Mechanisms of population establishment in insect invasions : Drosophilidae as a model system

Opperman, Elizabeth Johanna (2018-03)

Thesis (MScConsEcol)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The mechanisms and traits influencing insect invasions are generally poorly understood. Drosophilids are an excellent model system for studying invasions and especially the adaptive processes occurring during invasions since the family has short generation times, diverse functional traits and variation in geographic distributions while possessing several notable invasive species. While there are many studies of environmental stress resistance or life-history traits and how these might influence population dynamics or geographic range limits in Drosophilidae, these studies have several potential shortcomings. Chief among these are perhaps concerns surrounding their use of Stock Centers (laboratory cultures) of varying time in culture and sometimes unknown geographic origins to infer trait-environment associations, niche requirements or evolutionary adaptive capacity. Traits can respond rapidly to laboratory rearing with laboratory cultures typically losing stress resistance and increasing fecundity and/or development rates. In this study, I sought to determine whether there is a significant and systematic effect of time spent in culture on estimates of environmental stress resistance and its thermal acclimation (i.e. phenotypic plasticity) of two wild-caught Drosophila species (Drosophila melanogaster and Zaprionus vittiger) between newly established lines (in the F2 generation) and a later timepoint (F8-F10 generation) in the laboratory under standard, controlled rearing conditions. A further objective was to identify the nature and magnitude of basal and plastic estimates of environmental stress resistance traits among four populations of D. melanogaster collected from different areas within South Africa to assess if geographic origin influences trait and plasticity estimates substantially within a single species. This was done by measuring traits of upper and lower thermal activity limits (CTMAX and CTMIN, respectively), the proportion of individuals surviving after 24 hours after exposure to a potentially lethal temperature (heat and cold survival survival), desiccation resistance, starvation resistance and the plasticity thereof in response to thermal acclimation at three temperatures (18, 23, 28 ˚C). There was significant variation in resistance to environmental stressors between earlier and later generations for D. melanogaster and Z. vittiger. Drosophila melanogaster generally increased resistance to environmental stressors after spending ten generations in the laboratory whilst Z. vittiger had decreased resistance. There was also significant variation in both thermal and survival traits and the plasticity thereof between the four populations of D. melanogaster. Thus, it is clear that conditions at time of sampling and the species or population’s geographic source can strongly mediate trait and plasticity assessments in laboratory cultures. Consequently, environmental stress resistance measured from Stock Centers lines or species may give a biased view which could influence tests of climate or niche matching and risk assessments. The divergent, idiosyncratic responses noted between my study species’ means that more species would need to be assessed to understand the generality of the outcomes described here.

AFRIKAANSE OPSOMMING: Meganismes en eienskappe wat suksesvolle indringings te weeg bring word nie goed verstaan nie. Die Drosofiliede verteenwoordig ‘n belangrike model sisteem vir die studie van indringerspesis en die adaptiewe prosesse wat gepaard gaan met suksesvolle indringings, omdat die familie diverse funksionele eienskappe besit, ‘n wye geografiese verspreding het en terselfdetyd uit verskeie kenmerkende indringerspesies bestaan. Daar bestaan verskeie studies wat die omgewings stres weerstands eienskappe van die Drosofiliede gemeet het, maar hierdie studies het verskeie tekortkominge. Een van die belangrikste tekortkominge is moontlik bekommernisse rondom die gebruik van ‘Stock Centers’ (laboratorium kulture) van verkeie ouderdomme en geografiese afkoms om omgewings assosiasies te maak of om evolutionêre adaptiewe kapasiteit te bepaal. Spesies eienskappe reageer vinnig op laboratorium kondisies en verloor tipies hulle weerstand tot omgewings kondisies met ‘n gepaardgaande toename in voortplanting. Die doel van hierdie studie was om te bepaal of die aantal tyd wat spandeer word in kultuur ‘n beduidende effek het op beramings van omgewings stress weerstand en termiese akklimasie (fenotiepiese plastisiteit) tussen nuutgestigte (F2 generasie) Drosofilied spesies (Drosophila melanogaster en Zaprionus vittiger) en na die spesies tien generasies (F10 generasie) in kultuur onder standaard grootmaak praktyke spandeer het. ‘n Verdere doel van die studie was om die natuur en omvang van basale en plastiese beramings van omgewings stress weerstand eienskappe te identifiseer binne vier populasies van D. melanogaster wat gevang was in verskeie dele van Suid Afrika om sodoende te bepaal of geografiese afkoms ‘n beduidende impak kan hê op beramings van omgewingseinskappe en plastisiteit binne ‘n spesifieke spesies. Hierdie was gedoen deur die boonste en onderste termiese limiete (CTMAX en CTMIN), die proporsie van individue wat oorleef het 24 uur na blootstelling aan ‘n potensiële dodelike temperatuur (hitte en koue skok) sowel as oorlewing na uitdroging en uithongering, en hulle plastisiteit in reaksie tot termiese akklimasie by drie temperature (18°C, 23°C en 28°C), te bepaal. Daar was betekenisvolle en teenstrydige weerstand tot omgewing stressors tussen vroeër en latere generasies van D. melanogaster en Z. vittiger. Drosophila melanogaster het ‘n algemene toename in weerstand tot omgewing stressors gehad na tien generasies in die laboratorium, terwyl Z. vittiger ‘n afname ondergaan het. Daar was ook betekenisvolle verskille in beide termiese en oorlewingseienskappe sowel as hulle plastiese reaksie tussen die vier populasies van D. melanogaster, en dus is dit duidelik dat die geografiese oorsprong van ‘n spesies of populasie ‘n effek kan hê op die assessering van eienskappe en hulle plastisiteit in laboratorium kulture. Dus is dit duidelik dat die kondisies tydens steekproefneming sowel as die spesies of populasie se geografiese oorsprong ‘n verdere invloed het op omgewingseienskap en platisiteit assesserings in laboratorium culture. As ‘n gevolg sal omgewings stress eienskappe wat gemeet is vanaf ‘Stock Centers’ ‘n bevoordeelde uitkyk gee en hierdie vooroordele kan ondersoeke van klimaat en nis ooreenstemmings beïnvloed en sodoende risiko assessering beïnvloed. Die uiteenlopende eienaardige reaksies opgemerk tussen my studie spesies beteken dat meer spesies geasseseer sal moet word om die algemeenheid van die uitkomste wat hier beskryf word te verstaan.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103832
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