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Download Lecture 16 – Hybridization and Introgression An overlooked effect of
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Lecture16–HybridizationandIntrogression Anoverlookedeffectofinvasivespeciesoccursatthelevelofthegenome.When speciesareintroducedthatarecloselyrelatedtonativespecies,theymayhybridize withthenatives.Thesegeneticcrossescanhavemanydifferentoutcomes. Hybridization ◦ Interbreedingbetweengeneticallydistinctpopulations ◦ Oftenrestrictedtointerbreedingbetweenspeciesbutsubspecific populationsmaybeconsidered. Introgression ◦ Geneflowbetweenpopulationswhoseindividualshybridize,and wherehybridsbackcrosstooneorbothparentpopulations (1) Maynotproduceoffspringthatcansurviveorproduceveryunfitoffspring: thisundesirablefromthenativestandpointasitrepresentslostreproductive effortanddiminishedfitness. (2) Hybridscanreproduceandviableoffspringarecreated (1) Preferotherhybrids(=apotentiallynovelthirdspecies) (2) Backcrosswitheitherorbothparents (3) Introgression,repeatedgeneflowbetweenpopulationswhose individualshybridize,andwherehybridsbackcrosstooneorboth parentpopulationscanreducethegeneticintegrityoftheparent population.Overtime,repeatedbackcrossingandhybridization movesDNAfromonepopulationintotheother.Bothpopulationscan losetheirdistinctidentitybutwithinvasivespecies,thegeneflow oftenisasymmetric,withthenativepopulationbeingsubsumed. • Theendresultisalossofnativediversitythroughchangestothegenome. Examples - NorthAmericantamariskpopulationsarecomposedoftwoseparatespecies andavarietyofdifferenthybrids.Ahybridspeciesisthemostdominant genotypeonthelandscapeinNorthAmerica. - Spartinacordgrass–nativetoeastcoastofNorthAmerica.Movedto Englandwhereithybridizedwiththenativecreatinganinfertile,clonally reproducingthirdspecies.Afterawhile,aspontaneousdoublingof chromosomescreatedasexuallyreproducingfourthspecieswhichbecamea super-invaderinEngland,Europe,andelsewhere. - Mallardducksassimilatenativeanatidduckpopulationseverywherethey havebeenintroduced.Geneflowisdisproportionatelyfrommallardsto nativeduckswhichgraduallylosetheiridentityafterrepeatedly backcrossingwithvarioushybridforms - Californiasalamandershybridizewithintroducedbarredsalamanders creatinghybridswarms.Thehybridsarebiggerandmoreaggressivethan eitherparentpopulationandtheynowdominatewetlandsinpartsofthe - centralvalley.Theirlargersizeincreasespredationonco-occurringnewts andnativefrogs Phragmites–nativegenotypesofPhragmitesaustralishavebeenvirtually eliminatedbyaEuropeangenotype(haplotypeM)whichisbroadlytolerant ofmanymorehabitats.
