Measuring probabilistic reaction norms for age and size at maturation

We present a new probabilistic concept of reaction norms for age and size at maturation that is applicable when observations are carried out at discrete time intervals. This approach can also be used to estimate reaction norms for age and size at metamorphosis or at other ontogenetic transitions. Such estimations are critical for understanding phenotypic plasticity and life-history changes in variable environments, assessing genetic changes in the presence of phenotypic plasticity, and calibrating size- and age-structured population models. We show that previous approaches to this problem, based on regressing size against age at maturation, give results that are systematically biased when compared to the probabilistic reaction norms. The bias can be substantial and is likely to lead to qualitatively incorrect conclusions; it is caused by failing to account for the probabilistic nature of the maturation process. We explain why, instead, robust estimations of maturation reaction norms should be based on logistic regression or on other statistical models that treat the probability of maturing as a dependent variable. We demonstrate the utility of our approach with two examples. First, the analysis of data generated for a known reaction norm highlights some crucial limitations of previous approaches. Second, application to the northeast arctic cod (Gadus morhua) illustrates how our approach can be used to shed new light on existing real-world data.     

The selective advantage of reaction norms for environmental tolerance

A tolerance curve defines the dependence of a genotype's fitness on the state of an environmental gradient. It can be characterized by a mode (the genotype's optimal environment) and a width (the breadth of adaptation). It seems possible that one or both of these characters can be modified in an adaptive manner, at least partially, during development. Thus, we extend the theory of environmental tolerance to include reaction norms for the mode and the width of the tolerance curve. We demonstrate that the selective value of such reaction norms increases with increasing spatial heterogeneity and between-generation temporal variation in the environment and with decreasing within-generation temporal variation. Assuming that the maintenance of a high breadth of adaptation is costly, reaction, norms are shown to induce correlated selection for a reduction in this character. Nevertheless, regardless of the magnitude of the reaction norm, there is a nearly one to one relationship between the optimal breadth of adaptation and the within-generation temporal variation perceived by the organism. This suggests that empirical estimates of the breadth of adaptation may provide a useful index of this type of environmental variation from the organism's point of view.     

Plasticity in probabilistic reaction norms for maturation in a salmonid fish

The relationship between body size and the probability of maturing, often referred to as the probabilistic maturation reaction norm (PMRN), has been increasingly used to infer genetic variation in maturation schedule. Despite this trend, few studies have directly evaluated plasticity in the PMRN. A transplant experiment using white-spotted charr demonstrated that the PMRN for precocious males exhibited plasticity. A smaller threshold size at maturity occurred in charr inhabiting narrow streams where more refuges are probably available for small charr, which in turn might enhance the reproductive success of sneaker precocious males. Our findings suggested that plastic effects should clearly be included in investigations of variation in PMRNs.     

The environmental and genetic control of seasonal polyphenism in larval color and its adaptive significance in a swallowtail butterfly

Seasonal polyphenism, in which different forms of a species are produced at different times of the year, is a common form of phenotypic plasticity among insects. Here I show that the production of dark fifth-instar caterpillars of the eastern black swallowtail butterfly, Papilio polyxenes, is a seasonal polyphenism, with larvae reared on autumnal conditions being significantly darker than larvae reared on midsummer conditions. Both rearing photoperiod and temperature were found to have individual and synergistic effects on larval darkness. Genetic analysis of variation among full-sibling families reared on combinations of two different temperatures and photoperiods is consistent with the hypothesis that variation in darkness is heritable. In addition, the genetic correlation in larval darkness across midsummer and autumnal environments is not different from zero, suggesting that differential gene expression is responsible for the increase in larval darkness in the autumn. The relatively dark autumnal form was found to have a higher body temperature in sunlight than did the lighter midsummer form, and small differences in temperature were found to increase larval growth rate. These results suggest that this genetically based seasonal polyphenism in larval color has evolved in part to increase larval growth rates in the autumn.     

Matching field and laboratory environments: effects of neglecting daily temperature variation on insect reaction norms

The tropical butterfly, Bicyclus anynana, exhibits seasonal polyphenism. The wet season form has large eyespots and a pale band while these characters are much less conspicuous or absent in the dry season form. This plasticity is induced in the laboratory by use of a standard series of constant temperatures in the larval stage yielding a continuous norm of reaction. Butterflies in this study were reared from hatchling larvae in seven regimes which differed with respect to thermoperiod or photoperiod. The effect of rearing treatment on the phenotypic plasticity of the adult wing pattern, on life history traits and on larval feeding rhythms was investigated. Photoperiod had little effect except that constant light produced a higher mortality and tended to produce a longer development time. Thermoperiod had a major effect on the life history traits in comparison to a constant temperature regime with the same daily mean: development time was shorter with higher growth rates. The faster development was associated with a substantial shift in the wing pattern towards the wet season form. Larvae feed mostly at night both under constant and thermoperiod (cool nights) conditions. The results are discussed with respect to the necessity of matching field and laboratory environments in studies of norms of reaction or of life history traits where the adaptive significance of the variation is important. Fluctuating conditions in nature, especially with respect to thermoperiod, must be taken into account.     

Quantitative genetic analysis of responses to larval food limitation in a polyphenic butterfly indicates environment‐ and trait‐specific effects

Different components of heritability, including genetic variance (V_G), are influenced by environmental conditions. Here, we assessed phenotypic responses of life‐history traits to two different developmental conditions, temperature and food limitation. The former represents an environment that defines seasonal polyphenism in our study organism, the tropical butterfly Bicyclus anynana, whereas the latter represents a more unpredictable environment. We quantified heritabilities using restricted maximum likelihood (REML) procedures within an “Information Theoretical” framework in a full‐sib design. Whereas development time, pupal mass, and resting metabolic rate showed no genotype‐by‐environment interaction for genetic variation, for thorax ratio and fat percentage the heritability increased under the cool temperature, dry season environment. Additionally, for fat percentage heritability estimates increased under food limitation. Hence, the traits most intimately related to polyphenism in B. anynana show the most environmental‐specific heritabilities as well as some indication of cross‐environmental genetic correlations. This may reflect a footprint of natural selection and our future research is aimed to uncover the genes and processes involved in this through studying season and condition‐dependent gene expression.     

Seasonal selection and resource dynamics in a seasonally polyphenic butterfly

Seasonal polyphenisms are widespread in nature, yet the selective pressures responsible for their evolution remain poorly understood. Previous work has largely focussed either on the developmental regulation of seasonal polyphenisms or putative ‘top‐down’ selective pressures such as predation that may have acted to drive phenotypic divergence. Much less is known about the influence of seasonal variation in resource availability or seasonal selection on optimal resource allocation. We studied seasonal variation in resource availability, uptake and allocation in Araschnia levana L., a butterfly species that exhibits a striking seasonal colour polyphenism consisting of predominantly orange ‘spring form’ adults and black‐and‐white ‘summer form’ adults. ‘Spring form’ individuals develop as larvae in the late summer, enter a pupal diapause in the fall and emerge in the spring, whereas ‘summer form’ individuals develop directly during the summer months. We find evidence for seasonal declines in host plant quality, and we identify similar reductions in resource uptake in late summer, ‘spring form’ larvae. Further, we report shifts in the body composition of diapausing ‘spring form’ pupae consistent with a physiological cost to overwintering. However, these differences do not translate into detectable differences in adult body composition. Instead, we find minor seasonal differences in adult body composition consistent with augmented flight capacity in ‘summer form’ adults. In comparison, we find much stronger signatures of sex‐specific selection on patterns of resource uptake and allocation. Our results indicate that resource dynamics in A. levana are shaped by seasonal fluctuations in host plant nutrition, climatic conditions and intraspecific interactions.     

Allometric plasticity in a polyphenic beetle

Abstract 1. Environmental conditions, such as variation in nutrition, commonly contribute to morphological variation among individuals by affecting body size and the expression of certain morphological traits; however the scaling relationship between a morphological trait and body size over a range of body sizes is generally assumed not to change in response to environmental fluctuation (allometric plasticity), but instead to be constant and diagnostic for a particular trait and species or population. The work reported here examined diet‐induced allometric plasticity in the polyphenic beetle Onthophagus taurus Schreber (1759) (Coleoptera: Scarabaeidae). 2. Male O. taurus vary in body size depending on larval nutrition. Only males above a critical body size threshold express fully developed horns; males smaller than this threshold develop only rudimentary horns or no horns at all. 3. Field populations that naturally utilise two different resources for feeding larvae (horse dung vs. cow manure) exhibited significant differences in the average scaling relationship between body size and male horn length over the same range of body sizes. Males collected from cow manure populations expressed consistently longer horns for a given body size than males collected from horse dung populations. 4. Males reared in the laboratory on horse dung or cow manure also exhibited significant differences in the average scaling relationship between body size and horn length. Differences between laboratory populations reared on horse dung or cow manure were of the same kind and magnitude as differences between field populations that utilise these different resources naturally. 5. These findings suggest that between‐population differences in scaling relationships between horn length and body size can be the product of differences in the quality of resources available to developing larvae. Results are discussed in the context of onthophagine mating systems and recent insights in the developmental and endocrine control of horn polyphenisms.     

Seasonal phenotypic plasticity of wing melanisation in the cabbage white butterfly, Pieris rapae L. (Lepidoptera: Pieridae)

Abstract.  1. Effective thermoregulation is crucial for the fitness of small flying insects. Phenotypic plasticity of the ventral hindwing of pierid butterflies is widely recognised as adaptive for effective thermoregulation. Butterflies eclosing in cooler environments have more heavily melanised wings that absorb solar radiation, thus allowing flight under these cool conditions.
2. Many pierids also exhibit phenotypic plasticity of dorsal forewing melanisation but in this case, cooler environments reduce melanisation. It has been hypothesised that this plasticity is also adaptive because it increases solar reflection from the wing surfaces onto the body in certain basking postures.
3. The degree of seasonal variation in ventral hindwing and dorsal forewing melanisation of wild-caught Pieris rapae was quantified to determine if it shows patterns of plasticity similar to that documented for other Pieris species.
4. Male wing melanisation on both wing surfaces shows the characteristic seasonal, adaptive plasticity. However, only some dorsal forewing pattern elements of females conformed to the predictions of the hypothesis of adaptive dorsal forewing melanisation. Sexual dimorphism of wing pattern plasticity may result from, and/or affect, sexual dimorphism of behaviour and physiology of these butterflies.     

Sex-specific reaction norms to intraspecific larval competition in the mosquito Aedes aegypti

As the relationship between a given life‐history trait and fitness is not necessarily the same for the two sexes, an ‘intersexual ontogenetic conflict’ may arise. We analysed the phenotypic reaction to intraspecific larval competition of the mosquito, Aedes aegypti, asking: (i) Do both sexes pay the cost of competition with the same life‐history traits and are they equal competitors? (ii) Is there a specific cost of competition beyond sharing food resources? We found that competition incurs a specific cost that was expressed differently by the two sexes. Indeed, each sex maintained the more important life‐history trait(s) for their fitness (developmental time for males and body weight and size for females) at the expense of other traits, thus minimizing the effects of competition on their fitness. The competition exerted by females was estimated as being more intense, probably linked with the greater importance of body size for their fitness.     

Variation in reaction norms: Statistical considerations and biological interpretation

Analysis of reaction norms, the functions by which the phenotype produced by a given genotype depends on the environment, is critical to studying many aspects of phenotypic evolution. Different techniques are available for quantifying different aspects of reaction norm variation. We examine what biological inferences can be drawn from some of the more readily applicable analyses for studying reaction norms. We adopt a strongly biologically motivated view, but draw on statistical theory to highlight strengths and drawbacks of different techniques. In particular, consideration of some formal statistical theory leads to revision of some recently, and forcefully, advocated opinions on reaction norm analysis. We clarify what simple analysis of the slope between mean phenotype in two environments can tell us about reaction norms, explore the conditions under which polynomial regression can provide robust inferences about reaction norm shape, and explore how different existing approaches may be used to draw inferences about variation in reaction norm shape. We show how mixed model‐based approaches can provide more robust inferences than more commonly used multistep statistical approaches, and derive new metrics of the relative importance of variation in reaction norm intercepts, slopes, and curvatures.     

Dimorphic growth patterns and sex‐specific reaction norms in the butterfly Lycaena hippothoe sumadiensis

Directly developing larvae of the butterfly Lycaena hippothoe sumadiensis exhibited two growth strategies with one cohort passing four larval instars at high growth rates, and the other five instars at lower growth rates. The 4‐instar‐cohort displayed decreased development times, in combination with slightly reduced pupal and adult weights. In addition to adjustment of growth rate, omitting a larval instar may comprise a further mechanism to decrease development time when needed. Using the 4‐instar‐cohort, sex‐related differences in reaction norms were investigated over a temperature gradient. At high temperatures, protandrous males showed early emergence at a reduced size, whereas weight of females remained similar throughout. These differences suggest that large size is more important for female than for male fitness. The pattern is similar to that previously reported for alpine L. tityrus, indicating that sex‐specific reaction norms might be widespread in species living under severe time constraints.     

An empirical test for a zone of canalization in thermal reaction norms

Theoretical models on the evolution of phenotypic plasticity predict a zone of canalization where reaction norms cross, and genetic variation is minimized in the environment a population most frequently encounter. Empirical tests of this prediction are largely missing, in particular for life‐history traits. We addressed this prediction by quantifying thermal reaction norms of three life‐history traits (somatic growth rate, age and size at maturation) of a Norwegian population of Daphnia magna and testing for the occurrence of an intermediate temperature (T_m) at which genetic variance in the traits is minimized. Size at maturation changed relatively little with temperature compared to the other traits, and there was no genetic variance in the shape of the reaction norm. Consequently, age at maturation and somatic growth rate were strongly negatively correlated. Both traits showed a strong genotype–environment interaction, and the estimated T_m was 14 °C for both age at maturation and growth rate. This value of T_m corresponds well with mean summer temperatures experienced by the population and suggests that the population has evolved under stabilizing selection in temperatures that fluctuate around this mean temperature. These results suggest local adaptation to temperature in the studied population and allow predicting evolutionary trajectories of thermal reaction norms under changing thermal regimes.     

Host plant exodus and larval wandering behaviour in a butterfly: diapause generation larvae wander for longer periods than do non‐diapause generation larvae

1. Prior to pupation, lepidopteran larvae enter a wandering phase lasting up to 30 h before choosing a pupation site. Because stillness is important for concealment, this behaviour calls for an adaptive explanation. 2. The explanation most likely relates to the need to find a suitable pupation substrate, especially in terms of shelter from predation, and given that many predators and parasitoids use host plants as prey‐location cues, mortality probably decreases with distance from the host plant. Hence, remaining on the host includes a long‐term risk, while moving away from the host introduces an increased risk during locomotion. 3. Bivoltine species that overwinter in the pupal stage produce two kinds of pupae; non‐diapausing pupae from which adults emerge after 1–2 weeks, or diapausing pupae that overwinter with adults emerging after 8–10 months. 4. Given the hypothesis of distance‐from‐host‐plant‐related predation, this should select for phenotypic plasticity with larvae in the diapausing generation having a longer wandering phase than larvae under direct development, if there is a trade‐off between mortality during the wandering phase and accumulated mortality during winter. 5. Here this prediction is tested by studying the duration of the wandering period in larvae of the partially bivoltine swallowtail butterfly, Papilio machaon, under both developmental pathways. 6. The results are in agreement with the predictions and show that the larval wandering phase is approximately twice as long under diapause development. The authors suggest that the longer duration of the wandering phase in the diapause generation is a general phenomenon in Lepidoptera.     

The evolution of life histories in spatially heterogeneous environments: Optimal reaction norms revisited

Summary Natural populations live in heterogeneous environments, where habitat variation drives the evolution of phenotypic plasticity. The key feature of population structure addressed in this paper is the net flow of individuals from source (good) to sink (poor) habitats. These movements make it necessary to calculate fitness across the full range of habitats encountered by the population, rather than independently for each habitat. As a consequence, the optimal phenotype in a given habitat not only depends on conditions there but is linked to the performance of individuals in other habitats. We generalize the Euler-Lotka equation to define fitness in a spatially heterogeneous environment in which individuals disperse among habitats as newborn and then stay in a given habitat for life. In this case, maximizing fitness (the rate of increase over all habitats) is equivalent to maximizing the reproductive value of newborn in each habitat but not to maximizing the rate of increase that would result if individuals in each habitat were an isolated population. The new equation can be used to find optimal reaction norms for life history traits, and examples are calculated for age at maturity and clutch size. In contrast to previous results, the optimal reaction norm differs from the line connecting local adaptations of isolated populations each living in only one habitat. Selection pressure is higher in good and frequent habitats than in poor and rare ones. A formula for the relative importance of these two factors allows predictions of the habitat in which the genetic variance about the optimal reaction norm should be smallest.     

Testing the status-dependent ESS model: population variation in fighter expression in the mite Sancassania berlesei

The conditional evolutionarily stable strategy (ESS) with status-dependent tactics is the most commonly invoked ESS for alternative reproductive tactics within the sexes. Support for this model has recently been criticized as apparent rather than real. We address key predictions of the status-dependent ESS in three populations of the male dimorphic mite Sancassania berlesei. In S. berlesei'fighter' males are characterized by a thickened pair of legs used for killing rivals; 'scramblers' are benign. Most males in each population could be manipulated to become fighters by decreasing density, fulfilling the prediction that males make a 'decision'. There was evidence of genetic covariance between sire status and offspring morph, but also a strong effect of sire morph on offspring morph ratio. This was consistent with considerable genetic variation for the status-dependent switch point as a breeding experiment found no support for single-locus inheritance. We also found evidence that switch points evolve independently of distributions of status. This study supports the current status-dependent ESS model.     

Artificial selection on the shape of reaction norms for eyespot size in the butterfly Bicyclus anynana: direct and correlated responses

The tropical butterfly Bicyclus anynana shows phenotypic plasticity in its ventral wing pattern as an adaptive response to wet‐dry seasonality. Wet season form individuals have large eyespots, whereas individuals of the dry season generation have small eyespots. In the laboratory these forms can be obtained by rearing larvae at high and low temperatures, respectively. To study the extent to which the shape of the nearly linear reaction norms for eyespot size can be changed we applied four generations of sib selection by rearing full‐sib families across three temperatures. In addition, we measured ecdysteroid titres shortly after pupation in the final generation. Although phenotypic variation in shape was present initially, the experiment yielded lines with reaction norms with similar shapes but different elevations. High, positive genetic correlation across temperatures can explain this lack of response. Differences in ecdysteroid titres did not readily relate to differences in eyespot size.     

Seasonal plasticity in growth and development of the speckled wood butterfly, Pararge aegeria (Satyrinae)

Regulation of growth and development by photoperiod was studied in a population of the speckled wood butterfly, Purarge aegeria L. (Lepidoptera: Satyrinae), from southern Sweden. Individuals were reared in a range of photoperiodic regimes (9L. to 22L) and temperatures (13°C to 21° C). Plasticity was found for important life-history traits- generation time, growth rate and final weight and seasonal regulation of development in response to photoperiod was found to occur at two levels. Purarge aegeria hibernates as a third instar larva or in the pupal stage, cantering one of four major developmental pathways in response to photoperiod: (1) direct development in both the larval and pupal stages, (2) pupal winter diapause with or (3) without a preceding larval summer diapause, or (4) larval winter diapause. In addition to this high-level regulation of individual development, larval growth rate and pupal development rate also appear to be finally regulated by photoperiod within each major pathway. As photoperiods decreased from 22 h to 17 h at 17° C, growth rate among directly developing larvae increased progressively, as was the case for larva? developing according to a univoltine life cycle from 17 h to 14 h. At two photoperiods, 13 h and 16 h (corresponding to shifts between major pathways), both larval and pupal development were extremely variable with the fastest individuals developing directly and the slowest developing with a diapause. This indicates a gradual nature of diapause itself, suggesting that the two level may not he fundamentally different.     

The diapause decision as a cascade switch for adaptive developmental plasticity in body mass in a butterfly

Switch‐induced developmental plasticity, such as the diapause decision in insects, is a major form of adaptation to variable environments. As individuals that follow alternative developmental pathways will experience different selective environments the diapause decision may evolve to a cascade switch that induces additional adaptive developmental differences downstream of the diapause decision. Here, we show that individuals following alternative developmental pathways in a Swedish population of the butterfly, Pararge aegeria, display differential optimization of adult body mass as a likely response to predictable differences in thermal conditions during reproduction. In a more northern population where this type of selection is absent no similar difference in adult mass among pathways was found. We conclude that the diapause decision in the southern population appears to act as a cascade switch, coordinating development downstream of the diapause decision, to produce adult phenotypes adapted to the typical thermal conditions of their expected reproductive period.