Immunological larval polyphenism in the map butterfly Araschnia levana reveals the photoperiodic modulation of immunity

The bivoltine European map butterfly (Araschnia levana) displays seasonal polyphenism characterized by the formation of two remarkably distinct dorsal wing phenotypes: The spring generation (A. levana levana) is predominantly orange with black spots and develops from diapause pupae, whereas the summer generation (A. levana prorsa) has black, white, and orange bands and develops from subitaneous pupae. The choice between spring or summer imagoes is regulated by the photoperiod during larval and prepupal development, but polyphenism in the larvae has not been investigated before. Recently, it has been found that the prepupae of A. levana display differences in immunity‐related gene expression, so we tested whether larvae destined to become spring (short‐day) or summer (long‐day) morphs also display differences in innate immunity. We measured larval survival following the injection of a bacterial entomopathogen (Pseudomonas entomophila), the antimicrobial activity in their hemolymph and the induced expression of selected genes encoding antimicrobial peptides (AMPs). Larvae of the short‐day generation died significantly later, exhibited higher antibacterial activity in the hemolymph, and displayed higher induced expression levels of AMPs than those of the long‐day generation. Our study expands the seasonal polyphenism of A. levana beyond the morphologically distinct spring and summer imagoes to include immunological larval polyphenism that reveals the photoperiodic modulation of immunity. This may reflect life‐history traits that manifest as trade‐offs between immunity and fecundity.     

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.     

The conceptual and practical implications of interpreting diet breadth mechanistically in generalist predatory insects

Seasonal polyphenism in animal colour patterns indicates that temporal variation in selection pressures maintains phenotypic plasticity. Spring generation of the polyphenic European map butterfly Araschnia levana has an orange–black fritillary‐like pattern whilst individuals of the summer generation are black with white bands across the wings. What selects for the colour difference is unknown. Because predation is a major selection pressure for insect coloration, we first tested whether map butterfly coloration could have a warning function (i.e. whether the butterflies are unpalatable to birds). In a following field experiment with butterfly dummies we tested whether the spring form is better protected than the summer form from predators in the spring, and vice versa in the summer. The butterflies were palatable to birds (blue tits Cyanistes caeruleus) and in the field the spring and summer form dummies were attacked equally irrespective of season. Therefore, we found no evidence that the map butterfly is warning‐coloured or that seasonal polyphenism is an adaptation to avian predation. Because insect coloration has multiple functions and map butterfly coloration is linked to morphology, life history and development it is likely that the interplay of several selection pressures explains the evolution of colour polyphenism. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Differential Bird Predator Attack Rate on Seasonal Forms of the Map Butterfly (Araschnia levana L.): Does the Substrate Matter?

The European map butterfly (Araschnia levana L.) is a striking example of seasonal plasticity. Individuals of the spring generation are reddish with a fritillary‐like colour pattern, whereas the summer generation is black with a white dorsal stripe. Proximate factors explaining the development of the forms are well known, but ultimate explanations have not been tested experimentally yet. The reddish spring form is assumed to have a warning coloration, as found in other nymphalid butterflies that are unpalatable (Aglais urticae). We tested for differential predation by a visually hunting predator (Parus major) in a laboratory experiment using artificial butterflies designed to represent the spring and summer form. Birds were released individually in a flight cage where the alternative forms were presented. Summer forms were more frequently attacked than spring forms, which may point to some aversion against the reddish spring form. But there was also a strong effect of the interaction between seasonal form and type of substrate. Spring forms were much better protected from attacks on the brown substrate of dead leaves compared to the green substrate of nettle leaves. On the latter substrate, latency times before attacking spring forms were on average 2.5 times longer than for summer forms. Experiments with artificial butterflies simplify complex predator–prey interactions because they exclude potential taste or odour effects and they also exclude behavioural responses and interactions of the butterfly. However, our results based on static visual signals provide a promising first step to test the functional significance of this striking seasonal polyphenism.     

Hormonal control of seasonal morphs by the timing of ecdysteroid release in Araschnia levana L. (Nymphalidae: Lepidoptera)

Araschnia levana L. occurs in two seasonal morphs. Larvae reared under short-day conditions become diapause pupae and emerge as red spring-morph butterflies. Long-day larvae become non-diapause pupae, which emerge as black and white summer morphs. Pupae reared under these different conditions were joined in parabiosis. Both underwent adult development without diapause and the long-day animals developed into the summer morph as normal. The morph of short-day animals depended on the time of parabiosis. When they were joined to fresh long-day pupae 1 day after their own pupation, summer morphs resulted. When parabiosis began 4 days after pupation or later, spring morphs resulted. Extirpation of the brain-corpora cardiaca-allata complex from long-day pupae affected neither non-diapause development nor the summer morph. Adult development could be prevented by removal of head and prothorax. When adult development was initiated in the remaining bodies by 20-hydroxyecdysone 14 days after pupation, the spring morph resulted.—Injection of 20-dydroxyecdysone into 3-day-old short-day pupae initiated adult development and led to the summer morph. Injections into 10-day-old short-day pupae led to the spring morph. The same was true in diapause pupae deprived of their brain-corpora cardiaca-allata complex. These results indicate that seasonal diphenism in A. levana is controlled only by the timing of ecdysteroid release, which initiates adult development. There is no direct influence of the brain on wing coloration.     

Daphnia magna microRNAs respond to nutritional stress and ageing but are not transgenerational

Maternal effects, where the performance of offspring is determined by the condition of their mother, are widespread and may in some cases be adaptive. The crustacean Daphnia magna shows strong maternal effects: offspring size at birth and other proxies for fitness are altered when their mothers are older or when mothers have experienced dietary restriction. The mechanisms for this transgenerational transmission of maternal experience are unknown, but could include changes in epigenetic patterning. MicroRNAs (miRNAs) are regulators of gene expression that have been shown to play roles in intergenerational information transfer, and here, we test whether miRNAs are involved in D. magna maternal effects. We found that miRNAs were differentially expressed in mothers of different ages or nutritional state. We then examined miRNA expression in their eggs, their adult daughters and great granddaughters, which did not experience any treatments. The maternal (treatment) generation exhibited differential expression of miRNAs, as did their eggs, but this was reduced in adult daughters and lost by great granddaughters. Thus, miRNAs are a component of maternal provisioning, but do not appear to be the cause of transgenerational responses under these experimental conditions. MicroRNAs may act in tandem with egg provisioning (e.g., with carbohydrates or fats), and possibly other small RNAs or epigenetic modifications.     

Novel transcriptome data analysis implicates circulating microRNAs in epigenetic inheritance in mammals

Experimental evidence supports a role of mobile small non-coding RNAs in mediating soma to germline hereditary information transfer in epigenetic inheritance in plants and worms. Similar evidence in mammals has not been reported so far. In this bioinformatic analysis, differentially expressed microRNAs (miRNAs) or mRNAs reported previously in genome level expression profiling studies related to or relevant in epigenetic inheritance in mammals were examined for circulating miRNA association. The reported sets of differentially expressed miRNAs or miRNAs that are known to target the reported sets of differentially expressed genes, in that order, showed enrichment of circulating miRNAs across environmental factors, tissues, life cycle stages, generations, genders and species. Circulating miRNAs commonly representing the expression profiles enriched various epigenetic processes. These results provide bioinformatic evidence for a role of circulating miRNAs in epigenetic inheritance in mammals.     

Seasonal changes of gastrointestinal nematode populations in yearling beef cattle in Louisiana with emphasis on prevalence of inhibition in Ostertagia ostertagi

Investigation of seasonal changes in the composition of nematode populations, principally Ostertagia oslertagi, was conducted over 3 years at three locations in Louisiana. This is the most commonly occurring parasite of cattle in the state. Naturally infected yearling cattle were killed monthly over extended periods and tracer calves were grazed for monthly intervals from late autumn to summer at two locations in 1978–1979. Major objectives were to determine seasonal incidence of common gastrointestinal nematodes and for O. ostertagi, in particular, the time period during which larval inhibition was prevalent, circumstances under which larvae were conditioned to inhibition, and the duration of inhibition. Small numbers of inhibited O. ostertagi were recovered between November and February. Large numbers were found initially in March and increased numbers in April and May. Both normally developing and inhibition prone larvae were acquired during late winter-early spring, with the proportion of the latter being more prevalent in April and May. Evidence from tracer calves indicated that few O. ostertagi larvae were acquired after early June. Large burdens of inhibited larvae persisted in yearling cattle through summer; numbers of developing larvae and adults were minimal. Maturation of inhibited larvae occurs from August to October and in one instance was associated with cases of clinical parasitism. Factors responsible for inhibition were not defined, but increasing temperatures of late winter-early spring, host resistance, and density-dependence of populations were considered. Other abomasal genera were most prevalent in spring while intestinal genera were most common during autumn through spring.