Resistance to the extreme stresses of high temperature-desiccation and low temperature were compared among six species of the melanogaster species subgroup of Drosophila. D. melanogaster was the most resistant to all stresses. The cosmopolitan species, D. melanogaster and D. simulans, were more resistant to cold stresses than the four endemic species, D. mauritiana, D. teissieri, D. yakuba and D. erecta. D. simulans, D. mauritiana and D. teissieri showed similar resistance to heat and desiccation, while D. yakuba and especially D. erecta were sensitive to these stresses. A prerequisite for the spread of the cosmopolitan species into temperate zones thus appears to have involved a level of resistance to cold stress exceeding that of the endemics. A greater resistance to heat and desiccation stresses, such as that found in D. melanogaster, may also be necessary for the invasion of more extreme environments. All species with the exception of D. erecta and to a lesser extent D. yakuba can survive 6 h at the extreme temperature of 34�C at 95% RH, and in addition most of those flies surviving stresses of this type are fertile. This suggests that these species can survive short stress periods in a humid microhabitat in the wild.

D. simulans adults show an increase in longevity when exposed to 0.5-3.0% atmospheric ethanol in the absence of other food. The offspring produced in atmospheric ethanol also developed best at 0.5-3.0%. D. melanogaster adults show longevity increases, when exposed to 0.5-9.0% ethanol, which always exceed D. simulans; these increases are maximal in the 0.5-3.0% range. Larvae of D. melanogaster developed at all concentrations of ethanol, and at 0.5% to the adult stage. For development times, both species show minima at similar low ethanol concentrations. Therefore both species show maximum fitness at low alcohol concentrations, but only D. melanogaster can utilize high concentrations. This parallels the field situation, since D. simulans coexists with D. melanogaster at low concentrations, while only the latter is found at high concentrations.

On the basis of life-span, the threshold ranking at which ethanol and acetic acid ceased to be resources and became stresses for 3 sympatric, cosmopolitan Drosophila species was: D. melanogaster Mg. > D. simulans Sturt. > D. immigrans Sturt. The threshold ranking between larval attraction and avoidance followed the same sequence. An Adh-null mutant of D. melanogaster utilised ethanol to an extremely low threshold, while acetic acid was utilised to a threshold close to that of the D. melanogaster population; this predictable result was paralleled by larval attraction to acetic acid but not ethanol. It can therefore be concluded that there is an association between biochemical and behavioural phenotypes relating to resources commonly available in nature. Acetic acid acts as an attractant to larvae of all species at concentrations down to 1/1000 of the concentrations of ethanol attracting flies, which suggests that acetic acid may be a resource recognition compound, as well as a food resource.

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