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.     

Developmental time,body size and wing loading in Drosophila buzzatii from lowland and highland populations in Argentina

Genotype-by-temperature interaction is a necessary condition for adaptive evolution of fitness traits as a response to temperature. Several fitness-related traits (developmental time, pre-adult survival, thorax and wing lengths, and wing loading) were measured in laboratory-reared D. buzzatii from four populations sampled at different altitudes in north-western Argentina: a lowland population (407 m a.s.l.), two populations from intermediate altitude (780 to 950 m a.s.l.), and a highland population (2380 m a.s.l.). Temperature is the main climatic difference between the collection sites: lowland but not highland populations are exposed to physiologically high temperatures during both spring and summer in nature. Three growth temperatures (20, 25 and 30 degrees C) were used to test for population-by-temperature interactions. Both developmental time and pre-adult survival exhibit highly significant population-by-temperature interaction. Pre-adult survival at 30 degrees C is significantly higher in lowland than in highland populations, but not so at lower growth temperatures (20 and 25 degrees C). Both wing length and wing loading show no population-by-temperature interaction, indicating that these traits are not the direct targets of thermal adaptation in nature. Wing loading is higher in highland than in lowland populations, suggesting that flight performance is subject to stronger selection in the highland population. This hypothesis is consistent with ecological observations in both types of populations. There is no obvious among-population relationship between developmental time and body size, even though both traits are related within populations in a well-known trade-off. Overall, thermal adaptation is evident for developmental time and pre-adult survival but not for size-related traits.     

Butterfly selected lines explore the hormonal basis of interactions between life histories and morphology

Hormonal mechanisms underlie many life-history traits and their interactions. We studied the role of ecdysteroids with regard to wing pattern and development time of the polyphenic butterfly Bicyclus anynana. Ecdysteroid titers and sensitivity to ecdysone injection were assayed for two-trait selected lines (ventral eyespot size and development time concurrently). These two traits are genetically and phenotypically coupled, having a common endocrinal basis. Two-trait selection had been applied both antagonistically (opposite the correlation) and synergistically (in the same direction as the correlation). Although selected lines had diverged most in eyespot size, the widest differences in timing of ecdysteroid titers were observed between the development time selection regimes; fast selected lines had an earlier hormonal increase after pupation than slow selected lines (even when corrected for differential pupal times). This endocrine peak was also earlier for females than for males. Furthermore, sensitivity to ecdysone injection as measured by a subsequent decrease in pupal time was significantly lower for slow selected lines than for fast or unselected lines. We conclude that the observed response in eyespot size to artificial selection must have been achieved via alteration of, or selection on, other developmental mechanisms, because the dynamics of the alternative, hormonal, pathway were dictated by development time selection. The developmental system is flexible enough to allow evolution in directions opposing the correlation between wing pattern and developmental time, and responses to selection are not constrained by a shared hormonal system.     

Simultaneous selection on two fitness-related traits in the butterfly Bicyclus anynana

Abstract. Theory about the role of constraints in evolution is abundant, but few empirical data exist to describe the consequences a bias in phenotypic variation has for micro evolution. Responses to natural selection can be severely hampered by a genetic correlation among a suite of traits. Constraints can be studied using antagonistic selection experiments, that is, two-trait selection in opposition to this correlation. The two traits studied here were development time and wing pattern (eyespot size) in the butterfly Bicyclus anynana , both of which have a clear adaptive significance. Rates of response were higher for eyespot size than for development time, but were independent of the concurrent selection (either in the same direction as the correlation or perpendicular to it). Regimes differed in both traits in all directions after 11 generations of selection. The uncoupling lines had higher relative responses than the synergistic lines in development time and equal relative responses in eyespot size. The patterns for eyespot size (reaction norms) were consistent across different rearing temperatures. Differences in lines selected for fast and slow development time were more pronounced at lower temperatures, irrespective of the direction of joint wing pattern selection. Furthermore, correlated responses in pupal weight and growth rate were observed; lines selected for a slower development had higher pupal weights, especially at lower temperatures. The response of the uncoupling lines was not hampered by a lack of selectable genetic variation, and the relative response in the development time was larger than expected based on response in the coupled direction and quantitative genetic predictions. This suggests that the structure of the genetic architecture does not constrain the short-term, independent evolution of both wing pattern and development time.     

Thermal evolution of pre-adult life history traits, geometric size and shape, and developmental stability in Drosophila subobscura

Replicated lines of Drosophila subobscura originating from a large outbred stock collected at the estimated Chilean epicentre (Puerto Montt) of the original New World invasion were allowed to evolve under controlled conditions of larval crowding for 3.5 years at three temperature levels (13, 18 and 22 degrees C). Several pre-adult life history traits (development time, survival and competitive ability), adult life history related traits (wing size, wing shape and wing-aspect ratio), and wing size and shape asymmetries were measured at the three temperatures. Cold-adapted (13 degrees C) populations evolved longer development times and showed lower survival at the highest developmental temperature. No divergence for wing size was detected following adaptation to temperature extremes (13 and 22 degrees C), in agreement with earlier observations, but wing shape changes were obvious as a result of both thermal adaptation and development at different temperatures. However, the evolutionary trends observed for the wing-aspect ratio were inconsistent with an adaptive hypothesis. There was some indication that wing shape asymmetry has evolutionarily increased in warm-adapted populations, which suggests that there is additive genetic variation for fluctuating asymmetry and that it can evolve under rapid environmental changes caused by thermal stress. Overall, our results cast strong doubts on the hypothesis that body size itself is the target of selection, and suggest that pre-adult life history traits are more closely related to thermal adaptation.     

Laboratory evolution of longevity in the bean weevil (Acanthoscelides obtectus)

In this study we examined the effects of long-term selection on early and late reproduction in the bean weevil. The pure lines and the hybrids between the lines within a selection regime were compared for longevity, early and late female fecundity, male mating ability, pre-adult developmental time and wet adult weight. Comparison of hybrid with pure lines provided some evidence for inbreeding despression in the lines from both selection regimes. We found that virgin and mated adults of both sexes from the “old” lines lived longer than “young” line beetles. Comparisons of the hybrid “young” with hybrid “old” lines revealed a trade-off between early and late fecundity of females. For noncompetitive mating ability of males there was no difference between the lines with different rates of senescence when the males were young. But, when the males were older, beetles from the lines selected for delayed senescence expressed superior mating ability. In addition, the “old” line beetles take longer to develop and are heavier than those from “young” line beetles. Although these data suggest that shorter pre-adult developmental time may imply more rapid senescence, there is the possibility of inadvertent selection for rapid development in the “young” lines and this complicates the interpretation of the observed trade-off between the pre-adult and adult performances.     

Selection for developmental time in bean weevil (Acanthoscelides obtectus): correlated responses for other life history traits and genetic architecture of line differentiation

In this article we investigate the direct and correlated responses to selection for developmental time in order to discern differences between lines in several preadult and adult life history traits of Acanthoscelides obtectus (Coleoptera, Bruchidae). Selection for fast development was about five times as effective as selection for slow development, as judged by realized heritabilities. The correlated responses on the following life‐history traits were studied: egg size, hatching success, embryonic developmental time, egg‐to‐adult viability, body weight, first day of egg laying, total fecundity, and longevity. Analyses of the terminal generation of selection showed that all life history traits examined, except for hatching success, were affected by selection. The findings suggest that body weight, total fecundity, and longevity traded off to preadult developmental time. Unlike the adult traits, none of the preadult traits showed negative correlations with developmental time. We also present data concerning the underlying genetic basis that produces changes in preadult developmental time, body weight, and egg‐to‐adult viability in the lines selected for fast and slow preadult developmental time. Additive‐dominance genetic architecture for both preadult developmental time and body weight was found. In addition, it appears that the responses to selection for preadult developmental time involved between 10 and 28 loci, which were correlated with at least one to four genes for body weight. Epistasis makes a significant contribution to genetic divergence between fast and slow selected lines only with respect to preadult viability. The observed levels of dominance and epistasis underscore the important role of nonadditive genetic effects to the adaptive diversifications of bean weevil populations.     

How to become large quickly: quantitative genetics of growth and foraging in a flush feeding lepidopteran larva

Rapid larval growth in insects may be selected for by rapid ephemeral phenological changes in food resources modifying the structure of phenotypic and genetic (co)variation in and among individual traits. We studied the relative effects of three processes which can modify expression of additive genetic and nongenetic variation in traits. First, natural selection tends to erode genetic variation in fitness-related traits. Second, there may be high variance even in traits closely coupled with fitness, if these traits are themselves products of variable lower level traits. Third, traits may be canalized by developmental processes which reduce phenotypic variation. Moreover, we investigated the phenotypic and genetic role played by the underlying traits in attaining simultaneously both large size and short development time. We measured phenotypic and genetic (co)variation in several pre- and post-ingestive foraging traits, growth, development rate, development time and size, together forming a hierarchical network of traits, in the larvae of a flush feeding geometrid, Epirrita autumnata. Rapid larval growth rate and high pupal mass are closely related to fitness in E. autumnata. Traits closely associated with larval growth displayed low levels of additive genetic variation, indicating that genetic variability may have been exhausted by selection for rapid growth. The body size of E. autumnata, in spite of its close correlation with fitness, exhibited a significant additive genetic variation, possiblye because caterpillar size is the outcome of many underlying heritable traits. The low level traits in the hierarchical net, number (indicating larval movements) and size of feeding bouts in leaves, relative consumption rate and efficiency of conversion of ingested food, displayed high levels of residual variation. High residual variation in consumption and physiological ability to handle leaf material resulted from their flexibility which reduced variation in growth rate, i.e. growth rate was canalized. We did not detect a trade-off between development time and final size. On the contrary, large pupal masses were attained by short larval periods, and this relationship was strongly genetically determined, suggesting that both developmental time and final size are expressions of the same developmental process (vigorous growth) and the same genes (or linkage disequilibrium).     

Correlation between characters as related to developmental pattern in Drosophila

In order to better understand the genetic basis of some body traits and their correlations in Drosophila, in relation to their developmental history, a biometrical study was performed on three lines selected for short wing (fourth vein) and a control strain.The correlated response to selection for short wing and four body traits (thorax length and width, scutellar length, head width) and of eight other dimensional wing traits was considered.The results show a strong correlated response to selection of all wing traits, low correlations for the thorax characters, while head width remains relatively constant. Two groups of wing characters, corresponding to compartments of development, show different levels of covariation with the selected trait, the covariation being greater when the characters included in the same compartment of the selected trait are considered.The results are discussed in terms of developmental genetics of Drosophila and suggest that quantitative studies may be suitable for studying the rôle of interactions between sets of genes controlling development.     

JOINT ALLELIC EFFECTS ON FITNESS AND METRIC TRAITS

Theoretical explanations of empirically observed standing genetic variation, mutation, and selection suggest that many alleles must jointly affect fitness and metric traits. However, there are few direct demonstrations of the nature and extent of these pleiotropic associations. We implemented a mutation accumulation (MA) divergence experimental design in Drosophila serrata to segregate genetic variants for fitness and metric traits. By exploiting naturally occurring MA line extinctions as a measure of line‐level total fitness, manipulating sexual selection, and measuring productivity we were able to demonstrate genetic covariance between fitness and standard metric traits, wing size, and shape. Larger size was associated with lower total fitness and male sexual fitness, but higher productivity. Multivariate wing shape traits, capturing major axes of wing shape variation among MA lines, evolved only in the absence of sexual selection, and to the greatest extent in lines that went extinct, indicating that mutations contributing wing shape variation also typically had deleterious effects on both total fitness and male sexual fitness. This pleiotropic covariance of metric traits with fitness will drive their evolution, and generate the appearance of selection on the metric traits even in the absence of a direct contribution to fitness.     

THE RESPONSE TO SELECTION FOR FAST LARVAL DEVELOPMENT IN DROSOPHILA MELANOGASTER AND ITS EFFECT ON ADULT WEIGHT: AN EXAMPLE OF A FITNESS TRADE-OFF

A selection experiment using Drosophila melanogaster revealed a strong trade-off between adult weight and larval development time (LDT), supporting the view that antagonistic pleiotropy for these two fitness traits determines mean adult size. Two experimental lines of flies were selected for a shorter LDT (measured from egg laying to pupation). After 15 generations LDT was reduced by an average of 7.9%. The response appeared to be controlled primarily by autosomal loci. A correlated response to the selection was a reduction in adult dry weight: individuals from the selected populations were on average 15.1% lighter than the controls. The lighter females of the selected lines showed a 35% drop in fecundity, but no change in longevity. Thus, there is no direct relationship between LDT and adult longevity. The genetic correlation between weight and LDT, as measured from their joint response to selection, was 0.86. Although there was weak evidence for dominance in LDT, there was none for weight, making it unlikely that selection acting on this antagonistic pleiotropy could lead to a stable polymorphism. In all lines, sex differences in weight violated expectations based on intrasex genetic correlations: Females, being larger than males, ought to require a longer LDT, whereas there was a slight trend in the opposite direction. Because the sexual dimorphism in size was not significantly altered by selection, it appears that the controlling loci are either invariant or have very limited pleiotropic effect on developmental time. It is suggested that they probably control some intrinsic, energy-intensive developmental process in males.     

EVOLUTIONARY EFFECTS OF SELECTION ON AGE AT REPRODUCTION IN LARVAL AND ADULT: DROSOPHILA MELANOGASTER

Two sets of three replicate lines of Drosophila melanogaster were artificially selected by reproduction at either a ‘young’ or an ‘old’ age. The pure lines, the hybrids between the lines within a selection regimen and the base stock from which the lines were derived were compared for longevity, early and late fertility, development time, larval viability and adult thorax length. Comparison of hybrid with pure lines showed some evidence for inbreeding depression in the lines from both selection regimes. Comparison of hybrid lines with the base stock did not provide evidence for any trade-off in either males or females between early fertility on the one hand and late life fertility and longevity on the other. Nor was there any clear evidence of a trade-off between pre-adult and adult fitness components. There was evidence of inadvertent selection for rapid development in both selection regimens, especially in the females of the ‘young’ lines, and this complicated the interpretation of the responses and correlated responses to selection. An improvement in adult performance in the ‘old’ line males relative to the base stock appeared to be attributable to reversal of mutation accumulation. Comparison of the hybrid ‘young’ and ‘old’ lines with the base stock did not support the idea that the superior longevity and late life fertility of the ‘old’ lines relative to the ‘young’ lines could be accounted for by the effects of mutation accumulation in the ‘young’ lines. The results point to the need to compare selected lines with their base stock when deducing responses and correlated responses to selection and to avoid unintentional selection. In this type of experiment, larval density should be standardized during selection, and adults should not be under pressure for rapid maturation.     

Response of fluctuating and directional asymmetry to selection on wing shape in Drosophila melanogaster

We tested whether directional selection on an index-based wing character in Drosophila melanogaster affected developmental stability and patterns of directional asymmetry. We selected for both an increase (up selection) and a decrease (down selection) of the index value on the left wing and compared patterns of fluctuating and directional asymmetry in the selection index and other wing traits across selection lines. Changes in fluctuating asymmetry across selection lines were predominantly small, but we observed a tendency for fluctuating asymmetry to decrease in the up-selected lines in both replicates. Because changes in fluctuating asymmetry depended on the direction of selection, and were not related to changes in trait size, these results fail to support existing hypotheses linking directional selection and developmental stability. Selection also produced a pattern of directional asymmetry that was similar in all selected lines whatever the direction of selection. This result may be interpreted as a release of genetic variance in directional asymmetry under selection.     

ANTAGONISTIC PLEIOTROPIC EFFECT OF SECOND-CHROMOSOME INVERSIONS ON BODY SIZE AND EARLY LIFE-HISTORY TRAITS IN DROSOPHILA BUZZATII

A simple way to think of evolutionary trade-offs is to suppose genetic effects of opposed direction that give rise to antagonistic pleiotropy. Maintenance of additive genetic variability for fitness related characters, in association with negative correlations between these characters, may result. In the cactophilic species Drosophila buzzatii, there is evidence that second-chromosome polymorphic inversions affect size-related traits. Because a trade-off between body size and larval developmental time has been reported in Drosophila, we study here whether or not these inversions also affect larva-adult viability and developmental time. In particular, we expect that polymorphic inversions make a statistically significant contribution to the genetic correlation between body size (as measured by thorax length) and larval developmental time. This contribution is expected to be in the direction predicted by the trade-off, namely, those flies whose karyotypes cause them to be genetically larger should also have a longer developmental time than flies with other karyotypes. Using two different experimental approaches, a statistically significant contribution of the second-chromosome inversions to the phenotypic variances of body size and developmental time in D. buzzatii was found. Further, these inversions make a positive contribution to the total genetic correlation between the traits, as expected by the suggested trade-off. The data do not provide evidence as to whether the genetic correlation is due to antagonistic pleiotropic gene action or to gametic disequilibrium of linked genes that affect one or both traits. The results do suggest, however, a possible explanation for the maintenance of inversion polymorphism in this species.     

The genetics of phenotypic plasticity. III. Genetic correlations and fluctuating asymmetries

We examined the relationship of three aspects of development, phenotypic plasticity, genetic correlations among traits, and developmental noise, for thorax length, wing length, and number of sternopleural bristles in Drosophila melanogaster. We used 14 lines which had previously been selected on either thorax length or plasticity of thorax length in response to temperature. A half-sib mating design was used and offspring were raised at 19° C or 25° C. We found that genetic correlations were stable across temperatures despite the large levels of plasticity of these traits. Plasticities were correlated among developmentally related traits, thorax and wing length, but not among unrelated traits, lengths and bristle counts. Amount of developmental noise, measured as fluctuating asymmetry and within-environmental variation, was positively correlated with amount of plasticity only for some traits, thorax length and bristle number, and only at one temperature, 25° C.     

Repair of X-Irradiation Damage in TRIBOLIUM CASTANEUM Strains Selected for Differences in Developmental Rates

Lines previously selected for different speed of development were found to differ in their spontaneous and induced wing abnormalities after single exposure to X-ray (10 KR) in Tribolium castaneum at 33 degrees and 70% relative humidity. At the time of irradiation the mean developmental time (from egg to first day pupa) was 16.9, 18.8, 22.8 and 18.4 days for fast, intermediate, slow and unselected lines, respectively. There was no difference between males and females in their responses. The spontaneous rate (angular values) of wing abnormalities was 5.3%, 11.2%, 3.5% and 7.2%, and the corresponding induced values were 16.7%, 20.5%, 11.7% and 25.2% for fast, intermediate, slow and unselected lines, respectively. The difference between the spontaneous (due to selection) and the induced (due to selection and irradiation) rates indicates that slowing down development will probably increase the efficiency of the repair of the irradiation damage. This difference (angular values) was 11.4%, 9.4% and 8.1% for fast, intermediate and slow lines, respectively. Increasing developmental time by one day resulted in repairing 0.56% of the radiation damage. These results indicate that both the genetic and the developmental state of the line influence both induction and repair of the X-irradiation damage. The relationship between development, cell cycle, protein synthesis and repair is discussed.     

Correlational selection leads to genetic integration of body size and an attractive plumage trait in dark-eyed juncos

When a trait's effect on fitness depends on its interaction with other traits, the resultant selection is correlational and may lead to the integration of functionally related traits. In relation to sexual selection, when an ornamental trait interacts with phenotypic quality to determine mating success, correlational sexual selection should generate genetic correlations between the ornament and quality, leading to the evolution of honest signals. Despite its potential importance in the evolution of signal honesty, correlational sexual selection has rarely been measured in natural populations. In the dark-eyed junco (Junco hyemalis), males with experimentally elevated values of a plumage trait (whiteness in the tail or "tail white") are more attractive to females and dominant in aggressive encounters over resources. We used restricted maximum-likelihood analysis of a long-term dataset to measure the heritability of tail white and two components of body size (wing length and tail length), as well as genetic correlations between pairs of these traits. We then used multiple regression to assess directional, quadratic, and correlational selection as they acted on tail white and body size via four components of lifetime fitness (juvenile and adult survival, mating success, and fecundity). We found a positive genetic correlation between tail white and body size (as measured by wing length), which indicates past correlational selection. Correlational selection, which was largely due to sexual selection on males, was also found to be currently acting on the same pair of traits. Larger males with whiter tails sired young with more females, most likely due to a combination of female choice, which favors males with whiter tails, and male-male competition, which favors both tail white and larger body size. To our knowledge, this is the first study to show both genetic correlations between sexually selected traits and currently acting correlational sexual selection, and we suggest that correlational sexual selection frequently may be an important mechanism for maintaining the honesty of sexual signals.     

The costs of being dark: the genetic basis of melanism and its association with fitness‐related traits in the sand cricket

Melanism is an important component of insect cuticle and serves numerous functions that enhance fitness. Despite its importance, there is little information on its genetic basis or its phenotypic and genetic correlation with fitness‐related traits. Here, we examine the heritability of melanism in the wing dimorphic sand cricket and determine its phenotypic and genetic correlation with wing morphology, gonad mass and size of the dorso‐longitudinal muscles (the principle flight muscles). Previously demonstrated trade‐offs among these traits are significant factors in the evolution of life history variation. Using path analysis, we show that melanization is causally related to gonad mass, but not flight muscle mass. Averaged over the sexes, the heritability of melanism was 0.61, the genetic correlation with gonad mass was ?0.36 and with wing morph was 0.51. The path model correctly predicted the ranking of melanization score in lines selected for increased ovary mass, increased flight muscle mass, an index that increased both traits and an unselected control. Our results support the general hypothesis that melanization is costly for insects and negatively impacts investment in early reproduction.     

Correlated changes in circadian clocks in response to selection for faster pre-adult development in fruit flies Drosophila melanogaster

Although, circadian clocks are believed to be involved in the regulation of life-history traits such as pre-adult development time and lifespan in fruit flies Drosophila melanogaster, there is very little unequivocal evidence either to support or refute this. Here we report the results of a long-term study aimed at examining the role of circadian clocks in the temporal regulation of pre-adult development in D. melanogaster. We employed laboratory selection protocol for faster pre-adult development on four large, outbred, random mating populations of Drosophila. We assayed pre-adult development time and circadian period of locomotor activity rhythm of these flies at regular intervals of 5–10 generations. After 50 generations of selection, the overall egg-to-adult duration in the selected stocks was reduced by ~29 h (~12.5 %) relative to controls, with the selected populations showing a concurrent reduction in time taken to hatching, pupation and wing pigmentation, by ~2, ~16, and ~25.2 h, respectively. Furthermore, selected populations showed a concomitant reduction in the circadian period of locomotor activity rhythm, implying that circadian clocks and development time are correlated. Thus, our study provides the first ever unequivocal evidence for the evolution of circadian clocks as a correlated response to selection for faster pre-adult development, suggesting that circadian clocks and development are linked in fruit flies D. melanogaster.     

Directional and correlated effects of selection on amylase activity,weight and developmental time in Tribolium confusum (Coleoptera,Tenebrionidae)

The interrelationships of amylase activity, weight and developmental time in a strain of Tribolium confusum were studied by selecting, in turn, for one of these traits and monitoring other, non-target characters, in all lines.Experimental lines were selected for high and low amylase activity (AH, AL), for heavy and light adult weight (WH, WL) and for fast and slow development (DF, DS). The variables monitored every generation in all lines were mean amylase activity (measured colorimetrically), mean individual adult weight, median egg-to-pupa developmental time, mean productivity (offspring per fertile pair) and % sterile pairs.The six lines may be grouped by their response to selection in a meaningful way. AH, WH, and DS all had higher amylase activity, heavier adult weight, slower development and lower fitness (lower productivity, higher percent sterility) than the corresponding AL, WL, and DF, although a different variable was under selection in each pair of lines. These results demonstrate that amylase activity, developmental time and individual weight are intercorrelated.