Developmental plasticity and acclimation both contribute to adaptive responses to alternating seasons of plenty and of stress in <Emphasis Type="Italic">Bicyclus</Emphasis> butterflies

Plasticity is a crucial component of the life cycle of invertebrates that live as active adults throughout wet and dry seasons in the tropics. Such plasticity is seen in the numerous species of Bicyclus butterflies in Africa which exhibit seasonal polyphenism with sequential generations of adults with one or other of two alternative phenotypes. These differ not only in wing pattern but in many other traits. This divergence across a broad complex of traits is associated with survival and reproduction either in a wet season that is favourable in terms of resources, or mainly in a dry season that is more stressful. This phenomenon has led us to examine the bases of the developmental plasticity in a model species, B. anynana, and also the evolution of key adult life history traits, including starvation resistance and longevity. We now understand something about the processes that generate variation in the phenotype, and also about the ecological context of responses to environmental stress. The responses clearly involve a mix of developmental plasticity as cued by different environments in pre-adult development, and the acclimation of life history traits in adults to their prevailing environment.     

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.     

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.     

Translating environmental gradients into discontinuous reaction norms via hormone signalling in a polyphenic butterfly

Polyphenisms—the expression of discrete phenotypic morphs in response to environmental variation—are examples of phenotypic plasticity that may potentially be adaptive in the face of predictable environmental heterogeneity. In the butterfly Bicyclus anynana, we examine the hormonal regulation of phenotypic plasticity that involves divergent developmental trajectories into distinct adult morphs for a suite of traits as an adaptation to contrasting seasonal environments. This polyphenism is induced by temperature during development and mediated by ecdysteroid hormones. We reared larvae at separate temperatures spanning the natural range of seasonal environments and measured reaction norms for ecdysteroids, juvenile hormones (JHs) and adult fitness traits. Timing of peak ecdysteroid, but not JH titres, showed a binary response to the linear temperature gradient. Several adult traits (e.g. relative abdomen mass) responded in a similar, dimorphic manner, while others (e.g. wing pattern) showed a linear response. This study demonstrates that hormone dynamics can translate a linear environmental gradient into a discrete signal and, thus, that polyphenic differences between adult morphs can already be programmed at the stage of hormone signalling during development. The range of phenotypic responses observed within the suite of traits indicates both shared regulation and independent, trait-specific sensitivity to the hormone signal.     

Effects of temperature on reproductive output, egg provisioning, juvenile hormone and vitellogenin titres in the butterfly Bicyclus anynana

Environmentally induced phenotypic plasticity is common in nature. Hormones, affecting multiple traits and signaling to a variety of distant target tissues, provide a mechanistic link between environments, genes and trait expression, and may therefore well be involved in the regulation phenotypic plasticity. Here, we investigate whether in the tropical butterfly Bicyclus anynana temperature-mediated plasticity in egg size and number, with fewer but larger eggs produced at lower temperatures and vice versa, is under control of juvenile hormone, and whether different temperatures cause differences in egg composition. Female B. anynana butterflies showed the expected response to temperature, however, we found no evidence for an involvement of juvenile hormone. Neither haemolymph JH II and JH III titres nor vitellogenin levels differed across temperatures. The smaller eggs produced at the higher temperature contained relatively higher amounts of water, free carbohydrates and proteins, but relatively lower amounts of lipids. While these smaller eggs had a lower absolute energy content, total reproductive investment was higher at the higher temperature (due to a higher fecundity). Overall, our study indicates that temperature-mediated plasticity in reproduction in B. anynana is mechanistically related to a biophysical model, with oocyte production (differentiation) and oocyte growth (vitellogenesis) having differential temperature sensitivities.     

Phenotypic plasticity in field populations of the tropical butterfly Hypolimnas bolina (L.) (Nymphalidae)

Phenotypic plasticity may enable organisms to maximize their fitness in seasonally variable environments. However, in butterflies, seasonal polyphenism is often striking but functionally obscure. This paper addresses the possible adaptive significance of phenotypic variation in the tropical butterfly Hypolimnas bolina (L.) (Nymphalidae). Plasticity in body size and wing coloration can be elicited in this species under laboratory conditions, however it is not known how this plasticity is expressed in the wild. Moreover, adult H. bolina spend the winter dry season in a reproductive diapause, which allows certain predictions regarding the occurrence of seasonal plasticity. Based on consideration of the requirements of diapausing and directly developing individuals, we predicted that if seasonal plasticity in phenotype were adaptive, then overwintering individuals should be larger and darker than their directly developing counterparts. This prediction was largely - although not entirely - fulfilled. Dry season butterflies were duller and darker than their wet season counterparts (this plasticity was superimposed on a genetic colour polymorphism), however size plasticity varied geographically. Dry season adults were consistently larger than wet season adults in the tropical north, but not in the south. We use these findings to discuss the possible adaptive significance of seasonal variation in the colour and size of this tropical butterfly.     

Environmental determinants of population divergence in life‐history traits for an invasive species: climate,seasonality and natural enemies

Invasive species cope with novel environments through both phenotypic plasticity and evolutionary change. However, the environmental factors that cause evolutionary divergence in invasive species are poorly understood. We developed predictions for how different life‐history traits, and plasticity in those traits, may respond to environmental gradients in seasonal temperatures, season length and natural enemies. We then tested these predictions in four geographic populations of the invasive cabbage white butterfly (Pieris rapae) from North America. We examined the influence of two rearing temperatures (20 and 26.7 °C) on pupal mass, pupal development time, immune function and fecundity. As predicted, development time was shorter and immune function was greater in populations adapted to longer season length. Also, phenotypic plasticity in development time was greater in regions with shorter growing seasons. Populations differed significantly in mean and plasticity of body mass and fecundity, but these differences were not associated with seasonal temperatures or season length. Our study shows that some life‐history traits, such as development time and immune function, can evolve rapidly in response to latitudinal variation in season length and natural enemies, whereas others traits did not. Our results also indicate that phenotypic plasticity in development time can also diverge rapidly in response to environmental conditions for some traits.     

On the fate of seasonally plastic traits in a rainforest butterfly under relaxed selection

Many organisms display phenotypic plasticity as adaptation to seasonal environmental fluctuations. Often, such seasonal responses entails plasticity of a whole suite of morphological and life‐history traits that together contribute to the adaptive phenotypes in the alternative environments. While phenotypic plasticity in general is a well‐studied phenomenon, little is known about the evolutionary fate of plastic responses if natural selection on plasticity is relaxed. Here, we study whether the presumed ancestral seasonal plasticity of the rainforest butterfly Bicyclus sanaos (Fabricius, 1793) is still retained despite the fact that this species inhabits an environmentally stable habitat. Being exposed to an atypical range of temperatures in the laboratory revealed hidden reaction norms for several traits, including wing pattern. In contrast, reproductive body allocation has lost the plastic response. In the savannah butterfly, B. anynana (Butler, 1879), these traits show strong developmental plasticity as an adaptation to the contrasting environments of its seasonal habitat and they are coordinated via a common developmental hormonal system. Our results for B. sanaos indicate that such integration of plastic traits – as a result of past selection on expressing a coordinated environmental response – can be broken when the optimal reaction norms for those traits diverge in a new environment.     

Rearing Temperature Influences Adult Response to Changes in Mating Status

Rearing environment can have an impact on adult behavior, but it is less clear how rearing environment influences adult behavior plasticity. Here we explore the effect of rearing temperature on adult mating behavior plasticity in the butterfly Bicyclus anynana, a species that has evolved two seasonal forms in response to seasonal changes in temperature. These seasonal forms differ in both morphology and behavior. Females are the choosy sex in cohorts reared at warm temperatures (WS butterflies), and males are the choosy sex in cohorts reared at cooler temperatures (DS butterflies). Rearing temperature also influences mating benefits and costs. In DS butterflies, mated females live longer than virgin females, and mated males live shorter than virgin males. No such benefits or costs to mating are present in WS butterflies. Given that choosiness and mating costs are rearing temperature dependent in B. anynana, we hypothesized that temperature may also impact male and female incentives to remate in the event that benefits and costs of second matings are similar to those of first matings. We first examined whether lifespan was affected by number of matings. We found that two matings did not significantly increase lifespan for either WS or DS butterflies relative to single matings. However, both sexes of WS but not DS butterflies experienced decreased longevity when mated to a non-virgin relative to a virgin. We next observed pairs of WS and DS butterflies and documented changes in mating behavior in response to changes in the mating status of their partner. WS but not DS butterflies changed their mating behavior in response to the mating status of their partner. These results suggest that rearing temperature influences adult mating behavior plasticity in B. anynana. This developmentally controlled behavioral plasticity may be adaptive, as lifespan depends on the partner’s mating status in one seasonal form, but not in the other.     

Differential Expression of Ecdysone Receptor Leads to Variation in Phenotypic Plasticity across Serial Homologs

Bodies are often made of repeated units, or serial homologs, that develop using the same core gene regulatory network. Local inputs and modifications to this network allow serial homologs to evolve different morphologies, but currently we do not understand which modifications allow these repeated traits to evolve different levels of phenotypic plasticity. Here we describe variation in phenotypic plasticity across serial homologous eyespots of the butterfly Bicyclus anynana, hypothesized to be under selection for similar or different functions in the wet and dry seasonal forms. Specifically, we document the presence of eyespot size and scale brightness plasticity in hindwing eyespots hypothesized to vary in function across seasons, and reduced size plasticity and absence of brightness plasticity in forewing eyespots hypothesized to have the same function across seasons. By exploring the molecular and physiological causes of this variation in plasticity across fore and hindwing serial homologs we discover that: 1) temperature experienced during the wandering stages of larval development alters titers of an ecdysteroid hormone, 20-hydroxyecdysone (20E), in the hemolymph of wet and dry seasonal forms at that stage; 2) the 20E receptor (EcR) is differentially expressed in the forewing and hindwing eyespot centers of both seasonal forms during this critical developmental stage; and 3) manipulations of EcR signaling disproportionately affected hindwing eyespots relative to forewing eyespots. We propose that differential EcR expression across forewing and hindwing eyespots at a critical stage of development explains the variation in levels of phenotypic plasticity across these serial homologues. This finding provides a novel signaling pathway, 20E, and a novel molecular candidate, EcR, for the regulation of levels of phenotypic plasticity across body parts or serial homologs.     

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.     

Restoring riparian meadows currently dominated by Artemisia using alternative state concepts ‐ the establishment component

Abstract. We evaluated the potential for restoring riparian grass and sedge meadows currently dominated by Artemisia tridentata var. tridentata with an experiment in which we burned sites with low, intermediate, and high water tables, i.e., dry, intermediate, and wet sites. To define the alternative states and thresholds for these ecosystems, we examined burning and water table effects on both abiotic variables and establishment of grasses adapted to relatively high (Poa se‐cunda ssp. juncifolia), intermediate (Leymus triticoides), or low (L. cinereus) water tables. Wet sites had lower soil temperatures and higher soil water contents than dry sites. Burning increased soil temperatures on all sites. Undershrub microsites on control plots had the lowest temperatures, while former undershrub microsites on burn plots had the highest temperatures. Surface soil water was low on burn plots early in the growing season due to desiccation, but higher at deeper depths after active plant growth began. Emergence was generally greater on wet sites, but survival was microsite‐ and species‐specific. Undershrub microsites on control plots facilitated emergence and first‐year survival, but seedlings that survived initially harsh conditions on burn plots had similar numbers alive at the end. In general, favorable environments and establishment of species adapted to mesic conditions indicate that wet sites represent an alternative state of the naturally occurring dry meadow ecosystem type, and can be restored to grass and sedge meadows. Harsh environments and lack of establishment of species adapted to mesic conditions indicate that dry sites have crossed a threshold and may represent a new ecosystem type. Understory vegetation and seed banks on dry sites have been depleted, and restoration will require burning and reseeding with species adapted to more xeric conditions.     

Seasonal variation in basal and plastic cold tolerance: Adaptation is influenced by both long‐ and short‐term phenotypic plasticity

Understanding how thermal selection affects phenotypic distributions across different time scales will allow us to predict the effect of climate change on the fitness of ectotherms. We tested how seasonal temperature variation affects basal levels of cold tolerance and two types of phenotypic plasticity in Drosophila melanogaster. Developmental acclimation occurs as developmental stages of an organism are exposed to seasonal changes in temperature and its effect is irreversible, while reversible short‐term acclimation occurs daily in response to diurnal changes in temperature. We collected wild flies from a temperate population across seasons and measured two cold tolerance metrics (chill‐coma recovery and cold stress survival) and their responses to developmental and short‐term acclimation. Chill‐coma recovery responded to seasonal shifts in temperature, and phenotypic plasticity following both short‐term and developmental acclimation improved cold tolerance. This improvement indicated that both types of plasticity are adaptive, and that plasticity can compensate for genetic variation in basal cold tolerance during warmer parts of the season when flies tend to be less cold tolerant. We also observed a significantly stronger trade‐off between basal cold tolerance and short‐term acclimation during warmer months. For the longer‐term developmental acclimation, a trade‐off persisted regardless of season. A relationship between the two types of plasticity may provide additional insight into why some measures of thermal tolerance are more sensitive to seasonal variation than others.     

The role of El Niño–Southern Oscillation in the dynamics of a savanna large herbivore population

Our understanding of large‐scale climatic phenomena and dynamics of large herbivore populations comes principally from research in northern regions with temperate, seasonal climate and animal communities with relatively low species diversity. To assess the generality of that perspective, we investigated effects of El Niño–Southern Oscillation (ENSO) on population dynamics of African buffalo Syncerus caffer inhabiting a semi‐arid savanna with variable rainfall. We used linear and nonlinear‐threshold models to investigate relationships between population parameters and explanatory variables affecting forage conditions (seasonal rainfall, Southern Oscillation Index [SOI]). El Niño‐related droughts in 1982–1983 and 1991–1992 were associated with strongly negative population change, a pattern expected to coincide with a decrease in normally high and constant adult survival. Consistent with that nonlinear pattern, we detected threshold relationships between wet‐season rainfall and population change. Juvenile recruitment was described best by linear relationships with dry‐season. Because ENSO operates primarily through wet‐season rainfall, whereas population dynamics were also related to dry‐season rainfall, SOI did not have the predictive ability of individual weather components.     

A natural heating experiment: Phenotypic and genotypic responses of plant phenology to geothermal soil warming

Under global warming, the survival of many populations of sedentary organisms in seasonal environments will largely depend on their ability to cope with warming in situ by means of phenotypic plasticity or adaptive evolution. This is particularly true in high‐latitude environments, where current growing seasons are short, and expected temperature increases large. In such short‐growing season environments, the timing of growth and reproduction is critical to survival. Here, we use the unique setting provided by a natural geothermal soil warming gradient (Hengill geothermal area, Iceland) to study the response of Cerastium fontanum flowering phenology to temperature. We hypothesized that trait expression and phenotypic selection on flowering phenology are related to soil temperature, and tested the hypothesis that temperature‐driven differences in selection on phenology have resulted in genetic differentiation using a common garden experiment. In the field, phenology was related to soil temperature, with plants in warmer microsites flowering earlier than plants at colder microsites. In the common garden, plants responded to spring warming in a counter‐gradient fashion; plants originating from warmer microsites flowered relatively later than those originating from colder microsites. A likely explanation for this pattern is that plants from colder microsites have been selected to compensate for the shorter growing season by starting development at lower temperatures. However, in our study we did not find evidence of variation in phenotypic selection on phenology in relation to temperature, but selection consistently favoured early flowering. Our results show that soil temperature influences trait expression and suggest the existence of genetically based variation in flowering phenology leading to counter‐gradient local adaptation along a gradient of soil temperatures. An important implication of our results is that observed phenotypic responses of phenology to global warming might often be a combination of short‐term plastic responses and long‐term evolutionary responses, acting in different directions.     

Seasonal metabolism of a small, arboreal monitor lizard, Varanus scalaris, in tropical Australia

The field metabolic rates (FMR) and water fluxes of Varanus scalaris were measured during the wet and dry seasons by the doubly-labelled water technique. Seasonal measurements of standard (night-time) metabolism (SMR) and resting (daytime) metabolism (RMR) were made in the laboratory at 18, 24, 30 and 36°C, and maximal oxygen consumption was measured at 36°C on a motorized treadmill. This population was active throughout the year. In the wet season, the mean FMR was 7.8 kJ day⁻¹ (128.0 kJkg⁻¹ day⁻¹; mean mass = 66.4 g, n = 13), and during the dry season the mean was 5.0 kJ day⁻¹ (67.6 kJ kg⁻¹ day⁻¹; mean mass = 77.4 g, n = 17). The mean water flux rates for these animals were 3.6 and 1.2 ml day⁻¹, respectively (60.4 and 16.6 ml kg⁻¹ day⁻¹). The seasonal means of FMR and water flux were significantly different by ANCOVA ( P < 0.0001). Measurements of SMR and RMR were significantly higher in the wet season (ANCOVA: P < 0.0001), but we found no difference in the maximal oxygen consumption between seasons (ANCOVA: P = 0.6). The maximal oxygen consumption of the lizards on the treadmill (2.9 ml min⁻¹= 1.8 ml g⁻¹ h⁻¹), mean mass = 97.4 g, n = 16) was 20 times that of the SMR at the same temperature during the dry season, and 11 times that of the SMR during the wet season. The seasonal differences in FMR were attributable to: changes in SMR (12.2%) and RMR (16.4%); differences in night-time body temperatures (11.3) and daytime body temperatures (16.4%); and activity (broadly defined to include locomotion, digestion, and reproductive costs (43.7%).     

Phenotypic plasticity, seasonal climate and the population biology of Bicyclus butterflies (Satyridae) in Malawi

Abstract.

• 1 Seasonal polyphenism is studied in a community of five African butterflies of the genus Bicyclus at the transition between a wet and a dry season from May to July.
• 2 Butterflies characterized by large eyespots and, especially in B.sufitza (Hewitson), a pale band (the wet season form) are replaced over this period by butterflies lacking conspicuous wing markings (the dry season form, dsf). The latter butterflies also tend to be larger, but more variable in size. Butterflies of an intermediate phenotype are recruited over a comparatively short interim period.
• 3 This turnover coincides with a period of declining temperature and drying of the habitat, including the grasses on which larvae feed. Butterflies are progressively more likely to rest on brown leaf litter rather than on green herbage.
• 4 A relationship with temperature is supported by laboratory experiments with B.saJitza and B.anynana (Butler) showing that increasingly extreme dsf butterflies develop with decreasing rearing temperature in the final larval instar.
• 5 Some differences in behaviour and activity were observed between the seasonal forms. Butterflies of the dsf develop ovarian dormancy and fat bodies. They can survive to reproduce at the beginning of the rains in November.
• 6 Capture-recapture experiments showed that the adult butterflies have a comparatively long life expectancy and are quite sedentary.
• 7 The results are discussed in relation to a hypothesis linking the polyphenism to seasonal changes in resting background and selection for crypsis.

     

Seasonality of reproduction by liverbearing fishes in tropical rainforest streams

Reproductive ecology, population structure, and diets of three common livebearing poeciliid fishes (Alfaro cultratus, Phallichthys amates, Poecilia gilli) from rainforest streams in Costa Rica were investigated over ten continuous months. The region experiences little annual temperature variation, and although monthly rainfall is continuous each year, two brief dry seasons typically occur. Monthly changes in indices of ovarian condition, percentages of females with developing embryos, and population size structure revealed that reproductive output by females of all three species varied seasonally. Based on testicular condition, males were reproductively active year-round, however the mean gonadal index for males of two algivorous species showed low levels of seasonal cycling that largely coincided with female variation in reproductive effort. All three species had seasonal differences in the female size-brood size relationship, whereby larger females tended to carry more embryos during the wet season. Several important adult and neonate food resources are more available in the flooded forest during the wet season, which is also the period when conspecifics and predators are at their lowest per-area densities. Three hypotheses are discussed: (1) brood size in relation to conspecific density-mating frequency, (2) reproductive allocation in response to variation in adult food resources, and (3) selection for greater reproductive effort during conditions optimal for juvenile growth and survival. Data for Alfaro were consistent with the latter two hypotheses. In Phallichthys and Poecilia, diets were poorer during wet seasons, indicating that reproductive effort does not coincide with availability of adult food resources, and that selection probably favors greater reproductive effort during periods optimal for juvenile growth and survival.     

Metabolic rate and food availability of the Antarctic amphipod <Emphasis Type="Italic">Gondogeneia antarctica</Emphasis> (Chevreux 1906): seasonal variation in allometric scaling and temperature dependence

Among the few existing works on seasonal variation in metabolic rate of polar species, most have been conducted during summer due to logistic constraints and have been focused on species that cease feeding during winter. In this work, we present the first extensive data set on the seasonal variation in metabolic rate of G. antarctica, an abundant amphipod that feeds throughout the year, and its relationship with body size, potential food availability and temperature. We measured the resting metabolic rate (RMR) of groups of individuals during 6 months from late summer through winter at 4 experimental temperatures and for a wide range of body size. RMR had a negative allometric scaling with body size and showed a tendency to increase with temperature as expected. However, temperature and body size effects on RMR showed a significant temporal variation, and an increase in temperature decreased scaling exponents. RMR at the mean seawater temperature throughout the study showed a strong seasonal variation following food availability: RMR decreased from the end of summer through winter, coinciding with a reduction in microphytobenthos stock, but recovered summer values in August, when an epontic algae boom occurred. The seasonal factorial aerobic scope (×2.37) is lower than benthic Antarctic invertebrates that cease feeding during winter, in agreement with what is expected based on theoretical grounds. Results suggest that seasonal variation of RMR would allow G. antarctica to achieve a high efficiency in energy utilization, while maintaining the ability to exploit sudden changes in food supply.