Interspecific competition in Drosophila I. Reversal of competitive superiority due to varying concentration of ethanol

The experiments reported here were designed to investigate the outcome of interspecific competition between Drosophila melanogaster and D. simulans in environments varying in one factor only. The factor chosen was ethanol concentration in the medium; the strains used were D. melanogaster SM5 (Series I), D. melanogasterw, b , (Series II) and D. simulans v (both Series).
The results of competition over a number of generations, the longest experiment lasting 100 days, showed clear differences in the relative competitive abilities of the two species between the different environments. With no ethanol, D. simulans won in all replicates; with 896 ethanol, D. melanogaster won in all replicates; at intermediate (496) concentration, the winning species was not consistent. Experiments in Series II showed very similar results to their counterparts in Series I despite distinct genetic differences between the strains of D. melanogaster used.
These results tie in with published work, on (i) the relative susceptibility of single–species cultures of D. melanogaster and D. simulans to high ethanol concentrations, and (ii) distributional patterns observed in some field populations exposed to alcohols in their environment.     

Segment number,body length,and latitude in geophilomorph centipedes: a ‘converse‐Bergmann’ pattern

There is a negative relationship between trunk segment number and latitude among geophilomorph centipedes in general. A similar relationship is known to exist within the most intensively‐studied geophilomorph species, Strigamia maritima, and also within several other species from this group. Previously, it was considered that this relationship did not involve body length; instead, individuals of S. maritima with more segments were considered to be more finely subdivided (not longer) than those with fewer segments. This incorrect interpretation arose from the difficulty of reliably separating post‐embryonic stages and thus of making a simple and direct comparison. In the present study, we build on recent work that facilitates such comparisons; and we show conclusively that individuals with more segments are longer. Our finding means that it is now possible to connect the work on S. maritima in particular, and on geophilomorph centipedes in general, with the debate about Bergmann's ‘rule’: the proposal that body size increases with increasing latitude. There is a clear ‘converse‐Bergmann’ pattern, as has been found in several other taxa. We propose an adaptive hypothesis that may explain why geophilomorphs show this pattern. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Frequency-dependent competitive abilities and differential resource use in competition between Drosophila hydei and D. melanogaster

Several experiments, each involving competition between Drosophila melanogaster and D. hydei in population cages, were set up and allowed to run for up to 50 weeks. The population sizes of both species, and hence the species frequencies, were monitored once a fortnight, i.e. approximately once per generation. Coexistence of the two species was observed in cages containing resource bottles with 5 g of food medium; cages whose resource bottles contained only 1.5 g resulted in competitive exclusion of D. hydei. Competitive abilities were frequency-dependent in the former case but not in the latter. Tests of larval depth distributions revealed that D. hydei larvae feed at a deeper level in the food medium than larvae of D. melanogaster. The explanation of the contrasting results of competition when bottles contained 5 g and 1.5 g of resources lies in the production of frequency-dependent competitive abilities by larval resource partitioning in the bottles with 5 g, and the preclusion of such partitioning in the 1.5 g bottles because of the very limited depth of food medium then available. The relevance of these results to a model of competition is discussed, and the potential generality of differential resource use as a stabilizing mechanism in both interspecific and intergenotypic competition is noted.