The interaction of season length and development time alters size at maturity

Shaped by the Sun: the effect of exposure to sunlight on the evolution of spider bodies

Body temperature can strongly influence fitness. Some Sun-exposed ectotherms thermoregulate by adjusting body posture according to the Sun's position. In these species, body elongation should reduce the risk of heat stress by allowing the exposure of a smaller body area to sunlight. Therefore, selection should favour more elongated bodies in Sun-exposed than in Sun-protected species. Diurnal orb-web spider species that sit on their webs are more likely to be Sun-exposed, on average, than nocturnal or diurnal shelter-building species. We measured the body elongation of orb-web spiders (Araneae, Araneidae) across 1024 species and classified them as Sun-protected or exposed based on the literature. We found that Sun-exposed species evolved more elongate bodies than Sun-protected ones. Further, we built a model combining traditional heat transfer models with models of thermoregulatory postures in orb-web spiders and meteorological data. The model indicates that body elongation in large orb-web spiders decreases the risk of high body temperatures. Overall, our results suggest that Sun exposure influenced the evolution of body shapes of orb-web spiders.

Sex-specific developmental trajectories in an extremely sexually size dimorphic spider

Adult body size, development time, and growth rates are components of organismal life histories, which crucially influence fitness and are subject to trade-offs. If selection is sex-specific, male and female developments can eventually lead to different optimal sizes. This can be achieved through developmental plasticity and sex-specific developmental trajectories. Spiders present suitable animals to study differences in developmental plasticity and life history trade-offs between the sexes, because of their pronounced sexual dimorphism. Here, we examine variation in life histories in the extremely sexually size dimorphic African hermit spider (Nephilingis cruentata) reared under standardized laboratory conditions. Females average 70 times greater body mass (and greater body size) at maturity than males, which they achieve by developing longer and growing faster. We find a small to moderate amount of variability in life history traits to be caused by family effects, comprising genetic, maternal, and early common environmental effects, suggesting considerable plasticity in life histories. Remarkably, family effects explain a higher variance in male compared to female life histories, implying that female developmental trajectories may be more responsive to environment. We also find sex differences in life history trade-offs and show that males with longer development times grow larger but exhibit shorter adult longevity. Female developmental time also correlates positively with adult body mass, but the trade-offs between female adult mass, reproduction, and longevity are less clear. We discuss the implications of these findings in the light of evolutionary trade-offs between life history traits.

Web wars: males of the golden orb-web spider invest more in fights for mated females

In addition to resource value, the cost of finding mates may affect how much males invest in fights for females. The cost of finding females may be imposed through natural factors extrinsic to males, such as female spatial distribution and predation pressure, which can be challenging to simulate in laboratory conditions. Therefore, studies under natural conditions may be suitable for understanding how the costs of finding mating partners affect male investment in fights. We used the spider Trichonephila clavipes to evaluate the hypotheses that males in field conditions invest more in contests for access to 1) unmated and 2) more fecund females and 3) when access to females is harder. To test these hypotheses, we recorded the occurrence, duration, and escalation of induced contests between males located in webs of females that differed in reproductive status (estimated by female life stage), fecundity (estimated by female abdominal area), and spatial distribution (i.e., isolated or aggregated with webs of other females). The occurrence and duration of contests were unrelated to female value or search costs. However, the probability for escalation was higher when males were fighting for adult (and probably mated) females. We also found that males tended to start a contest more often in aggregated webs. These results indicate that males of T. clavipes adjust investment in contests but contrarily to what we expected. We suggest that males invest more in contests for adult females because they are defending females that they previously fertilized to avoid sperm competition.

Giant and dwarf females: how to explain the fourfold variation in body size and fecundity in Trichonephila senegalensis (Aranea: Nephilidae)

Variation in life-history traits within a population is caused by genetic, maternal and environmental factors. We explored the high variability in development time, adult body weight and fecundity in females of the sexually size dimorphic spider Trichonephila senegalensis. Their mothers originated from two habitats—strongly seasonal Namibia and mildly seasonal South Africa—and we reared F1 females under standardized laboratory conditions. We found that a considerable part of the variability in recorded life-history traits is caused by family-specific effects, comprising genetic, maternal and early environmental influences. Furthermore, we show population differences in development time, where females originating from Namibia matured within shorter periods than females from South Africa. Also, the relationship between development time and adult weight differs between the two populations, as a significant correlation is only found in females with Namibian origin. Against common wisdom, there was a weak overall correlation between adult weight and clutch mass. We also found that females make different life-history decisions under increasing rather than under decreasing daylength. Although a considerable part of variability in life-history traits is family-specific, we discuss how the between-population differences in life histories and their trade-offs reflect adaptation to diverse habitats.

Exploring diversification drivers in golden orbweavers

Heterogeneity in species diversity is driven by the dynamics of speciation and extinction, potentially influenced by organismal and environmental factors. Here, we explore macroevolutionary trends on a phylogeny of golden orbweavers (spider family Nephilidae). Our initial inference detects heterogeneity in speciation and extinction, with accelerated extinction rates in the extremely sexually size dimorphic Nephila and accelerated speciation in Herennia, a lineage defined by highly derived, arboricolous webs, and pronounced island endemism. We evaluate potential drivers of this heterogeneity that relate to organisms and their environment. Primarily, we test two continuous organismal factors for correlation with diversification in nephilids: phenotypic extremeness (female and male body length, and sexual size dimorphism as their ratio) and dispersal propensity (through range sizes as a proxy). We predict a bell-shaped relationship between factor values and speciation, with intermediate phenotypes exhibiting highest diversification rates. Analyses using SSE-class models fail to support our two predictions, suggesting that phenotypic extremeness and dispersal propensity cannot explain patterns of nephilid diversification. Furthermore, two environmental factors (tropical versus subtropical and island versus continental species distribution) indicate only marginal support for higher speciation in the tropics. Although our results may be affected by methodological limitations imposed by a relatively small phylogeny, it seems that the tested organismal and environmental factors play little to no role in nephilid diversification. In the phylogeny of golden orbweavers, the recent hypothesis of universal diversification dynamics may be the simplest explanation of macroevolutionary patterns.

Fitness implications of sex-specific catch-up growth in Nephila senegalensis, a spider with extreme reversed SSD

BACKGROUND

Animal growth is often constrained by unfavourable conditions and divergences from optimal body size can be detrimental to an individual's fitness, particularly in species with determinate growth and a narrow time-frame for life-time reproduction. Growth restriction in early juvenile stages can later be compensated by means of plastic developmental responses, such as adaptive catch-up growth (the compensation of growth deficits through delayed development). Although sex differences regarding the mode and degree of growth compensation have been coherently predicted from sex-specific fitness payoffs, inconsistent results imply a need for further research. We used the African Nephila senegalensis, representing an extreme case of female-biased sexual size dimorphism (SSD), to study fitness implications of sex-specific growth compensation. We predicted effective catch-up growth in early food-restricted females to result in full compensation of growth deficits and a life-time fecundity (LTF) equivalent to unrestricted females. Based on a stronger trade-off between size-related benefits and costs of a delayed maturation, we expected less effective catch-up growth in males.

METHODS

We tracked the development of over one thousand spiders in different feeding treatments, e.g., comprising a fixed period of early low feeding conditions followed by unrestricted feeding conditions, permanent unrestricted feeding conditions, or permanent low feeding conditions as a control. In a second experimental section, we assessed female fitness by measuring LTF in a subset of females. In addition, we tested whether compensatory development affected the reproductive lifespan in both sexes and analysed genotype-by-treatment interactions as a potential cause of variation in life-history traits.

RESULTS

Both sexes delayed maturation to counteract early growth restriction, but only females achieved full compensation of adult body size. Female catch-up growth resulted in equivalent LTF compared to unrestricted females. We found significant interactions between experimental treatments and sex as well as between treatments and family lineage, suggesting that family-specific responses contribute to the unusually large variation of life-history traits in Nephila spiders. Our feeding treatments had no effect on the reproductive lifespan in either sex.

DISCUSSION

Our findings are in line with predictions of life-history theory and corroborate strong fecundity selection to result in full female growth compensation. Males showed incomplete growth compensation despite a delayed development, indicating relaxed selection on large size and a stronger trade-off between late maturation and size-related benefits. We suggest that moderate catch-up growth in males is still adaptive as a 'bet-hedging' strategy to disperse unavoidable costs between life-history traits affected by early growth restriction (the duration of development and adult size).

Seasonal reproduction leads to population collapse and an Allee effect in a stage-structured consumer-resource biomass model when mortality rate increases

Many populations collapse suddenly when reaching low densities even if they have abundant food conditions, a phenomenon known as an Allee effect. Such collapses can have disastrous consequences, for example, for loss of biodiversity. In this paper, we formulate a stage-structured consumer-resource biomass model in which adults only reproduce at the beginning of each growing season, and investigate the effect of an increasing stage-independent background mortality rate of the consumer. As the main difference with previously studied continuous-time models, seasonal reproduction can result in an Allee effect and consumer population collapses at high consumer mortality rate. However, unlike the mechanisms reported in the literature, in our model the Allee effect results from the time difference between the maturation of juveniles and the reproduction of adults. The timing of maturation plays a crucial role because it not only determines the body size of the individuals at maturation but also influences the duration of the period during which adults can invest in reproductive energy, which together determine the reproductive output at the end of the season. We suggest that there exists an optimal timing of maturation and that consumer persistence is promoted if individuals mature later in the season at a larger body size, rather than maturing early, despite high food availability supporting rapid growth.

An RNA Virome Associated to the Golden Orb-Weaver Spider Nephila clavipes

The golden orb-weaver spider Nephila clavipes, known for its sexual size dimorphism, is abundant and widespread in the New World. The first annotated genome of orb-weaver spiders, exploring N. clavipes, has recently been reported. The study, focused primarily on the diversity of silk specific genes, shed light into the complex evolutionary history of spiders. Furthermore, a robust transcriptome analysis provided a massive resource for N. clavipes RNA survey. Here, I present evidence of viral sequences corresponding to the first 10 extant virus species associated to N. clavipes and indeed, nephilids. The putatively new species are linked to ssRNA positive-strand viruses, such as Picornavirales, and to ssRNA negative-strand and dsRNA viruses. In addition, I detected sequence data of new strains of two recently reported arthropod viruses, which complemented and extended the corresponding sequence references. The identified viruses appear to be complete, potentially functional, and presenting the typical architecture and consistent viral domains. The intrinsic nature of the detected sequences and their absence in the recently generated genome assembly, suggest that they correspond to bona fide RNA virus sequences. The available RNA data allowed for the first time to address a tissue/organ specific analysis of virus loads/presence in spiders, suggesting a complex spatial and differential distribution of the tentative viruses, encompassing the spider brain and also silk and venom glands. Until recently, the virus landscape associated to spiders remained elusive. The discovered viruses described here provide only a fragmented glimpse of the potential magnitude of the Aranea virosphere. Future studies should focus not only on complementing and expanding these findings, but also on addressing the potential ecological role of these viruses, which might influence the biology of these outstanding arthropod species.

Within-season variability of fighting behaviour in an Australian alpine grasshopper

Throughout the breeding season, changing environmental and biological conditions can lead to variation in the reproductive landscape of many species. In alpine environments temperature is a key driver of behaviour for small ectotherms such as insects, but variable biotic factors such as mate quality and availability can also influence behaviour. Kosicuscola tristis is a small semelparous grasshopper of the Australian alpine region. In a rare behaviour among grasshoppers, K. tristis males engage in vigorous fights over access to females, involving mandible displays, kicking, biting and grappling. In this study we describe the variation in fighting behaviour of K. tristis throughout the breeding season and test several hypotheses related to temperature, body size, mating behaviour, and female quality. We show that K. tristis males are more aggressive toward each other at the end of the breeding season than at the beginning. This increased aggression is associated with decreased daily average temperatures (from ~20°C to ~9°C), decreased mating activity, increased female fecundity, and an unexpected trend toward an increase in female-to-male aggression. These results suggest that K. tristis is likely under increased selective pressure to time key life cycle events with favourable biological and climatic conditions. The stochastic nature of alpine environments combined with a relatively short life span and breeding season, as well as limited mating opportunities toward the end of the season may have contributed to the evolution of this extraordinary mating system.

Socially cued developmental plasticity in web-building spiders

Background

Socially cued anticipatory plasticity (SCAP) has been proposed as a widespread mechanism of adaptive life-history shifts in semelparous species with extreme male mating investment. Such mating systems evolved several times independently in spiders and male reproductive success should critically depend on timely maturation and rapid location of a receptive and, ideally, virgin female. We experimentally investigated socially cued anticipatory plasticity in two sympatric, closely related Nephila species that share many components of their mating systems, but differ in the degree to which male reproductive success depends on mating with virgin females. Juveniles of both species were reared either in the presence or absence of virgin female silk cues. We predicted strong selection on socially cued plasticity in N. fenestrata in which males follow a highly specialized terminal investment strategy, but expected a weaker plastic response in N. senegalensis in which males lost the ability to monopolize females.

Results

Contrary to our predictions, N. fenestrata males presented with virgin female silk cues did not mature earlier than siblings reared isolated from such cues. Males in N. senegalensis, however, showed a significant response to female cues and matured several days earlier than control males. Plastic adjustment of maturation had no effect on male size.

Conclusions

Our results indicate that a strong benefit of mating with virgins due to first male sperm priority does not necessarily promote socially cued anticipatory plasticity. We emphasize the bidirectional mode of developmental responses and suggest that this form of plasticity may not only yield benefits through accelerated maturation, but also by avoiding costs of precipitate maturation in the absence of female cues.

Transcriptional changes during Daphnia pulex development indicate that the maturation decision resembles a rate more than a threshold

Maturation is a critical developmental process, and the age and size at which it occurs have important fitness consequences. Although maturation is remarkably variable, certain mechanisms, including a minimum size or state threshold, are proposed to underlie the process across a broad diversity of taxa. Recent evidence suggests that thresholds may themselves be developmentally plastic, and in the crustacean Daphnia pulex it is unclear whether maturation follows a threshold or is a gradual process more akin to a rate. Changes in gene expression across four instars before and during maturation were compared in a cDNA microarray experiment. Developmental stage was treated statistically both as a discontinuous and as a continuous variable, to determine whether genes showed gradual or discrete changes in expression. The continuous analysis identified a greater number of genes with significant differential expression (45) than the discontinuous analysis (11). The majority of genes, including those coding for histones, factors relating to transcription and cell cycle processes, and a putative developmental hormone showed continuous increases or decreases in expression from the first to the fourth instars that were studied, suggestive of a prolonged and gradual maturation process. Three genes coding for a fused vitellogenin/superoxide dismutase showed increases in expression following the second instar and coincided with the posited maturation threshold, but even their expression increased in a continuous fashion.

A molecular phylogeny of nephilid spiders: evolutionary history of a model lineage

The pantropical orb web spider family Nephilidae is known for the most extreme sexual size dimorphism among terrestrial animals. Numerous studies have made Nephilidae, particularly Nephila, a model lineage in evolutionary research. However, a poorly understood phylogeny of this lineage, relying only on morphology, has prevented thorough evolutionary syntheses of nephilid biology. We here use three nuclear and five mitochondrial genes for 28 out of 40 nephilid species to provide a more robust nephilid phylogeny and infer clade ages in a fossil-calibrated Bayesian framework. We complement the molecular analyses with total evidence analysis including morphology. All analyses find strong support for nephilid monophyly and exclusivity and the monophyly of the genera Herennia and Clitaetra. The inferred phylogenetic structure within Nephilidae is novel and conflicts with morphological phylogeny and traditional taxonomy. Nephilengys species fall into two clades, one with Australasian species (true Nephilengys) as sister to Herennia, and another with Afrotropical species (Nephilingis Kuntner new genus) as sister to a clade containing Clitaetra plus most currently described Nephila. Surprisingly, Nephila is also diphyletic, with true Nephila containing N. pilipes+N. constricta, and the second clade with all other species sister to Clitaetra; this "Nephila" clade is further split into an Australasian clade that also contains the South American N. sexpunctata and the Eurasian N. clavata, and an African clade that also contains the Panamerican N. clavipes. An approximately unbiased test constraining the monophyly of Nephilengys, Nephila, and Nephilinae (Nephila, Nephilengys, Herennia), respectively, rejected Nephilengys monophyly, but not that of Nephila and Nephilinae. Further data are therefore necessary to robustly test these two new, but inconclusive findings, and also to further test the precise placement of Nephilidae within the Araneoidea. For divergence date estimation we set the minimum bound for the stems of Nephilidae at 40 Ma and of Nephila at 16 Ma to accommodate Palaeonephila from Baltic amber and Dominican Nephila species, respectively. We also calibrated and dated the phylogeny under three different interpretations of the enigmatic 165 Ma fossil Nephila jurassica, which we suspected based on morphology to be misplaced. We found that by treating N. jurassica as stem Nephila or nephilid the inferred clade ages were vastly older, and the mitochondrial substitution rates much slower than expected from other empirical spider data. This suggests that N. jurassica is not a Nephila nor a nephilid, but possibly a stem orbicularian. The estimated nephilid ancestral age (40-60 Ma) rejects a Gondwanan origin of the family as most of the southern continents were already split at that time. The origin of the family is equally likely to be African, Asian, or Australasian, with a global biogeographic history dominated by dispersal events. A reinterpretation of web architecture evolution suggests that a partially arboricolous, asymmetric orb web with a retreat, as exemplified by both groups of "Nephilengys", is plesiomorphic in Nephilidae, that this architecture was modified into specialized arboricolous webs in Herennia and independently in Clitaetra, and that the web became aerial, gigantic, and golden independently in both "Nephila" groups. The new topology questions previously hypothesized gradual evolution of female size from small to large, and rather suggests a more mosaic evolutionary pattern with independent female size increases from medium to giant in both "Nephila" clades, and two reversals back to medium and small; combined with male size evolution, this pattern will help detect gross evolutionary events leading to extreme sexual size dimorphism, and its morphological and behavioral correlates.

Juvenile development, ecdysteroids and hemolymph level of metabolites in the spider Brachypelma albopilosum (Theraphosidae)

In the present work, juvenile development and physiological state of mygalomorph Brachypelma albopilosum were investigated by means of individual rearing under controlled conditions. Males required 4-5 years for development from first juvenile instar to adulthood, passing through 8 to 12 juvenile molts. Females developed to adults in 5-6 years with a variable juvenile molt number from 9 to 13. The development and growth of males and females took place in a similar way until the last juvenile molt leading to subadults. Ecdysteroids, total lipid, cholesterol, and protein concentrations increased along with the different development instars in both males and females. After the last juvenile molt, spiders presented morphological and biochemical sex differences. Subadult and adulthood males were smaller in size and weight than females; hemolymph levels of ecdysteroids, total lipids, cholesterol, and glucose were higher in males. These physiological and biochemical differences can be correlated to the different sexual development between males and females.

Phenological asynchrony between herbivorous insects and their hosts: signal of climate change or pre-existing adaptive strategy?

Climate change alters phenological relations between interacting species. We might expect the historical baseline, or starting-point, for such effects to be precise synchrony between the season at which a consumer most requires food and the time when its resources are most available. We synthesize evidence that synchrony was not the historical condition in two insect-plant interactions involving Edith's checkerspot butterfly (Euphydryas editha), the winter moth (Operophtera brumata) and their host plants. Initial observations of phenological mismatch in both systems were made prior to the onset of anthropogenically driven climate change. Neither species can detect the phenology of its host plants with precision. In both species, evolution of life history has involved compromise between maximizing fecundity and minimizing mortality, with the outcome being superficially maladaptive strategies in which many, or even most, individuals die of starvation through poor synchrony with their host plants. Where phenological asynchrony or mismatch with resources forms the starting point for effects of anthropogenic global warming, consumers are particularly vulnerable to impacts that exacerbate the mismatch. This vulnerability likely contributed to extinction of a well-studied metapopulation of Edith's checkerspot, and to the skewed geographical pattern of population extinctions underlying a northward and upward range shift in this species.

Phenological asynchrony between herbivorous insects and their hosts: signal of climate change or pre-existing adaptive strategy?

Climate change alters phenological relations between interacting species. We might expect the historical baseline, or starting-point, for such effects to be precise synchrony between the season at which a consumer most requires food and the time when its resources are most available. We synthesize evidence that synchrony was not the historical condition in two insect–plant interactions involving Edith's checkerspot butterfly ( Euphydryas editha ), the winter moth ( Operophtera brumata ) and their host plants. Initial observations of phenological mismatch in both systems were made prior to the onset of anthropogenically driven climate change. Neither species can detect the phenology of its host plants with precision. In both species, evolution of life history has involved compromise between maximizing fecundity and minimizing mortality, with the outcome being superficially maladaptive strategies in which many, or even most, individuals die of starvation through poor synchrony with their host plants. Where phenological asynchrony or mismatch with resources forms the starting point for effects of anthropogenic global warming, consumers are particularly vulnerable to impacts that exacerbate the mismatch. This vulnerability likely contributed to extinction of a well-studied metapopulation of Edith's checkerspot, and to the skewed geographical pattern of population extinctions underlying a northward and upward range shift in this species.

Cross-habitat variation in the phenology of a colonial spider: insights from a reciprocal transplant study

In species with widespread distribution, populations found in markedly different environments can show differences in developmental traits. This, in time, can have an effect on reproductive success. Sources of variation in developmental traits can be genetic or environmentally induced. I examined the relationship between environmental and genetic influences on juvenile development in populations of the colonial spider, Parawixia bistriata, located at sites with different moisture regimes and associated environmental variables (e.g., prey availability). It was expected that individuals from different populations would show differences in developmental traits and that those differences will be associated with lower reproductive success at dry sites. I recorded the phenology and developmental traits of native and transplanted individuals in the field and estimated reproductive success based on clutch size. Colonies from wet versus dry sites showed different phenologies, with individuals at dry sites maturing later. Transplant results suggest plasticity in instar duration caused by environmental effects. Despite differences in resources and spider phenology, clutch sizes of native dry and wet populations were similar. Transplanted individuals, however, were differentially affected. Transplants from wet to dry sites (WD) showed lower growth rates and smaller clutches, whereas transplants from dry to wet sites had larger clutch sizes than in native habitat. Delayed maturation and failure to reproduce in WD individuals is associated with a lower tendency to capture prey in groups and less aggressive interactions during prey capture. Thus, despite negative environmental effects on development, dry native individuals have evolved non-developmental traits that allow successful reproduction.

How dietary plant nectar affects the survival, growth, and fecundity of a cursorial spider Cheiracanthium inclusum (Araneae: Miturgidae)

We measured the effects of plant nectar consumption on Cheiracanthium inclusum (Hentz) (Miturgidae), an agriculturally important spider. Newly emerged spiderlings were reared on the eggs of Helicoverpa zea (Boddie) at four prey densities, 1, 5, 25, or 125 eggs, three times a week, with or without nectar. Nectar came from the extrafloral nectaries of Indian almond, Terminalia cattapa L. (Combretaceae). The addition of nectar to prey (1) allowed spiderlings on the 1-egg diet to survive longer and molt many more times; (2) allowed virtually all of the spiderlings on the 5-egg diet to become small adults and 50% to mate and reproduce versus those without nectar, none of which matured to adulthood; and (3) increased fecundity of females on 5-egg and 25-egg diets to the level of females fed five times the amount of prey. These results show that spiders that feed on nectar increase their fitness with increased survival, growth, and fecundity, particularly when density of prey is inadequate or marginal.

Correlated evolution of colour pattern and body size in polymorphic pygmy grasshoppers, Tetrix undulata

Theory posits that selection on functionally interrelated characters will promote physical and genetic integration resulting in evolution of favourable trait-value combinations. The pygmy grasshopper Tetrix undulata (Orthoptera: Tetrigidae) displays a genetically encoded polymorphism for colour pattern. Colour morphs differ in several traits, including behaviours, thermal biology and body size. To examine if these size differences may reflect phenotypic plasticity of growth and development in response to temperature we used a split brood-design and reared hatchlings from mothers belonging to different morphs in different thermal environments (warm or cold) until maturity. We found that time to maturity was longer in the cold compared with the warm treatment. In the warm (but not in the cold) treatment time to maturity also varied among individuals born to mothers belonging to different colour morphs. Although low temperature and long development time are normally accompanied by increased body size in ectotherms, our results revealed no difference in size at maturity between individuals reared in the two temperature treatments. There was also an increase (not a decrease) in adult body size with shortened time to maturity across families within each treatment. Taken together, this suggests that body size is canalized against environmental perturbations, and that early maturation does not necessarily trade off against a size-mediated decrease in fecundity. Heritability of body size was moderate in magnitude. Moreover, body size at maturity varied among individuals belonging to different morphs and was influenced also by maternal colour morph, suggesting that a genetic correlation exists between colour pattern and body size. These findings suggest that different characters have evolved in concert and that the various colour morphs represent different evolutionary strategies, i.e., alternative peaks in a multi-modal adaptive landscape.

Seasonally varying diet quality and the quantitative genetics of development time and body size in birch feeding insects

Genetic variance-covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the current theories of life-history evolution. However, the evolution of Gs in natural environments has been poorly documented. Resource quality and quantity for many animals and plants vary seasonally, which may shape genetic architectures of their life histories. In the mountain birch-insect herbivore community, leaf quality of birch for insect herbivores declines profoundly during both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage, we tested the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body size. In the species consuming mature birch leaves of stable quality, that is, without diet-imposed time constraints for development time, long development led to high body mass. This was revealed by the strongly positive phenotypic and genetic correlations between the traits. In the species consuming growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae from gaining high body mass after long development. In these species, the phenotypic and genetic correlations between development time and final mass were negative or zero. In the early-summer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obvious positive genetic relationship between development time and final body mass is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild-wide patterns in the genetic architectures brought about by seasonal variation in resource quality.