsexdifferentiation
ispredominantlycontrolledbyincubationtemperatureintuatara,somefish,turtles,and
lizardsandallcrocodilians(Bull1980;ConoverandKynard1981;EwertandNelson
1991;JanzenandPaukstis1991). Allothervertebratesappeartobegeneticallysex-
determined. Mcubationtemperaturecanalsoaffectbodysizeandgrowth(Crewsetal.
1998;JanesandKing,unpublisheddata),adultsexuality(GutzkeandCrews1988),
aggressivebehavior(RhenandCrews1999),andtheorganizationofneuralstructures
(Coomberetal.1997). Incubationtemperatureisknowntocausethedevelopmentofan
embryo'stestesorovaries;theproximatemechanismisstillunknown.
TherecognitionofESDasaderivedconditionissupportedbyreportsofGSDin
amphibians,abasaltaxontoreptiles. Nosurveyedamphibianshavedemonstratedasex-
determiningeffectofincubationtemperaturewithintherangeoftemperaturestowhich
theireggsarenaturallyexposed(Hayes1998). TheadaptivesignificanceofESDhas
3
eludedresearchers(JanzenandPaukstis1991). ConcensusfavorsESDaseitheran
adaptivemechanismforspeciesinwhichafitnessadvantageoscillatesbetweenmales
andfemales(Shine1999)orasaneutralmechanismthathasnotbeencounter-selected
(BullandChamov1989;GirondotandPieau1999).
Thisstudywilltesttheeffectsofenvirorunentalstressesonclutchsexratiosof
LeopardGeckos,Eublepharismacularius,areptilespeciesforwhichmuchhasbeen
writtenontheirsex-determiningresponsetoincubationtemperature. LeopardGeckos
liveinarangeofclimaticheterogeneity. Theirrangeextendsfromthegrasslandsof
southeasternTurkeytotheforestsofsouthwesternhidia(Smith1935).Clutchsexratios
aremale-biasedwhenincubatedat32.5''C. Coolerandwarmerincubationtemperatures
withinthefrillrangeofviableincubationtemperatures(25°to35°C)causefemale-biased
clutchsexratios(Vietsetal.1993). IftheESDmechanismbehavessimilarlythroughout
theclimaticallyheterogeneouszonesoftheirrangeandallothervariablesareirrelevant,
thendifferencesintheprimarysexratiosofE.maculariuscanbeexpectedasaresultof
differentincubationtemperaturesindifferentpartsoftheirrange(Vietsetal.1993;
Crewsetal.1996;Coomberetal.1997;Rhenetal.2000). Also,climatechangedueto
globalwarmingorothercausesmightcausedramaticshiftsinpopulationsexratiosofE.
maculariusandotherESDspecies.
Basedontheobservationsthat(a)amonghighervertebrates,thesexofseveral
speciesofreptilesandfishesappearstobecontrolledbyincubationtemperature,(b)
populationsofenvironmentallysex-determinedspeciescancoverrangesthatinclude
morethanoneclimatezone,and(c)sexratiosdonotappeartobeunbalancedatdifferent
extremesoftheirrange,Ihypothesizethatthesex-determiningmechanisminE.