Longevity and resistance to cold stress in cold‐stress selected lines and their controls in Drosophila melanogaster

Thermal environments can influence many fitness‐related traits including life span. Here, we assess whether longevity in Drosophila melanogaster can experimentally evolve as a correlated response to cold‐stress selection, and whether genotype‐by‐temperature and sex‐by‐temperature interactions are significant components of variation in life span. Three replicated S lines were cold‐stress selected and compared with their respective unselected controls (Clines) in the 16^th generation of thermal selection. Cold‐stress resistance exhibited a substantial direct response to selection, and also showed a significant interaction between sex and type of line. Mean longevity exhibited a significant interaction between adult test temperature (14 and 25 °C) and line (with suggestive evidence for increased longevity of S lines when tested at 14 °C), but there was no evidence for increased longevity in S lines at normal temperatures (i.e. 25 °C). Another temperature‐dependent effect was sex‐specific, with males being the longer lived sex at 25 °C but the less long‐lived sex at 14 °C. Additionally, we tested in an exploratory way the relationship between longevity and cold‐stress resistance by also measuring resistance to a prefreezing temperature before and after one generation of longevity selection at 14 °C (selection intensity, i = 1.47 for S lines, and 1.42 for C lines). In this longevity selection, we found that cold‐stress resistance increased by about 6% in S lines and 18% in C lines. However, taken together, the results indicate no simple relationship between longevity and cold‐stress resistance, with genotype‐by‐sex interactions in both traits. Temperature dependent interaction in longevity is apparent between S and C lines, and sex‐specific variation in mean longevity also depends on temperature.     

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.     

Rapid laboratory evolution of adult wing area in Drosophila melanogaster in response to humidity

Abstract We examined the evolutionary response of wing area (a trait highly correlated with other measures of body size) to relative humidity (RH), temperature, and their interaction in Drosophila melanogaster , using replicated lines that had been allowed to evolve at low or high humidity at 18°C or at 25°C. We found that after 20 weeks of selection (5–10 generations), low RH lines had significantly greater wing areas than high RH lines in both sexes. This evolutionary response may have resulted from selection of larger flies with a smaller surface area for water loss relative to their weight, or as a correlated response to selection on some other unidentified trait. There were no evolutionary effects of temperature on wing area or cell density. This may have been due to the short duration of the selection experiment, and/or counteracting selection pressures on body size at warm temperature.     

Costs of resistance in the Drosophila-macrocheles system: a negative genetic correlation between ectoparasite resistance and reproduction

Genetic variation for parasite resistance occurs in most host populations. Costs of resistance, manifested as reduced fitness of resistant genotypes in the absence of parasitism, can be an important factor contributing to the maintenance of this variation. One powerful tool for detecting costs of resistance is the study of correlated responses to artificial selection. Provided that experimental lines are recently derived from large outbreeding populations, and that inbreeding is minimized during the experiment, correlated responses to selection are expected to be strong indicators of pleiotropy. We artificially selected for elevated behavioral resistance against an ectoparasitic mite (Macrocheles subbadius) in replicate populations of the fly Drosophila nigrospiracula. Resistance was modeled as a threshold trait, and the realized heritability of resistance was estimated to be 12.3% (1.4% SE) across three replicate lines recently derived from nature. We contrasted the longevity and fecundity of resistant and control (unselected) flies under a variable thermal environment. We report that reduced fecundity is a correlated response to artificial selection for increased resistance, and that the strength of this effect increases from 25 degrees to 29 degrees C. In contrast, longevity differences were not detected between resistant and control lines at either temperature. These findings are robust as they were confirmed with an independent set of experimental lines. Thus, our results identify a negative genetic correlation between ectoparasite resistance and an important life-history trait. That a correlated response was only detected for fecundity, and not longevity, suggests that the genetic correlation is attributable to pleiotropic effects with narrower effects than reallocation of a general resource pool within the organism, although other interpretations are discussed. Combined with fluctuating parasite-mediated selection and temperature, the presence of this trade-off may contribute to the maintenance of genetic variation for resistance in natural populations.     

Temperature-induced shifts in selective pressure at a critical developmental transition

Selective mortality within a population, based on the phenotype of individuals, is the foundation of the theory of natural selection. We examined temperature-induced shifts in the relationships among early life history traits and survivorship over the embryonic and larval stages of a tropical damselfish, Pomacentrus amboinensis. Our experiments show that temperature determines the intensity of selective mortality, and that this changes with ontogeny. The size of energy stores determined survival through to hatching, after which egg size became a good indicator of fitness as predicted by theoretical models. Yet, the benefits associated with egg size were not uniform among test temperatures. Initial egg size positively influenced larval survival at control temperature (29 °C). However, this embryonic trait had no effect on post-hatching longevity of individuals reared at the higher (31 °C) and lower (25 °C) end of the temperature range. Overall, our findings indicate that the outcome of selective mortality is strongly dependent on the interaction between environment conditions and intrinsic developmental schedules.     

Nestling telomere length does not predict longevity,but covaries with adult body size in wild barn swallows

Telomere length and dynamics are increasingly scrutinized as ultimate determinants of performance, including age-dependent mortality and fecundity. Few studies have investigated longevity in relation to telomere length (TL) in the wild and none has analysed longevity in relation to TL soon after hatching, despite the fact that telomere shortening may mostly occur early in life. We show that TL in nestling barn swallows (Hirundo rustica) in the wild does not predict longevity. However, TL positively covaries with body size, suggesting that individuals with large TL can afford to grow larger without paying the cost of reduced TL, and/or that benign rearing conditions ensure both large body size and low rates of telomere shortening. Overall, our study hints at a role of TL in developmental processes, but also indicates a need for further analyses to assess the expectation that TL in young individuals predicts longevity in the wild.     

Association analysis of seed longevity in rice under conventional and high-temperature germination conditions

Seed longevity varies considerably in cultivated rice, but the underlying mechanism of longevity is not well understood. To measure seed longevity, we performed an aging treatment at 45 °C on seeds maintained at 14 % moisture content for 14 days. We measured the percentage germination of both treated and normal seeds at 25 °C as a control of seed longevity using four replications over 2 years. In total, 140 accessions from a core collection with diverse origins were genotyped using 204 SSR markers, which distributed into 12 chromosomes, to identify marker–trait associations with seed longevity. An analysis of the population structure revealed four subgroups. The r ² values ranged from 0.0 to 0.8901 for all intrachromosomal loci pairs, with an average of 0.0773. Linkage disequilibrium (LD) between linked markers decreased with distance and displayed a substantial drop in LD decay values between 20 and 50 cM. Marker–trait associations were investigated using a mixed linear model approach, considering both population structure (Q) and kinship (K). Twelve marker–trait associations (P < 0.01) were common between the two germination treatments and over the 2-year study, explaining more than 10 % of the total variation. These ten different markers were distributed on five chromosomes. The significant associated SSR markers identified will be useful to seed-bank managers to ensure collections are maintained at high levels of viability to avoid loss of genotypes from the population and for marker-assisted selection.     

Differential adult survival and phenotypic variation in a chromosomally polymorphic population of the grasshopper Sinipta dalmani

Small size results from homozygosity for the polymorphic M4 inversion in the grasshopper Sinipta dalmani. There is directional sexual selection in males favouring the standard sequence and larger individuals. Here I report differential adult male survival in relation to body size and shape in a chromosomally polymorphic population of S. dalmani. Adult male selection was examined by scoring the same five morphometric traits in two consecutive age classes (young and aged males). Multivariate analysis showed significant differences in size between the two samples so that young males are larger for total, third (3 degrees) femur and thorax lengths while aged males show a phenotypic superiority for the lengths of third (3 degrees) tibia and tegmina. Size-adjusted scores for each trait revealed that there are significant differences between young and aged males in shape. In S. dalmani phenotypic selection associated with male mating success may be explained by means of directional selection on body size, while phenotypic selection connected with differential adult male survival may be related to both size and shape.     

Selection does not favor larger body size at lower temperature in a seed-feeding beetle

Body size of many animals increases with increasing latitude, a phenomenon known as Bergmann's rule (Bergmann clines). Latitudinal gradients in mean temperature are frequently assumed to be the underlying cause of this pattern because temperature covaries systematically with latitude, but whether and how temperature mediates selection on body size is unclear. To test the hypothesis that the "relative" advantage of being larger is greatest at cooler temperatures we compare the fitness of replicate lines of the seed beetle, Stator limbatus, for which body size was manipulated via artificial selection ("Large,"Control," and "Small" lines), when raised at low (22 degrees C) and high (34 degrees C) temperatures. Large-bodied beetles (Large lines) took the longest to develop but had the highest lifetime fecundity, and highest fitness (r(C)), at both low and high temperatures. However, the relative difference between the Large and Small lines did not change with temperature (replicate 2) or was greatest at high temperature (replicate 1), contrary to the prediction that the fitness advantage of being large relative to being small will decline with increasing temperature. Our results are consistent with two previous studies of this seed beetle, but inconsistent with prior studies that suggest that temperature-mediated selection on body size is a major contributor to the production of Bergmann clines. We conclude that other environmental and ecological variables that covary with latitude are more likely to produce the gradient in natural selection responsible for generating Bergmann clines.     

Genetic variation in spontaneous ovulation rate and LH receptor induction in mice

Selection for litter size (Line S1) and for post-weaning body weight gain (Line G) increased spontaneous ovulation rate in mature females by 69 and 73%, respectively, over that of randomly bred control mice (Line C). Inbreeding from S1 mice with selection for litter size produced highly inbred lines with elevated ovulation rates. Inbreeding from Line C mice produced a 21% divergence among lines, but did not depress the mean ovulation rate. Crosses of these lines revealed little heterosis in ovulation rate. LH receptors were induced by treating females from 22 days of age with diethylstilboestrol for 4 days and FSH for 2 days. The in-vitro binding of 125I-labelled hCG per microgram DNA decreased 56% in response to selection for litter size and increased 57% in response to selection for body weight gain, indicating high susceptibility of this trait to genetic change. Inbreeding from Line C mice produced a 135% divergence amongst lines, but did not depress the mean LH receptor induction. Body weight had significant effects on ovulation rate and LH receptor induction. These results show that selection for litter size and for rapid post-weaning body weight gain increases ovulation rate, but we suggest that different mechanisms are involved in these responses.     

Interactions between size and temperature influence fecundity and longevity of a tortricid moth,Zeiraphera canadensis

Females of Zeiraphera canadensis Mut. & Free., the spruce bud moth, were reared in the laboratory at constant and alternating temperatures, and in an outdoor insectary, to (1) determine the effects of temperature, age and size on several reproductive parameters and, (2) to test the hypothesis that body size-temperature interactions influence longevity and realized fecundity. Egg maturation was linearly related to age and large moths developed eggs at a higher rate than small ones. Mcan lifetime oviposition rate reached a maximum and remained stable at temperatures 20° C while the mean lifetime rate of egg maturation increased linearly with temperature, indicating that higher temperatures adversely affect oviposition. The production of nonviable eggs increased with age but also with temperature, suggesting high temperature (25° C) reduces egg quality and/or hinders fertilization. The realized fecundity and longevity of females reared under an alternating temperature regime (mean 20° C) was significantly less than that of females reared at constant 20° C. Similar realized fecundity, longevity and mean lifetime oviposition rates for females reared at temperatures alternating between 10 and 25° C (mean 20° C) and those at constant 25° C reflected the inability of females to recover from elevated diurnal temperatures. Longevity was positively related to female body size at constant 15 and 20° C but the relationships were negative for moths exposed to diurnal temperatures equal to or exceeding 25° C. Due to the reduced longevity of large moths at high temperatures, linear regressions between size and realized fecundity were only significant at constant temperatures 20° C. At higher temperatures, the size-fecundity relationship became curvilinear as a result of the diminished reproductive output of large individuals. Reduced fecundity and longevity of large females at high temperatures may have been due to elevated internal temperatures of large-bodied moths. Large females in a controlled-environment chamber maintained at 25° C developed an internal temperature excess (i.e. temperature above ambient) of nearly 2° C while small-bodied females exceeded ambient by only 0.3° C. However, when held at 20° C, the temperature excess of large-bodied moths was much less than 1° C and small-bodied females did not differ from ambient. Such interactions between temperature and body size suggest that there should be stabilizing selection toward moderate-sized individuals and may explain the absence of size-related effects on fecundity and longevity previously reported for several other lepidopterans.     

Effect of different thermal conditions on the pre-imaginal biology of Culex apicinus (Philippi, 1865) (Diptera: Culicidae).

The time of development and survival of the pre-imaginal period and the adult body size of Culex apicinus were analyzed in individuals reared from the 1st instar larva to adult emergence under laboratory and field conditions. In the laboratory, insects were exposed to three constant temperatures (15, 20, and 25 degrees C) and a photoperiod of 14:10 (L:D). In the field, temperature and photoperiod were not manipulated; during the study period water temperature ranged between 15.5 and 24.2 degrees C, and photoperiod changed from 13:11 to 14:10 (L:D). Survival to the imaginal stage at 15 degrees C (85.4%) and in the field (88.8%) was higher than that at 20 degrees C (45.8%) and 25 degrees C (8.3%). Based on femur and wing length measurements, the mosquitoes reared under field conditions (mean water temperature = 20.5 degrees C) were smaller than those laboratory-reared, even when the time of development in the field (29.5 - 36 days) was longer than that observed at 15 degrees C (24 days), 20 degrees C (15 days), and 25 degrees C (11 days). Male emergence was earlier than that of females (protandry) only in the field. Although laboratory conditions included a photoperiod and temperature range comparable with the observed values in the field, the disagreements between field and laboratory results suggest that the characteristics examined in this work could be affected by the joint variation of several factors not controlled in field conditions, such as photoperiod, temperature regime, and/or food quality of larval habitat.     

Heat knockdown resistance and chill‐coma recovery as correlated responses to selection on mating success at high temperature in Drosophila buzzatii

Reproduction and related traits such as mating success are strongly affected by thermal stress. We tested direct and correlated responses to artificial selection in replicated lines of Drosophila buzzatii that were selected for mating success at high temperature. Knockdown resistance at high temperature (KRHT) and chill‐coma recovery (CCR) were tested as correlated selection responses. Virgin flies were allowed to mate for four hours at 33°C in three replicated lines (S lines) to obtain the selected flies and then returned at 25°C to lay eggs. Other three replicated lines were maintained at 25°C without any selection as control (C lines). After 15 selection generations, KRHT and CCR were measured. Both traits were assessed in flies that did not receive any hardening pretreatments as well as in flies that were either heat or cold hardened. Thermotolerance traits showed significant correlated responses with higher KRHT in S than in C lines, both with a heat‐hardening pretreatment and without a heat‐hardening pretreatment. CCR time was longer in S than in C lines both with a cold‐hardening pretreatment and without a cold‐hardening pretreatment. Hardening treatments improved both KRHT and CCR in all cases excepting KRHT in C lines. Overall, KRHT and CCR showed an antagonistic pattern of correlated responses to our selection regime, suggesting either pleiotropy or tightly linked trait‐specific genes partially affecting KRHT and CCR.     

Anhydrobiotic potential and long-term storage of entomopathogenic nematodes (Rhabditida: Steinernematidae)

Anhydrobiosis is considered to be an important means of achieving storage stability of entomopathogenic nematodes that are used in biological control. This study explored the effects of anhydrobiosis on longevity and infectivity of infective juveniles (IJs) of three species of entomopathogenic nematodes Steinernema carpocapsae, Steinernema feltiae, and Steinernema riobrave at 5 and 25 degrees C. Anhydrobiosis was induced in water-dispersible granules (WG) at 0.966-0.971 water activity and 25 degrees C following a 7-day preconditioning of IJs at 5 degrees C in tap water. Survival and infectivity of the desiccated (anhydrobiotic) IJs was compared with non-desiccated IJs stored in water for different periods. Anhydrobiosis increased longevity of S. carpocapsae IJs by 3 months and of S. riobrave by 1 month in WG at 25 degrees C as compared with IJs stored in water. However, desiccation decreased S. feltiae longevity at 25 degrees C and of all three species at 5 degrees C. These results demonstrate a shelf-life of 5 months for S. carpocapsae at 25 degrees C and 9 months at 5 degrees C in WG with over 90% IJ survival. For S. feltiae, over 90% survival occurred only for 2 months at 25 degrees C and 5 months at 5 degrees C in WG. Steinernema riobrave had over 90% survival only for 1 month at 25 degrees C and the survival dropped below 85% within 1 month at 5 degrees C. Induction of anhydrobiosis in WG resulted in 85, 79 and 76% reduction in oxygen consumption by S. carpocapsae, S. feltiae, and S. riobrave IJs, respectively. Differences in IJ longevity among three species in water at 25 degrees C were related both to the initial lipid content and the rate of lipid utilisation, but not at 5 degrees C. The one-on-one infection bioassays indicated that desiccation had no negative effect on the infectivity of any of the nematode species suggesting no harmful effect on the IJs and/or their symbiotic bacteria. The species differences in IJ longevity and desiccation survival at different temperatures are discussed in relation to their foraging strategy and temperature adaptation.     

The quantitative genetics of fluctuating asymmetry

Fluctuating asymmetry (subtle departures from identical expression of a trait across an axis of symmetry) in many taxa is under stabilizing selection for reduced asymmetry. However, lack of reliable estimates of genetic parameters for asymmetry variation hampers our ability to predict the evolutionary outcome of this selection. Here we report on a study, based on analysis of variation within and between isofemale lines and of generation means (line-cross analysis), designed to dissect in detail the quantitative genetics of positional fluctuating asymmetry (PFA) in bristle number in natural populations of Drosophila falleni. PFA is defined as the difference between the two sides of the body in the placement or position of components of a meristic trait. Heritability (measured at 25 degrees C) of two related measures of PFA were 13% and 21%, both of which differed significantly from zero. In contrast, heritability estimates for fluctuating asymmetry in the total number of anterior (0.7%) and transverse (2.4%) sternopleural bristles were smaller, not significant, and in quantitative agreement with previously published estimates. Heritabilities for bristle number (trait size) were considerably greater than that for any asymmetry measure. The experimental design controlled for the potentially confounding effects of common familial environment, and repeated testing revealed that PFA differences between lines were genetically stable for up to 16 generations in the laboratory at 25 degrees C. We performed line cross analysis between strains at the extremes of the PFA distribution (highest and lowest values); parental strains, F1, F1r (reciprocal), F2, backcross, and backcross reciprocal generations were represented. The inheritance of PFA was described best by additive and dominance effects localized to the X-chromosomes, whereas autosomal dominance effects were also detected. Epistatic, maternal, and cytoplasmic effects were not detected. The inheritance of trait size was notably more complex and involved significant autosomal additive, dominance, and epistatic effects; maternal dominance effects; and additive and dominance effects localized to the X-chromosomes. The additive genetic correlation between PFA and its associated measure of trait size was negative (-0.049), but not statistically significant, indicating that the loci contributing additive genetic effects to these traits are probably different. It is suggested that PFA may be a sensitive measure of developmental instability because PFA taps the ability of an organism to integrate interconnected developmental pathways.     

Direct and correlated responses to selection for longevity in Drosophila buzzatii

The possible associations between longevity, early fecundity, and stress-resistance traits were explored using artificial selection on longevity in a laboratory population of Drosophila buzzatii . Three replicated lines were selected for increased lifespan (L lines) and compared with the respective unselected controls (C lines) after the 14th generation of selection. Mean longevity exhibited a significant response to selection. The baseline mortality tended to decrease in the L lines and a negative correlated response to longevity selection was found for early fecundity. Egg-to-adult developmental time increased in L lines. Longevity selection increased stress resistance for both high and low temperatures, as measured by heat knockdown resistance and chill-coma recovery. Starvation resistance also tended to be higher in L than in C lines. The results obtained are consistent with the hypothesis of trade-offs between longevity and early fecundity, and also suggest a trade-off association between adult longevity and developmental time. Correlated selection responses were generally consistent with correlations among the traits previously inferred from altitudinal clines for longevity and stress-resistance phenotypes.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 738–748.     

Latitudinal Variation in Starvation Resistance is Explained by Lipid Content in Natural Populations of Drosophila melanogaster

One of the most common environmental stressors is a shortage or suboptimal quality of food, thus all animals deal with periods of starvation. In the present study we examine variation in starvation resistance, longevity and body lipid content and the correlations between traits along an environmental gradient using isofemale lines recently derived from natural populations of Drosophila melanogaster from South America. The use of isofemale lines and controlled rearing laboratory conditions allows us to investigate within and among population components of genetic variation and the potential associations among starvation resistance, longevity and body lipid content. All these traits were analyzed separately in females and males, improving our understanding of sexual dimorphism. Our results revealed significant differences among populations in starvation resistance and longevity. Actually, the opposing latitudinal cline detected for starvation resistance suggests that natural selection played an essential role in shaping the pattern of geographic variation in this trait. Moreover, we also detected a positive relationship between starvation resistance and body lipid content in both sexes, providing evidence for a physiological and/or evolutionary association between these traits. Conversely, starvation resistance was not correlated with longevity indicating that these traits might be enabled to evolve independently. Finally, our study reveals that there is abundant within population genetic variation for all traits that may be maintained by sex-specific effects.     

Mapping quantitative trait loci controlling seed longevity in rice ( Oryza sativa L.)

Quantitative trait loci (QTLs) controlling seed longevity in rice were identified using 98 backcross inbred lines (BILs) derived from a cross between a japonica variety Nipponbare and an indica variety Kasalath. Seeds of each BIL were kept for 12 months at 30 degrees C in dry conditions to promote loss of viability. To measure seed longevity, we performed an additional aging-processing treatment for 2 months at 30 degrees C maintaining seeds at 15% moisture content. We measured the germination percent of these treated seeds at 25 degrees C for 7 days as the degree of seed longevity. The germination of BILs ranged from 0 to 100% with continuous variation. Three putative QTLs for seed longevity, qLG-2, qLG-4 and qLG-9, were detected on chromosome 2, 4 and 9, respectively. Kasalath alleles increased the seed longevity at these QTLs. The QTL with the largest effect, qLG-9, explained 59.5% of total phenotypic variation in BILs. The other two QTLs, qLG-2 and qLG-4, explained 13.4 and 11.6% of the total phenotypic variation, respectively. We also verified the effect of the Kasalath allele of qLG-9 using chromosome segment substitution lines. Furthermore, QTLs for seed dormancy were identified on chromosomes 1, 3, 5, 7 and 11. Based on the comparison of the chromosomal location of QTLs for seed longevity and seed dormancy, these traits seem to be controlled by different genetic factors.     

Environmental origins of sexually selected variation and a critique of the fluctuating asymmetry-sexual selection hypothesis

Identifying sources of phenotypic variability in secondary sexual traits is critical for understanding their signaling properties, role in sexual selection, and for predicting their evolutionary dynamics. The present study tests for the effects of genotype, developmental temperature, and their interaction, on size and fluctuating asymmetry of the male sex comb, a secondary sexual character, in Drosophila bipectinata Duda. Both the size and symmetry of elements of the sex comb have been shown previously to be under sexual selection in a natural population in northeastern Australia. Two independent reciprocal crosses were conducted at 25 degrees and 29 degrees C between genetic lines extracted from this population that differed in the size of the first (TC1) and third (TC3) comb segments. These temperatures are within the documented range experienced by the species in nature. Additive and dominance genetic effects were detected for TC1, whereas additive genetic, and Y-chromosomal effects were detected for TC3. TC2 and TC3 decreased sharply with increasing temperature, by 10% and 22%, respectively. In contrast, positional fluctuating asymmetry (PFA) significantly increased with temperature, by up to 38%. The results (1) document an important source of environmental variance in a sexual ornament expected to reduce trait heritability in field populations, and thus act to attenuate response to sexual selection, (2) suggest that variation in ornament size reflects differences in male condition; and (3) support the general hypothesis that asymmetry in a sexual ornament is indicative of developmental instability arising from environmental stress. The "environmental heterogeneity" (EH) hypothesis is proposed, and supportive evidence for it presented, to explain negative size-FA correlations in natural populations. Data and theory challenge the use of negative size-FA correlations observed in nature to support the FA-sexual selection hypothesis, which posits that such correlations are driven by differences in genetic quality among individuals.     

Live fast, die young: trade-offs between fitness components and sexually antagonistic selection on weaponry in Soay sheep

Males are predicted to compete for reproductive opportunities, with sexual selection driving the evolution of large body size and weaponry through the advantage they confer for access to females. Few studies have explored potential trade-offs of investment in secondary sexual traits between different components of fitness or tested for sexually antagonistic selection pressures. These factors may provide explanations for observed polymorphisms in both form and quality of secondary sexual traits. We report here an analysis of selection on horn phenotype in a feral population of Soay sheep (Ovis aries) on the island of Hirta, St. Kilda, Scotland. Soay sheep display a phenotypic polymorphism for horn type with males growing either normal or reduced (scurred) horns, and females growing either normal, scurred, or no (polled) horns; further variation in size exists within horn morphs. We show that horn phenotype and the size of the trait displayed is subject to different selection pressures in males and females, generating sexually antagonistic selection. Furthermore, there was evidence of a trade-off between breeding success and longevity in normal-horned males, with both the normal horn type and larger horn size being associated with greater annual breeding success but reduced longevity. Therefore, selection through lifetime breeding success was not found to act upon horn phenotype in males. In females, a negative association of annual breeding success within the normal-horned phenotype did not result in a significant difference in lifetime fitness when compared to scurred individuals, as no significant difference in longevity was found. However, increased horn size within this group was negatively associated with breeding success and longevity. Females without horns (polled) suffered reduced longevity and thus reduced lifetime breeding success relative the other horn morphs. Our results therefore suggest that trade-offs between different components of fitness and antagonistic selection between the sexes may maintain genetic variation for secondary sexual traits within a population.