Allozyme genotype-environment relationships in natural populations of Drosophila buzzatii

Allozyme frequency data from natural populations of Drosophila buzzatii were analyzed for genotype-environment relationships. Allele frequency and heterozygosity at six loci polymorphic throughout eastern Australia and a number of environmental factors (both means and variabilities) were examined by a variety of multivariate techniques. Significant genotype-environment associations were found for five of the six loci, and after correcting for geographic location significant associations remained for Est-2 and Adh-1 gene frequencies and heterozygosities and for Pgm gene frequencies. The results are discussed in relation to selection and gene flow and provide the basis for laboratory studies to disentangle confounded effects of (1) environmental means and environmental variabilities and (2) allele frequency and heterozygosity, and thus to further test for and determine the nature of any natural selection at particular allozyme loci.This work was supported by a grant from the Australian Research Grants Committee to J. S. F. B.     

Genetic,ontogenetic, and tissue-specific variation of dipeptidases in Drosophila melanogaster

Three dipeptidases in Drosophila melanogaster are under independent genetic control and their structural genes have been localized, Dip-A to 2R and Dip-B and Dip-C to 3R (Voelker and Langley, 1978; Ohnishi and Voelker, 1981). These enzymes were characterized with respect to their substrate specificities, genetic variability (electrophoretic mobility and quantitative activity level), ontogeny (activity and isozyme pattern), and tissue localization. The dipeptide substrate specificities of DIP-A and DIP-B overlap each other considerably, but do not overlap with DIP-C. In natural populations, DIP-B and DIP-C are essentially monomorphic electrophoretically whereas DIP-A is polymorphic for three allozymes. Both DIP-A and DIP-B show quantitative genetic variation of activity level within an allozyme class. All three enzymes are expressed at all stages in the life cycle, but DIP-A and DIP-B activities vary considerably according to developmental stage and sex of adult. The tissue localizations of DIP-A and DIP-B activities show similar patterns and a nearly ubiquitous occurrence of both enzymes, but with particularly high values in larval and adult midguts and in the adult female reproductive system. These results suggest a general metabolic role for the enzymes, such as regulation of the concentrated pools of amino acids and oligopeptides found in Drosophila tissues.This work was supported by Public Health Service Grant GM 11546.Paper No. 7066 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh.     

Effects of fenoxycarb on ovarian development, spring fecundity and longevity in winterform pear psylla

The insect growth regulator fenoxycarb prompts ovarian development in diapausing winterform pear psylla, Cacopsylla pyricola (Förster) (Homoptera: Psyllidae). We applied fenoxycarb to caged psylla in September, November, and December to test whether premature ovarian development reduced overwintering survival, spring fecundity, or spring longevity. Fenoxycarb prompted ovarian development in all treated psylla, with the largest effects occurring in the September-treated insects. Recovery of live psylla in spring was 46–95% in treated insects and 72–92% in controls; overwintered insects from the fenoxycarb treatment survived field temperatures below –20°C despite having had mature ovaries. Fecundity and longevity of psylla were the same in treated and untreated insects, indicating that overwintering with mature ovaries did not cause reduced spring egglaying capacity. Several treated insects each deposited over 1900 eggs and survived more than 120 days. Due to their more advanced development, treated insects had higher oviposition rates than controls during the first 5 days after removal from the field. The largest impact on spring fitness was due to the effects of fenoxycarb on egg hatch. Eggs deposited on clean foliage by September-treated females were less likely to hatch than eggs deposited by controls, suggesting that fenoxycarb affected developing eggs within the female. Prospects for using fenoxycarb in fall to control pear psylla appear to be limited.     

The impact of genetic parental distance on developmental stability and fitness in Drosophila buzzatii

Measures of genetic parental distances (GPD) based on microsatellite loci (D (2) and IR), have been suggested to be better correlated with fitness than individual heterozygosity (H), as they contain information about past events of inbreeding or admixture. We investigated if GPD increased with increasing genetic divergence between parental populations in Drosophila buzzatii and if the measures indicate past events of admixture. Further we evaluated the relationship between GPD, fitness and fluctuating asymmetry (FA) of size and shape. We investigated three populations of Drosophila buzzati, from Argentina, Europe and Australia. From these populations two intraspecific hybridisation lines were made; one between the Argentinean and European populations, which have been separated 200 years and one between the populations from Argentina and Australia, which have been separated 80 years. By doing this we obtained hybrid progeny having different levels of GPD. We found that D (2) and H can be used as indicators of admixture when comparing hybrid individuals with their parentals. IR was not informative. Our results does not exclude the presence of genetic fitness correlations (GFC) over individuals with a broad fitness range from populations in equilibrium, but we doubt the presence of GFC using GPD measures in admixed populations. Shape FA could be a relevant measure for fitness, however, only when comparing populations, not at individual level.     

Longevity differences among lines artificially selected for developmental time and wing length in Drosophila buzzatii

We assessed the indirect response of longevity in lines selected for wing length (WL) and developmental time (DT). Longevity in selection lines was compared to laboratory control lines and the offspring of recently collected females. Wild flies (W lines), flies from lines selected for fast development (F lines), and for fast development and large wing length (L lines) outlived control laboratory lines (C lines) and lines selected for fast development and short wing (S lines). The decline in longevity in S lines is in line with the idea that body size and longevity are correlated and may be the result of the fixation of alleles at loci affecting pleiotropically the two traits under selection and longevity. In addition, inbreeding and artificial selection affected the correlation between wing length and longevity that occurs in natural populations of Drosophila buzzatii, suggesting that correlations between traits are not a perdurable feature in a population.     

A three-locus system of interspecific incompatibility underlies male inviability in hybrids between Drosophila buzzatii and D. koepferae

In hybrids between the sibling species D. buzzatii and D. koepferae, both sexes are more or less equally viable in the F₁: However, backcross males to D. buzzatii are frequently inviable, apparently because of interspecific genetic incompatibilities that are cryptic in the F₁. We have performed a genetic dissection of the effects of the X chromosome from D. koepferae. We found only two cytological regions, termed hmi-1 and hmi-2, altogether representing 9% of the whole chromosome, which when introgressed into D. buzzatii cause inviability of hybrid males. Observation of the pattern of asynapsis of polytene chromosomes (incomplete pairing, marking introgressed material) in females and segregation analyses were the technique used to infer the X chromosome regions responsible for this hybrid male inviability. The comparison of these results with those previously obtained with the same technique for hybrid male sterility in this same species pair indicate that in the X chromosome of D. koepferae there are at least seven times more regions that produce hybrid male sterility than hybrid male inviability. We have also found that the inviability brought about by the introgression of hmi-1 is suppressed by the cointrogression of two autosomal sections from D. koepferae. Apparently, these three regions conform to a system of species-specific complementary factors involved in an X-autosome interaction that, when disrupted in backcross hybrids by recombination with the genome of its sibling D. buzzatii, brings about hybrid male inviability.     

Estimating the number of genetic elements that defer senescence inDrosophila

Summary Although many different physiological and biochemical changes characterize the process of senescence, little is understood of the genetic elements that determine its age of onset. We provide here the first estimates of the number of genetic factors that extend longevity inDrosophila melanogaster. Life span was measured in F₁, F₂ and backcrosses of true-breeding long and short-lived stocks ofD. melanogaster, established by selection. Estimates of the number of effective factors delaying senescence range from about 0.3 to 1.5, indicating control by a single factor. The distribution of longevity shows this to arise as selection acts on the short-lived parental stock. Life span is extended at the cost of early fecundity.     

An adaptive chromosomal polymorphism affecting size-related traits,and longevity selection in a natural population ofDrosophila buzzatii

Size-related phenotypic variation among second-chromosome karyotypes inDrosophila buzzatii was examined in an Argentinian natural population. For all measured traits (thorax and wing length; wing, head and face width), this inversion polymorphism exhibited a significant and (additive) linear contribution to the phenotypic variance in newly emerged wild flies. The results suggest that only overall body size, and not body shape, is affected. as no karyotypic variation was found for any trait when the effects of differences in within-karyotype size were removed with Burnaby's method. Likewise, in an experiment of longevity selection in the wild, variation in chromosomal frequencies was verified in the direction predicted on the basis of: (i) previous studies on longevity selection for body size in the wild and (ii) the pattern of chromosomal effects we observed on size. The direction of such selection is consistent with a pattern of antagonistic selection detected in previous studies on the inversion polymorphism.     

Altitudinal and seasonal variation in microsatellite allele frequencies of Drosophila buzzatii

Variation in climate, particularly temperature, is known to affect the genetic composition of populations. Although there have been many studies of latitudinal variation, comparisons of populations across altitudes or seasons, particularly for animal species, are less common. Here, we study genetic variation (microsatellite markers) in populations of Drosophila buzzatii collected along altitudinal gradients and in different seasons. We found no differences in genetic variation between 2 years or between seasons within years. However, there were numerous cases of significant associations between allele frequencies or expected heterozygosities and altitude, with more than half showing nonlinear relationships. While these associations indicate possible selection and local altitudinal adaptation, direct tests gave strong evidence for selection affecting two loci and weaker evidence for five other loci. Two loci that are located within an inversion (including the one with strongest evidence for selection) show a linear increase in genetic diversity with altitude, likely due to thermal selection. Parallel associations with altitude here and with latitude in Australian populations indicate that selection is operating on chromosomal regions marked by some of the loci.     

Probing the evolution of senescence inDrosophila melanogaster withP-element tagging

Natural populations host a wealth of genetic variation in longevity and age-specific schedules of reproduction. This variation provides critical information for inferring the evolutionary origin of senescence. Patterns of mutational effects on age-specific fecundity and survival provide additional insight to distinguish alternative models of senescence. In this study,P-elements bearing thewhite minigene were inserted at random into a common genetic background, generating lines ofD. melanogaster with single, stable transposon inserts. A series of 48 single-P-element lines revealed statistically significant heterogeneity in both longevity and fecundity. Longevity and early fecundity were only weakly positively correlated (r=0.286,P=0.0398). Both the pooled sample and 30 of the individual lines exhibited a leveling of age-specific mortality at advanced ages, in opposition to the classical demographic models. To the extent that these mutational effects are representative of naturally-occurring mutations in heterogeneous populations, this result presents a problem for the evolutionary theory of senescence. Natural selection is inefficient at removing deleterious mutations that are expressed only at late ages, and selection may not differentiate between mutations whose effects on longevity are post-reproductive. A leveling of the mortality rate would also be seen if mutations whose expression is delayed until very late simply do not occur. A simulation of mutation-selection balance among the 48P-element tagged lines shows that the mean longevity declines monotonically with increasing mutation rate, consistent with the mutation-accumulation model.     

Evolution of delayed reproductive senescence in male fruit flies: Sperm competition

Populations ofDrosophila melanogaster that had been subjected to long-term selection favoring either delayed or rapid senescence were compared with respect to age-specific components of male reproductive success involving sperm competition. These components of reproductive success were divided into those related to sperm defense (protection of sperm from other males), and into those related to sperm offense (ability to mate with previously mated females and to displace the sperm of other males). Males were tested at four ages ranging from 1–2d to 5–6 wk after eclosion. Several aspects of sperm defense capability showed clear evidence of senescent decline. Furthermore, males from populations selected for delayed senescence were superior to males from control (rapid senescence) populations with regard to components of sperm defense. The superiority of males from populations with delayed senescence either increased as a function of male age, or was present at all ages tested. These results indicate that the rate of reproductive senescence in maleD. melanogaster can be altered in predictable directions by artificial selection. There were no differences between selection regimes with regard to sperm offense, and most components of sperm offense did not show clear evidence of senescence. The improved late-age reproductive success of males from populations selected for delayed senescence did not appear to entail any cost or trade-off at early ages with respect to the reproductive traits examined in these experiments.     

Size and fecundity hierarchies in an herbaceous perennial

Summary Inequalities in size in populations have potentially important effects on fitness but have rarely been examined in natural populations. I measured size (number of culms) and fecundity (number of spikelets) in five populations of the grass Danthonia spicata from 1981 to 1985. The populations were in sites in a Pinus-Quercus-Populus forest in northern lower Michigan, USA comprising a secondary succession sequence. The sites had been burned in 1980, 1954, 1948, 1936, and 1911, respectively. Mean sizes and fecundities and the amount of hierarchy in size and fecundity, measured by the Gini coefficient, were compared between the YOUNG population, 1980 burn site, and the OLD populations, 1954, 1948, 1936, and 1911 burn sites. I found large differences in mean size and fecundity between the YOUNG and OLD populations with much larger individuals in the YOUNG population. No differences in size hierarchies were found in either the first year of measurement or after five years. The fecundity hierarchies showed no significant difference among the populations in the first year but after five years the YOUNG population showed a significant decrease in amount of inequality. The longterm patterns of size and fecundity hierarchies differed because fecundity was a cumulative trait while size was not. Size inequalities may not always be a good measure of fecundity inequalities. Short-term measures of inequality in perennials may not be a good indicator of long-term values. In contrast to greenhouse studies, habitat light levels did not affect size hierarchies although they did affect fecundity hierarchies.     

Sequential gel electrophoretic analysis of esterase-2 in two populations of Drosophila buzzatii

Sequential electrophoresis, using three different buffer systems on cellulose acetate gels, was used to characterize the allelic variation for esterase-2 in two populations of D. buzzatii in Australia that are separated by 550 km. Twenty-five alleles were detected, of which nine were unique to one population, eight unique to the other, and only eight were common to both populations. Allele frequencies within each population were significantly different between the two major chromosome sequences (standard and j inversion), and for each chromosome sequence allele frequencies were significantly different between populations. Observed allelic frequency distributions were not significantly different from those predicted for selective neutrality using the homozygosity test statistic. However, estimates of the effective sizes of the populations derived from their observed differentiation, together with the history of the species in Australia, provide support for some form of balancing selection affecting at least some of the alleles.     

A study of wing morphology and fluctuating asymmetry in interspecific hybrids between Drosophila buzzatii and D. koepferae

In this work we investigate the effect of interspecific hybridization on wing morphology using geometric morphometrics in the cactophilic sibling species D. buzzatii and D. koepferae. Wing morphology in F₁ hybrids exhibited an important degree of phenotypic plasticity and differs significantly from both parental species. However, the pattern of morphological variation between hybrids and the parental strains varied between wing size and wing shape, across rearing media, sexes, and crosses, suggesting a complex genetic architecture underlying divergence in wing morphology. Even though there was significant fluctuating asymmetry for both, wing size and shape in F₁ hybrids and both parental species, there was no evidence of an increased degree of fluctuating asymmetry in hybrids as compared to parental species. These results are interpreted in terms of developmental stability as a function of a balance between levels of heterozygosity and the disruption of coadaptation as an indirect consequence of genomic divergence.     

Genetics and the environment in interspecific competition: a study using the sibling species Drosophila melanogaster and Drosophila simulans

The outcome of interspecific competition of two closely related species may depend upon genetic variation in the two species and the environment in which the experiment is carried out. Interspecific competition in the two sibling species, Drosophila melanogaster and D. simulans, is usually investigated using longterm laboratory stocks that often have mutant markers that distinguish them. To examine competition in flies that genetically more closely resemble flies in nature, we utilized freshly caught wildtype isofemale lines of the two species collected at the same site in San Carlos, Mexico. Under ordinary laboratory conditions, D. melanogaster always won in competition. However, in hotter and drier conditions, D. simulans competed much more effectively. In these environmental conditions, there were genetic differences in competitive ability among lines with the outcome of competition primarily dependent upon the line of D. melanogaster used but in some cases also influenced by the line of D. simulans used. Differences in the measures of productivity and developmental time did not explain the differences in competitive ability among lines. This suggests that the outcome of competition was not due to differences in major fitness components among the isofemale lines but to some other attribute(s) that influenced competitive ability. When lines of flies were combined, the outcome of competition was generally consistent with competitive outcomes between pairs of lines. In several cases, the combination of lines performed better than the best of the constituent lines, suggesting that competitive ability was combined heterotically and that the total amount of genetic variation was important in the outcome of interspecific competition.     

Protein synthesis patterns following stage-specific heat shock in early Drosophila embryos

Summary Very short heat shocks are administered to carefully staged early embryos of Drosophila melanogaster, and the effects on protein synthesis pattern investigated. A shock as short as 2 min will induce the heat shock response (reduction of normal protein synthesis, increased synthesis of the heat shock proteins) in syncytial blastoderm or later stages. Thus the initial events of the heat shock response must occur within 2 min, and not reverse upon rapid return to 22° C. A low level of synthesis of the 70 kDa heat shock protein is sometimes visible in unshocked animals, but may be induced by the labeling procedure. Survival following a short shock is not strictly correlated with a high level of heat shock response. Pre-blastoderm embryos do not produce significant heat shock protein, but survive a 2 min 43°C heat shock better than do heat shock response competent blastoderm embryos. The protein synthesis pattern prior to the blastoderm stage is very stable, possibly enhancing survival following a short shock. Shocks of 3 min or longer are more detrimental to pre-blastoderm embryos than to later stages, confirming the role of the heat shock response in survival following a longer shock. Stage-specific developmental defects (phenocopies) may be induced by heat shock at the blastoderm or later stages. Induction of these defects may require disruption of the normal protein synthesis pattern. Use of very short heat shocks to induce the heat shock response will be valuable in identifying the precise time at which a specific defect can be induced.     

Altitudinal variation for stress resistance traits and thermal adaptation in adult Drosophila buzzatii from the New World

Multiple stress resistance traits were investigated in the cactophilic fly Drosophila buzzatii. Adults from seven populations derived from North-Western Argentina were compared with respect to traits relevant for thermal stress resistance and for resistance to other forms of environmental stress. The populations were collected along an altitudinal gradient spanning more than 2000 m in height, showing large climatic differences. The results suggest that knock-down resistance to heat stress, desiccation resistance and Hsp70 expression at a relatively severe stressful temperature best reflect thermal adaptation in this species. Furthermore, cold resistance seemed to be of less importance than heat resistance, at least for the adult life stage, in these populations. Clinal variation in thermal resistance traits over short geographical distances suggests relatively strong adaptive differentiation of the populations. This study provides the first evidence for altitudinal differentiation in stress-related traits, and suggests that Hsp70 expression level can be related to altitudinal clines of heat-stress resistance.     

Influence of mite infestation on the longevity and fecundity of the mosquitoMansonia uniformis (Diptera: Insecta) under laboratory conditions

A study relating to the influence of mite infestation on the longevity and fecundity of the mosquito,Mansonia uniformis was carried out in the laboratory. It was found that the mite parasitism does influence the quantity of blood ingested, survivorship and the time lag between feeding and egg-laying. However fecundity and hatching percentages were not affected.     

Size-related sexual selection and yeast diet inDrosophila buzzatii (Diptera: Drosophilidae)

Size-related sexual selection (SRSS) was examined on four traits (thorax and wing length and head and face width) inDrosophila buzzatii, by scoring male copulatory status in two mass-mating experiments. Using axenic females, experiment 1 was carried out with axenic males, and experiment 2 with yeast-supplemented males. While there was no indication of SRSS in experiment 1, such selection was substantial in yeast-supplemented males, which transmitted yeasts to mating females. Multivariate analyses of selection indicated that face width is the measured trait on which directional SRSS essentially acted in yeast-supplemented males, resulting in indirect selection on body size. Because this selection was affected by yeast diet in males, its possible interaction with the yeast transmission from males to females during the courtship is discussed.     

Flight initiations in Drosophila melanogaster are mediated by several distinct motor patterns

We have monitored the patterns of activation of five muscles during flight initiation of Drosophila melanogaster: the tergotrochanteral muscle (a mesothoracic leg extensor), dorsal longitudinal muscles #3, #4 and #6 (wing depressors), and dorsal ventral muscle #Ic (a wing elevator). Stimulation of a pair of large descending interneurons, the giant fibers, activates these muscles in a stereotypic pattern and is thought to evoke escape flight initiation. To investigate the role of the giant fibers in coordinating flight initiation, we have compared the patterns of muscle activation evoked by giant fiber stimulation with those during flight initiations executed voluntarily and evoked by visual and olfactory stimuli. Visually elicited flight initiations exhibit patterns of muscle activation indistinguishable from those evoked by giant fiber stimulation. Olfactory-induced flight initiations exhibit patterns of muscle activation similar to those during voluntary flight initiations. Yet only some benzaldehyde-induced and voluntary flight initiations exhibit patterns of muscle activation similar to those evoked by giant fiber stimulation. These results indicate that visually elicited flight initiations are coordinated by the giant fiber circuit. By contrast, the giant fiber circuit alone cannot account for the patterns of muscle activation observed during the majority of olfactory-induced and voluntary flight initiations.Abbreviations DLM/DLMn dorsal longitudinal muscle/motor neuron - DVM/DVMn dorsal ventral muscle/motor neuron - GF(s) giant fiber interneuron (s) - PSI peripherally synapsing interneuron - TTM/TTMn tergotrochanteral muscle/motor neuron