Contact chemoreception of oviposition-stimulating glucosinolates and an oviposition-deterrent cardenolide in two subspecies of Pieris napi

Abstract. Electrophysiological responses to ten glucosinolates, oviposition stimulants isolated from their cruciferous host plants, were recorded from tarsal taste neurones of two subspecies of Pieris napi: P. napi napi L. and P. napi olerucea (Harris) (Lepidoptera: Pieridae). In the cluster of fourteen medial B-sensilla on the fifth tarsomere, strong responses to glucosinolates were recorded. Three receptors cells, identified on the basis of spike amplitude and shape, were typically active in these responses, with the cell producing the smallest spike predominating. The response intensity of the latter cell to a particular glucosinolate at 1 mm concentration depended both on the chemical structure of the stimulus and on the subspecies of insect. In P. napi napi , aromatic glucosinolates were just as effective as aliphatic glucosinolates, whereas in P. napi oleracea the long-chain aliphatic glucosinolates gluconapin and progoitrin were stronger stimuli. At 10 and 100 pg/ml concentrations, significant differences between the subspecies were found in the absolute response intensity to seven compounds. A second cell in P. napi olerucea , producing a medium-sized spike, was excited most by sinalbin, whereas in Pnupi nupi three aliphatic glucosinolates were equally effective stimulants for a similar cell. In both subspecies the cell producing the medium-sized spikes in response to glucosinolates was also highly sensitive to erychroside, an oviposition deterring cardenolide occurring in a host of P. napi olerncea. The evolutionary divergence between the two geographically separated subspecies is reflected by differences in chemosensory recognition mechanisms.     

Repeated larval diapause and diapause-free development in geographic strains of the burnet moth Zygaena trifolii Esp. (Insecta,Lepidoptera)

Summary Zygaena trifolii is a long-day insect with temperature-dependent photoperiodic responses. All larval instars are sensitive to photoperiod; however, diapause may occur at the third larval stage or any subsequent larval instars. There were quantitative differences within populations in the threshold photoperiod for diapause induction. The diapause response was polymorphic, so that larvae might enter diapause at different instars under the same culture conditions. Furthermore, decreasing photoperiods below a critical daylength shifted the diapausing instar towards earlier stages. Geographic strains of Z. trifolii showed discontinuous clinal variation. Near the northern edge of the distribution [Cologne (Köln), FRG], there is first an obligatory diapause, mainly during early instars, and additional facultative (repeat) diapauses during later larval instars in subsequent years. In the southern part of its distribution, this burnet moth is partially bivoltine in the field with a facultative first developmental arrest and a decreased capacity for repeated diapause (Valencia, Spain; Marseille, France). Further experiments indicated that the photoperiodically controlled diapause reaction is also influenced by the number of photoperiodic cycles experienced during the period spent in each larval instar, which depends on temperature. The adaptive significance of obligatory and facultative repeated diapause, varying even among the offspring of a single female, may be to buffer the populations against the more extreme and, from year to year, unpredictable fluctuations in climatic conditions at the northern edge of the distribution.Abbreviations L₃ feeding 3rd larval instar - L₄D diapausing 4th larval instar - L₅D2 repeat-diapausing larval instar with second diapause at the 5th larval stage - LD light-dark cycle - KT shortday conditions (e.g. LD 8:16) - LT long-day conditions (e.g. LD 16:8)     

Regulation of body size during larval development in the German cockroach, Blattella germanica

It may be advantageous for insects to attain a certain standard size when they become adult. Recent studies have demonstrated that in some species of insects, a threshold size must be reached in the last instar in order that the adult will attain the standard size. It has been revealed also, however, that there may be another type of regulation of body size during larval development in the German cockroach. Head width and other characteristics were measured in every larval instar up to the adult. Body size is distributed clearly into a small group and a large group in the 5th instar, and even in the 4th instar. This suggests that a larva has already determined the number of following moults by the late 3rd instar. Adult size is almost the same in 5- and 6-instar types. Among individuals which require the same number of instars to reach the adult stage, the coefficient of variation in body size increases until the 3rd instar then decreases to the adult stage. Smaller individuals in a particular instar after the 3rd tend to grow more than larger ones during that instar. These results suggest that a larva regulates its body size after the 3rd instar in order to attain the ‘norm’ for adult size.     

Effect of daylength and temperature on the larval diapause of the sunflower moth, Homoeosoma electellum

The effect of daylength and temperature on the regulation of the larval diapause of a central Missouri population of the sunflower moth, Homoeosoma electellum, was examined. Fully grown fourth-instar larvae exhibit a facultative diapause. Measurements of the effect of photoperiod on diapause induction revealed critical photoperiods of about 13 h 30 min light/day at 20°C, and between 11 h 45 min and 12 h light/day at 23°C. Third and fourth-instar larvae were shown to be the main sensitive stages for diapause determination. Daylength was also shown to be an important regulator of the rate of diapause development. A short day of LD 10:14 h permitted only a low rate of diapause development, whereas long days of LD 14:10 h and LD 16:8 h accelerated diapause development at 25 and 30°C. When long days were alternated with short days at 30°C the accelerating effect of long days on diapause development was not found. Systematic transfers of chilled diapausing larvae revealed an accelerated diapause development in groups transferred from 10 to 30°C LD 10:14 h, but diapause development was not accelerated in groups transferred from 10 to 30°C LD 16:8 h.     

Effects of photoperiod and temperature on diapause induction in Conogethes punctiferalis （Lepidoptera： Pyralidae）

The yellow peach moth, Conogethes punctiferalis （Guenee）, a multivoltine species that overwinters as diapausing larvae, is one of the most serious insect pests on maize in China. Effect of photoperiod and temperature on larval diapause was examined under empirical laboratory conditions. Short-day treatments caused larval diapause at 25℃, and the critical photoperiod was between 12 and 13 h （or 12 h 51 min） light per day. No sensitive instar was identified for diapause induction under alternated short- （L ： D 11 ： 13 h） and long-day （L ： D 14 ： 10 h） treatments at different larval stages. However, accumulative treatment of three instars and 10 d under short-day treatment was required for the induction of 50% larval diapause. All larvae entered diapause at 20℃, whereas less than 3% did so at 30℃, irrespective of the long- or short-day treatment. Furthermore, under the short-day treatment, more than 90% of larvae went into diapause with temperatures ≤ 25℃, but less than 17% did so at 28℃. In contrast, under the long-day treatment, less than 19% of larvae went into diapause with temperatures ≥23 ℃. The forward shift （5℃） of critical temperature under the long-day regime demonstrated the compensatory effect of temperature and photoperiod on diapause induction. In conclusion, C. punctiferalis had a temperature-dependent type Ⅰ photoperiodic diapause response; there was no sensitive instar for diapause determination, but the photoperiodic accumulation time countermeasures both of the short-day cycles and the number ofinstars exposed, and the photoperiodic diapause response, was a temperature-compensated phenomenon.     

Phenotypic plasticity in development and evolution: facts and concepts

This theme issue pursues an exploration of the potential of taking into account the environmental sensitivity of development to explaining the evolution of metazoan life cycles, with special focus on complex life cycles and the role of developmental plasticity. The evolution of switches between alternative phenotypes as a response to different environmental cues and the evolution of the control of the temporal expression of alternative phenotypes within an organism''s life cycle are here treated together as different dimensions of the complex relationships between genotype and phenotype, fostering the emergence of a more general and comprehensive picture of phenotypic evolution through a quite diverse sample of case studies. This introductory article reviews fundamental facts and concepts about phenotypic plasticity, adopting the most authoritative terminology in use in the current literature. The main topics are types and components of phenotypic variation, the evolution of organismal traits through plasticity, the origin and evolution of phenotypic plasticity and its adaptive value.     

Do plants and animals differ in phenotypic plasticity?

This paper compares the flexibility in the nexus between phenotype and genotype in plants and animals. These taxa although considered to be fundamentally different are found to be surprisingly similar in the mechanisms used to achieve plasticity. Although non-cognitive behaviour occurs in plants, its range is limited, while morphological and developmental plasticity also occur to a considerable extent in animals. Yet both plants and animals are subject to unique constraints and thus need to find unique solutions to functional problems. A true comparison between the plant and animal phenotype would be a comparison between plants and sessile photosynthesizing colonial invertebrates. Such comparisons are lacking. However, they would provide important insights into the adaptive significance of plasticity in these groups. It is also suggested that a comparison of inflexible traits in these groups would provide an understanding of the constraints, as well as the costs and benefits, of a plastic versus non-plastic phenotype in plants and animals.     

Influence of seasonal timing on thermal ecology and thermal reaction norm evolution in Wyeomyia smithii

Evolutionary changes in the seasonal timing of life-history events can alter a population's exposure to seasonally variable environmental factors. We illustrate this principle in Wyeomyia smithii by showing that: (1) geographic divergence in diapause timing reduces differences among populations in the thermal habitat experienced by nondiapause stages; and (2) the thermal habitat of the growing season is more divergent at high compared with low temperatures with respect to daily mean temperatures. Geographic variation in thermal reaction norms for development time was greater in a warm compared with a cool rearing treatment, mirroring the geographic trend in daily mean temperature. Geographic variation in body size was unrelated to geographic temperature variation, but was also unrelated to development time or fecundity. Our results suggest that proper interpretation of geographic trends may often require detailed knowledge of life-history timing.     

A mutant strain of Chymomyza costata (Diptera: Drosophilidae) insensitive to diapause-inducing action of photoperiod

ABSTRACT. From a Japanese population of Chymomyza costata which has been known to have a photoperiodic larval diapause, we selected a mutant strain which did not respond to photoperiod. However, about 70% of the individuals of this strain entered diapause at 11^oC irrespective of photoperiod, and about the same percentage of those of the photoperiod-sensitive strain also did so in continuous illumination at 11^oC. This indicates that low temperature induces diapause independently of photoperiod. On the other hand, a temperature drop from 18 or 25^oC to 15^oC and chilling at 4^oC did not induce diapause.     

The evolution of alternative developmental pathways: footprints of selection on life-history traits in a butterfly

Developmental pathways may evolve to optimize alternative phenotypes across environments. However, the maintenance of such adaptive plasticity under relaxed selection has received little study. We compare the expression of life-history traits across two developmental pathways in two populations of the butterfly Pararge aegeria where both populations express a diapause pathway but one never expresses direct development in nature. In the population with ongoing selection on both pathways, the difference between pathways in development time and growth rate was larger, whereas the difference in body size was smaller compared with the population experiencing relaxed selection on one pathway. This indicates that relaxed selection on the direct pathway has allowed life-history traits to drift towards values associated with lower fitness when following this pathway. Relaxed selection on direct development was also associated with a higher degree of genetic variation for protandry expressed as within-family sexual dimorphism in growth rate. Genetic correlations for larval growth rate across sexes and pathways were generally positive, with the notable exception of correlation estimates that involved directly developing males of the population that experienced relaxed selection on this pathway. We conclude that relaxed selection on one developmental pathway appears to have partly disrupted the developmental regulation of life-history trait expression. This in turn suggests that ongoing selection may be responsible for maintaining adaptive developmental regulation along alternative developmental pathways in these populations.     

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.     

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.     

Effect of photoperiod on the first-instar development in the lacewing Nineta pallida

ABSTRACT Nineta pallida (Schneider) (Neuroptera: Chrysopidae) larvae were reared at 21^CC under various stationary photoperiods. The duration of the first stadium varied with daylength. Long-day regimes (LD 18:6 and 16:8 h) induced the fastest development, whereas the first ecdysis was more or less delayed in LD 14:10 and 13:11 h. Medium days (LD 12:12 h) resulted in the longest duration of the first stadium and high mortality, and all shorter days (LD 10:14, 8:16 and 6:18 h) in a moderate delay in the first ecdysis in all individuals. The facultative delay in development is thought to be a diapause, and viewed as a factor contributing to the annual synchronization of the life cycle.