Avis C. James

Thermal evolution of rate of larval development in Drosophila melanogaster in laboratory and field populations

The duration of Drosophila melanogaster larval and pupal periods was measured in laboratory thermal lines and in populations collected along a latitudinal transect in eastern Australia. In replicated laboratory lines kept for 9 years at 16.5° C or 25° C the duration of larval development had continued to diverge compared with 4 and 5 years previously, with more rapid larval development, and hence reduced total duration of pre‐adult development, in the low temperature lines at both experimental temperatures. After 4 years of separate evolution, lines derived from the 25° C lines and subsequently cultured at 29° C showed no evidence of significant divergence in the duration of any part of the pre‐adult period. The geographic populations showed a decrease in the duration of larval development, and hence of the total pre‐adult period, with increasing latitude. In both laboratory and field populations, evolution at lower temperature was associated with more rapid larval development to a larger adult body size, the opposite genetic correlation between these traits to that found within a single temperature. The indications are that lower temperatures may be permissive of more efficient growth in D. melanogaster. It will be important to discover if evolution in response to temperature induces similar correlations in other ectotherms.

LATITUDINAL VARIATION OF WING:THORAX SIZE RATIO AND WING-ASPECT RATIO IN DROSOPHILA MELANOGASTER.

In dipterans, the wing‐beat frequency, and, hence, the lift generated, increases linearly with ambient temperature. If flight performance is an important target of natural selection, higher wing:thorax size ratio and wing‐aspect ratio should be favored at low temperatures because they increase the lift for a given body weight. We investigated this hypothesis by examining wing: thorax size ratio and wing‐aspect ratio in Drosophila melanogaster collected from wild populations along a latitudinal gradient and in their descendants reared under standard laboratory conditions. In a subset of lines, we also studied the phenotypic plasticity of these traits in response to temperature. To examine whether the latitudinal trends in wing:thorax size ratio and wing‐aspect ratio could have resulted from a correlated response to latitudinal selection on wing area, we investigated the correlated responses of these characters in lines artificially selected for wing area. In both the geographic and the artificially selected lines, wing:thorax size ratio and wing‐aspect ratio decreased in response to increasing temperature during development. Phenotypic plasticity for either trait did not vary among latitudinal lines or selective regimes. Wing:thorax size ratio and wing‐aspect ratio increased significantly with latitude in field‐collected flies. The cline in wing:thorax size ratio had a genetic component, but the cline in wing‐aspect ratio did not. Artificial selection for increased wing area led to a statistically insignificant correlated increase in wing:thorax size ratio and a decrease in wing‐aspect ratio. Our observations are consistent with the hypotheses that high wing‐thorax size ratio and wing aspect ratio are per se selectively advantageous at low temperatures.

EXPRESSION OF CYTOPLASMIC INCOMPATIBILITY IN DROSOPHILA SIMULANS AND ITS IMPACT ON INFECTION FREQUENCIES AND DISTRIBUTION OF WOLBACHIA PIPIENTIS

Abstract The aim of this study is to examine the expression of cytoplasmic incompatibility and investigate the distribution and population frequencies of Wolbachia pipientis strains in Drosophila simulans. Nucleotide sequence data from 16S rDNA and a Wolbachia surface protein coding sequence and cytoplasmic incompatibility assays identify four distinct Wolbachia strains: w Ha, w Ri, w Ma, and w Au. The levels of cytoplasmic incompatibility between six lines carrying these strains of bacteria and three control lines without bacteria are characterized. Flies infected with w Ha and w Ri are bidirectionally incompatible, and males that carry either strain can only successfully produce normal numbers of offspring with females carrying the same bacterial strain. Males infected with wAu do not express incompatibility. Males infected with the w Ma strain express intermediate incompatibility when mated to females with no bacteria and no incompatibility with females with any other Wolbachia strain. We conduct polymerase chain reaction/restriction fragment length polymorphism assays to distinguish the strain of Wolbachia and the mitochondrial haplotype to survey populations for each type and associations between them. Drosophila simulans is known to have three major mitochondrial haplotypes (si I, si II, and si III) and two subtypes (si IIA and si IIB). All infected lines of the si I haplotype carry w Ha, w No, or both; w Ma and w No are closely related and it is not clear whether they are distinct strains or variants of the same strain. Infected lines with the si IIA haplotype harbor w Ri and the si IIB haplotype carries w Au. The w Ma infection is found in si III haplotype lines. The phenotypic expression of cytoplasmic incompatibility and its relation to between‐population differences in frequencies of Wolbachia infection are discussed.

AGGREGATION AND THE COEXISTENCE OF MYCOPHAGOUS DROSOPHILA

SUMMARY (1) Intraspecific aggregation has been postulated to facilitate the coexistence of competing species that share patchy resources. Data on adult emergence numbers of four mycophagous species of Drosophila from field-collected mushrooms show that these species exhibit highly aggregated distributions, both in collections comprising several naturally occurring species of mushrooms and in those of a single species for which mushroom size, condition, exposure time, and location in the field were controlled. (2) The relation between the mean and the variance in the numbers of flies emerging per mushroom reveals that the mean level of larval crowding increases with overall population density. Aggregation of larvae of each species is due to (i) females laying eggs in clutches of more than one egg, and (ii) non-random distributions of ovipositing females across breeding sites. (3) The emergence number of different species of Drosophila tend to be positively correlated across mushrooms, especially between species belonging to the same species group. In fact, within species groups, intraspecific and interspecific aggregation of ovipositing females contribute about equally to the proportional increase in the number of competitors an individual larva shares a mushroom with. Thus, aggregation does not appear to play an important-role in the coexistence of species belonging to the same species group. (4) The probability of parasitism of emergent Drosophila by the nematode Howardula aoronymphium Welch is generally independent of emergence numbers. Thus, although density-dependent parasitism could promote coexistence of species that have independent aggregated distributions, we find little evidence of such density dependence in these species.

Cellular basis of wing size variation in Drosophila melanogaster: a comparison of latitudinal clines on two continents

We investigated the cellular basis of two extensive, continuous, latitudinal, genetic, body size clines of Drosophila melanogaster by measuring wing area and cell size in the wing blade of adult flies reared under standard, laboratory conditions. We report that the contribution of cell size to an Australian cline is much smaller than that to a South American cline. The data suggest that neither cell size nor cell number were the targets of selection, but rather wing area itself, or a trait closely related to it. We hypothesize that the differences between the continents were caused by differences in the initial pattern of genetic variation for the cell traits and/or by the direction of selection on the source populations of the clines. Despite large differences between continents in the cellular basis of the latitudinal variation, multiple regression analysis, using the individual variation within populations, showed that the relationship between cell size and cell number was changed with latitude in the same way in the two clines. The relative contribution of cell number to wing area variation increased with latitude, probably because of compensatory interactions with cell size as a consequence of the latitudinal increase in cell number. Our findings are discussed in relation to the cellular basis of evolutionary change in laboratory thermal selection lines and natural populations along latitudinal clines.

Effects of assay conditions in life history experiments with Drosophila melanogaster

Selection experiments with Drosophila have revealed constraints on the simultaneous evolution of life history traits. However, the responses to selection reported by different research groups have not been consistent. Two possible reasons for these inconsistencies are (i) that different groups used different environments for their experiments and (ii) that the selection environments were not identical to the assay environments in which the life history traits were measured. We tested for the effect of the assay environment in life history experiments by measuring a set of Drosophila selection lines in laboratories working on life history evolution with Drosophila in Basel, Groningen, Irvine and London. The lines measured came from selection experiments from each of these laboratories. In each assay environment, we measured fecundity, longevity, development time and body size. The results show that fecundity measurements were particularly sensitive to the assay environment. Differences between assay and selection environment in the same laboratory or differences between assay environments between laboratories could have contributed to the differences in the published results. The other traits measured were less sensitive to the assay environment. However, for all traits there were cases where the measurements in one laboratory suggested that selection had an effect on the trait, whereas in other laboratories no such conclusion would have been drawn. Moreover, we provide good evidence for local adaptation in early fecundity for lines from two laboratories.

Geographic variation in competitive ability in Drosophila melanogaster.

The aim of this study was to examine the latitudinal variation in preadult competitive ability of Drosophila melanogaster. Two pairs of populations from Queensland and Tasmania, Australia, were examined. Queensland flies are genetically smaller and develop more slowly than the Tasmanian flies. Survival and body size of flies raised at different temperatures and densities were compared when larvae were challenged with a common competitor. No latitudinal variation in larval survival was detected. Body size (measured as wing length) decreased with increasing temperature and larval density. Flies from the Tasmanian populations were more sensitive to the effects of temperature and density and to the joint effect of increased temperature and density. This could explain the evolution of greater growth efficiency and larger body size at lower temperatures.

On the causes of monophagy in Drosophila quinaria

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