Mechanisms of population establishment in insect invasions: Drosophilidae as a model system
Opperman, Elizabeth Johanna
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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 4 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.