Role of ageing and temperature in shaping reaction norms and fecundity functions in insects

The existing energy partitioning models assume that fecundity is constant throughout adult life. In insects, however, fecundity is a triangular function of time: after maturation, it initially sharply increases and after reaching its maximum it slowly declines as the mother ages. These models also fail to explain that empirical data generally indicate an increase in juvenile growth rate caused by improvement in food quality results in larger adults, whereas that caused by an increase in ambient temperature results in smaller adults. This ‘life history puzzle’ has worried many biologists for a long time. An energy‐partitioning model for insects is presented with soma and gonads as its components, which – contrary to other models – assumes ageing of soma. This model explains the triangular shape of the fecundity function, and also offers an explanation of the ‘life history puzzle’. The differential response in adult size to changes in food quality and temperature in nature may result from the differential responses of our model’s parameters to changes in these environmental parameters. Better food quality results in bigger adults, because food quality affects the assimilation rate, but not the rate of conversion of gonadal biomass into offspring, or the rate of senescence. In contrast, an increase in temperature speeds up all the processes. That is, temperature affects the assimilation rate, the conversion rate of gonadal biomass into offspring, and the rate of senescence equally. Therefore, an increase in temperature results in larger or smaller adults, depending on the shape of the senescence function.     

Individual variation in parental care reaction norms: integration of personality and plasticity

Personality (consistent differences between individuals in behavior) and plasticity (changes within individuals in behavior) are often viewed as separate and opposing phenomena. We tested this idea by analyzing parental care reaction norms in a bird that exhibits biparental care. Personality in provisioning behavior existed ([Formula: see text]) and persisted despite being reduced after accounting for individual differences in environment. Plasticity was also evident and differed between the sexes. Male visit rate was associated with changes in brood size and time of day, but female visit rate was associated with changes in nestling age and date. In both sexes changes in visit rate were positively correlated with changes in their partner's visit rate. Both sexes also exhibited multidimensional reaction norms; interaction terms revealed that within-individual visit rates increased more steeply with brood size when nestlings were older, and the effect of the partner's visit rate was sensitive to variation in date, precipitation, and the focal bird's age. Individuals also varied in how they responded (reaction norm slope) to changes in nestling age and partner visits. Moreover, parental personality was interdependent with individual plasticity in several ways. Individuals of both sexes with a high visit rate also responded more positively to changes in nestling age, and males also showed this pattern with changes in partner visit rate. Explicit use of the behavioral reaction norm integrated personality and plasticity, revealed that these are not opposing concepts, and stimulated new hypotheses about sexual conflict over care and provisioning as a life-history trait.     

Latitudinal variation in wild populations of Drosophila melanogaster: heritabilities and reaction norms

Large amounts of genetic variation for wing length and wing area were demonstrated both within and between Drosophila melanogaster populations along a latitudinal gradient in South America. Wing length and wing area showed a strong positive correlation with latitude in both wild flies and laboratory-raised descendants. Large population differences were observed for heritability and coefficient of variation of these two traits, whereas relatively small population differences were found for development time, viability, pupal mortality, sex ratio and their norms of reaction to four developmental temperatures. No clear-cut latitudinal clines were established for these life-history characters. These results are discussed in the light of Bergmann's Rule and the relation between larval development and adult body size.     

Latitudinal and voltinism compensation shape thermal reaction norms for growth rate

Latitudinal variation in thermal reaction norms of key fitness traits may inform about the response of populations to climate warming, yet their adaptive nature and evolutionary potential are poorly known. We assessed the contribution of quantitative genetic, neutral genetic and environmental effects to thermal reaction norms of growth rate for populations of the damselfly Ischnura elegans. Among populations, reaction norms differed primarily in elevation, suggesting that time constraints associated with shorter growth seasons in univoltine, high-latitude as well as multivoltine, low-latitude populations selected for faster growth rates. Phenotypic divergence among populations is consistent with selection rather than drift as Q(ST) was greater than F(ST) in all cases. Q(ST) estimates increased with experimental temperature and were influenced by genotype by environment interactions. Substantial additive genetic variation for growth rate in all populations suggests that evolution of trait means in different environments is not constrained. Heritability of growth rates was higher at high temperature, driven by increased genetic rather than environmental variance. While environment-specific nonadditive effects also may contribute to heritability differences among temperatures, maternal effects did not play a significant role (where these could be accounted for). Genotype by environment interactions strongly influenced the adaptive potential of populations, and our results suggest the potential for microevolution of thermal reaction norms in each of the studied populations. In summary, the observed latitudinal pattern in growth rates is adaptive and results from a combination of latitudinal and voltinism compensation. Combined with the evolutionary potential of thermal reaction norms, this may affect populations' ability to respond to future climate warming.     

Intraspecific variation in food plant favourability to phytophagous insects: psyllids on Eucalyptus blakelyi M.

Abstract.

• 1 A combination of variables reflecting leaf quality provided the best predictive model explaining differences in population trend of Glycaspis spp. among individuals of the food plant Eucalyptus blakelyi, measured under field conditions on an egg to egg basis. The equation incorporated the product of: (1) proportion of foliage in each age class; (2) probability of foliage escape from herbivory; (3) preference for the foliage class measured as the proportion of eggs oviposited on it by the female; these variables being summed over the young and mature age classes.
• 2 In addition, significant and consistent differences in insect survivorship among trees were demonstrated by transfer experiments. These experiments established that young foliage was preferable to mature foliage for first instar survival. Differences in insect survivorship also showed a consistent pattern among the study trees. The most probable explanation for this consistent pattern was the influence of the trees on the insects, particularly physiological or biochemical differences among trees. It was not, however, possible to identify the cause of the differences.

     

Intraspecific variation in M1 enamel development in modern humans: implications for human evolution

The timing and sequence of enamel development, as well as enamel thickness, was documented for individual cusps (protoconid, hypoconid, metaconid, entoconid) in 15 unworn permanent lower first molars (M(1)s) from a sample of modern human juveniles. These data were compared with previously published data for modern and fossil species reported in the literature. Crown formation in all teeth was initiated in the protoconid and completed in the hypoconid. These cusps had significantly longer formation times (2.91 and 2.96 yrs, respectively) than the metaconid and entoconid (2.52 and 2.38 yrs, respectively), as well as thicker enamel, and each represented between 92-95% of the total crown formation time. Rates of enamel secretion in all cusps increased significantly from 2.97 microm in the inner enamel to 4.47 microm in the outer enamel. Two cusps of one individual were studied in more detail and did not follow this typical trajectory. Rather, there was a sharp decrease in the middle of enamel formation and then a slow recovery of secretion rates from the mid- to outer enamel. This anomalous trajectory of enamel formation is discussed in the context of other nondental tissue responses to illness. Neither secretion rates nor periodicity differed significantly when compared between the cusps of each molar. Differences in cusp formation times, initiation, and completion suggest a relationship between the rates of enamel formation and enamel thickness. This fits with expectations about the mechanics of the chewing cycle and general lower molar morphology. A comparison with similar data for some nonhuman primates and fossil hominoids suggests this relationship may hold true across several primate taxa. Other aspects of enamel growth differed between this human sample and certain fossil species. The lower molars formed slowly over a longer period of time, which may reflect the extended growth period of modern humans. The methodological approach adopted in this study is discussed in the context of that used in other studies.     

Host plant adaptation and the evolution of thermal reaction norms

For most ectotherms, increasing the rearing temperature reduces the final (adult) body size, producing a negative slope for the thermal reaction norm. Recent studies show that this relationship may be reversed under conditions of low resource quality, producing a positive slope for the thermal reaction norm. If populations or species differ in the degree of evolutionary adaptation to a resource, how does this differential adaptation alter their thermal reaction norms? We used a common garden experiment with the tobacco hornworm, Manduca sexta, to address this question. We examined the thermal reaction norms for body size of two populations of M. sexta that differ in their evolutionary exposures to an atypical, low-quality resource (devil's claw; Proboscidea louisianica), but have comparable exposures to a typical, high-quality resource (tobacco; Nicotiana tabacum). Both populations had increased mean larval mortalities and development times when reared on devil's claw compared with tobacco, but the magnitudes of these increases differed between populations. Both populations had similar, negatively sloped thermal reaction norms on the typical, high-quality resource (tobacco), but had divergent, non-negative thermal reaction norms on the atypical, low-quality resource (devil's claw): the population with the longer evolutionary history of exposure to the atypical resource exhibited a flat (rather than positive) reaction norm. These results suggest that population differences in host plant adaptation can predictably influence the slopes of thermal reaction norms.     

Photoperiod modifies thermal reaction norms for growth and development in the red poplar leaf beetle Chrysomela populi (Coleoptera: Chrysomelidae)

Regression lines of development rate on temperature appeared significantly different between long (22 h) and short (12 h) day conditions and intersected each other at 23.8 °С. Thus, the rate of growth and development was higher at temperatures below the intersection point under short-day but above the intersection point it was higher under long day. Ecological relevance of this effect seems as follows: in autumn, as nights become longer and average daily temperature decreases, larvae have to speed up their development because it is only imago that overwinters. Conversely, midsummer offers long days and usually higher temperature, so again it is advantageous to develop as fast as possible in order to have at least one more generation per year. These results are compared with other studies showing interactions between photoperiod and temperature, and some possible general patterns are outlined. The lower thermal threshold for larval development depended on photoperiodic conditions; therefore rate isomorphy must be violated in this species. Development at higher temperatures generally resulted in smaller adults, as is usual with ectotherms according to the “temperature-size rule”, but body weight depended significantly on temperature only under short day. Our estimates of the lower temperature thresholds for growth and development in both cases did conform to the generalization made previously by Walters and Hassall (2006) in spite of another formula used by us. We briefly discuss this phenomenon and argue that relative position of these thresholds can be explained mathematically and per se may lack any biological sense.     

Relating environmental variation to selection on reaction norms: an experimental test

Theoretical models predict that selection on reaction norms should depend on the relative frequency of environmental states experienced by a population. We report a laboratory experimental test of this prediction for thermal performance curves of larval growth rate in Pieris rapae in relation to their thermal environment. We measured short-term relative growth rate (RGR) for each individual at a series of five temperatures, and then we assigned individuals randomly to warm or cool selection treatments, which differ in the frequency distributions of environmental temperatures. Selection gradient analyses of two independent experiments demonstrated significant positive selection for increasing RGR, primarily through its effects on survival to adulthood and on development rate. In both the warm and cool selection treatments, the magnitude of directional selection on RGR was consistently greater at lower (suboptimal) temperatures than at higher temperatures; differences in selection between the treatments did not match model predictions. The temporal order and duration of environmental conditions may affect patterns of selection on thermal performance curves and other continuous reaction norms, complicating the connections between variation in environment, phenotype, and fitness.     

Intraspecific variation for resource use in Drosophila

Individuals within natural populations of Drosophila tripunctata and D. falleni varied significantly in their responses to alternative types of food baits; no such variation was detected in D. putrida. The food type at which laboratory-reared D. melanogaster tended to settle in the field was influenced by the type of food on which they had been kept prior to release. Although flies apparently forgot their previous experience within hours, its effect on their behaviour can be manifested for at least 1 day in the field. Mark -recapture experiments on wild flies are shown to be very inefficient at detecting individual variation in behaviour.     

New approaches to an understanding of hemodynamic norms]

Tetrapolar rheography was used to investigate central hemodynamics parameters in a group of 80 healthy subjects aged 25-68. Hyperkinetic, eukinetic and hypokinetic types of circulation were determined by interval distribution according to cardiac index. The proportion of the examinees by their hemodynamic type was: 16.3:63.7:20.0%. In hyperkinetic type the optimal level of average blood pressure was provided by a relatively high cardiac output (CO) and low total and specific peripheral resistance (TPR, SPR). The hypokinetic type of hemodynamics was characterized by a low level of CO and a high value of TPR and SPR. Subjects with eukinetic hemodynamic type were in an intermediate position. It is suggested to take into consideration the hemodynamic type in hypertensive subjects for drug selection and treatment control.     

Rapid population divergence in thermal reaction norms for an invading species: breaking the temperature-size rule

The temperature-size rule is a common pattern of phenotypic plasticity in which higher temperature during development results in a smaller adult body size (i.e. a thermal reaction norm with negative slope). Examples and exceptions to the rule are known in multiple groups of organisms, but rapid population differentiation in the temperature-size rule has not been explored. Here we examine the genetic and parental contributions to population differentiation in thermal reaction norms for size, development time and survival in the Cabbage White Butterfly Pieris rapae, for two geographical populations that have likely diverged within the past 150 years. We used split-sibship experiments with two temperature treatments (warm and cool) for P. rapae from Chapel Hill, NC, and from Seattle, WA. Mixed-effect model analyses demonstrate significant genetic differences between NC and WA populations for adult size and for thermal reaction norms for size. Mean adult mass was 12-24% greater in NC than in WA populations for both temperature treatments; mean size was unaffected or decreased with temperature (the temperature-size rule) for the WA population, but size increased with temperature for the NC population. Our study shows that the temperature-size rule and related thermal reaction norms can evolve rapidly within species in natural field conditions. Rapid evolutionary divergence argues against the existence of a simple, general mechanistic constraint as the underlying cause of the temperature-size rule.     

Plasticity of size and growth in fluctuating thermal environments: comparing reaction norms and performance curves

Ectothermic animals exhibit two distinct kinds of plasticityin response to temperature: Thermal performance curves (TPCs),in which an individual's performance (e.g., growth rate) variesin response to current temperature; and developmental reactionnorms (DRNs), in which the trait value (e.g., adult body sizeor development time) of a genotype varies in response to developmentaltemperatures experienced over some time period during development.Here we explore patterns of genetic variation and selectionon TPCs and DRNs for insects in fluctuating thermal environments.First, we describe two statistical methods for partitioningtotal genetic variation into variation for overall size or performanceand variation in plasticity, and apply these methods to availabledatasets on DRNs and TPCs for insect growth and size. Our resultsindicate that for the datasets we considered, genetic variationin plasticity represents a larger proportion of the total geneticvariation in TPCs compared to DRNs, for the available datasets.Simulations suggest that estimates of the genetic variationin plasticity are strongly affected by the number and rangeof temperatures considered, and by the degree of nonlinearityin the TPC or DRN. Second, we review a recent analysis of fieldselection studies which indicates that directional selectionfavoring increased overall size is common in many systems—thatbigger is frequently fitter. Third, we use a recent theoreticalmodel to examine how selection on thermal performance curvesrelates to environmental temperatures during selection. Themodel predicts that if selection acts primarily on adult sizeor development time, then selection on thermal performance curvesfor larval growth or development rates is directly related tothe frequency distribution of temperatures experienced duringlarval development. Using data on caterpillar temperatures inthe field, we show that the strength of directional selectionon growth rate is predicted to be greater at the modal (mostfrequent) temperatures, not at the mean temperature or at temperaturesat which growth rate is maximized. Our results illustrate someof the differences in genetic architecture and patterns of selectionbetween thermal performance curves and developmental reactionnorms.     

Intraspecific variability in number of larval instars in insects

The number of larval instars varies widely across insect species. Although instar number is frequently considered to be invariable within species, intraspecific variability in the number of instars is not an exceptional phenomenon. However, the knowledge has remained fragmentary, and there are no recent attempts to synthesize the results of relevant studies. Based on published case studies, we show that intraspecific variability in the number of larval instars is widespread across insect taxa, occurring in most major orders, in both hemimetabolous and holometabolous insects. We give an overview of various factors that have been observed to affect the number of instars. Temperature, photoperiod, food quality and quantity, humidity, rearing density, physical condition, inheritance, and sex are the most common factors influencing the number of instars. We discuss adaptive scenarios that may provide ultimate explanations for the plasticity in instar number. The data available largely support the compensation scenario, according to which instar number increases in adverse conditions when larvae fail to reach a species-specific threshold size for metamorphosis. However, in Orthoptera and Coleoptera, there are some exceptional species in which instar number is higher in favorable conditions. In more specific cases, the adaptive value of the variability in instar number may be in reaching or maintaining the developmental stage adapted to hibernation, producing additional generations in multivoltine species, or increasing the probability of surviving in long-lasting adverse conditions.     

Temperature-induced plasticity in membrane and storage lipid composition: thermal reaction norms across five different temperatures

Temperature is a key environmental factor inducing phenotypic plasticity in a wide range of behavioral, morphological, and life history traits in ectotherms. The strength of temperature-induced responses in fitness-related traits may be determined by plasticity of the underlying physiological or biochemical traits. Lipid composition may be an important trait underlying fitness response to temperature, because it affects membrane fluidity as well as availability of stored energy reserves. Here, we investigate the effect of temperature on lipid composition of the springtail Orchesella cincta by measuring thermal reaction norms across five different temperatures after four weeks of cold or warm acclimation. Fatty acid composition in storage and membrane lipids showed a highly plastic response to temperature, but the responses of single fatty acids revealed deviations from the expectations based on HVA theory. We found an accumulation of C_18:2n6 and C_18:3n3 at higher temperatures and the preservation of C_20:4n6 across temperatures, which is contrary to the expectation of decreased unsaturation at higher temperatures. The thermal response of these fatty acids in O. cincta differed from the findings in other species, and therefore shows there is interspecific variation in how single fatty acids contribute to HVA. Future research should determine the consequences of such variation in terms of costs and benefits for the thermal performance of species.     

Intraspecific and interspecific genetic variation in Drosophila

Utilizing the technique of DNA-DNA hybridization, we have characterized the degree of genetic variability in single-copy DNA both within and between several species of Drosophila. The results of intraspecific variation studies indicate considerable variation both for levels of nucleotide heterozygosity (estimated to be over 2%) as well as for insertions-deletions. Interspecific studies confirm this great deal of variability and further establish an extreme heterogeneity within Drosophila genomes for rates of divergence. This heterogeneity is much more extreme than that seen between exons and introns. The degree of single-copy DNA divergence generally supports phylogenetic affinities deduced from more traditional methods. However, exceptions occur where single-copy DNA divergence is not correlated with other properties such as degree of chromosomal differentiation, morphology, or ability to form interspecific hybrids. We argue that single-copy DNA divergence as measured by DNA-DNA hybridization is an accurate indicator of phylogenetic relationships and therefore sheds light on the evolution of other biological properties. Many, if not most, evolutionary tests require an accurate phylogeny of the group being studied and DNA, because of the high information content inherent within the molecule, offers the best hope of deriving true phylogenies.