Factors and consequences of a non-functional alary polymorphism inPyrrhocoris apterus (Heteroptera: Pyrrhocoridae)

Sexual dimorphism of diapause regulation in the hemipteran bug Pyrrhocoris apterus

Diapause is one of the major strategies for insects to prepare for and survive harsh seasons. In females, the absence of juvenile hormone (JH) is a hallmark of adult reproductive diapause, a developmental arrest, which is much less characterized in males. Here we show that juvenile hormone III skipped bisepoxide (JHSB₃) titers in hemolymph remarkably differ between reproductive males and females of the linden bug Pyrrhocoris apterus, whereas no JH was detected in diapausing adults of both sexes. Like in females, ectopic application of JH mimic effectively terminated male diapause through the canonical JH receptor components, Methoprene-tolerant and Taiman. In contrast to females, long photoperiod induced reproduction even in males with silenced JH reception or in males with removed corpus allatum (CA), the JH-producing gland. JHSB₃ was detected in the accessory glands (MAG) of reproductive males, unexpectedly, even in males without CA. If there is a source of JHSB₃ outside CA or a long-term storage of JHSB₃ in MAGs remains to be elucidated. These sex-related idiosyncrasies are further manifested in different dynamics of diapause termination in P. apterus by low temperature. We would like to propose that this sexual dimorphism of diapause regulation might be explained by the different reproductive costs for each sex.

Complex Evolution of Insect Insulin Receptors and Homologous Decoy Receptors, and Functional Significance of Their Multiplicity

Evidence accumulates that the functional plasticity of insulin and insulin-like growth factor signaling in insects could spring, among others, from the multiplicity of insulin receptors (InRs). Their multiple variants may be implemented in the control of insect polyphenism, such as wing or caste polyphenism. Here, we present a comprehensive phylogenetic analysis of insect InR sequences in 118 species from 23 orders and investigate the role of three InRs identified in the linden bug, Pyrrhocoris apterus, in wing polymorphism control. We identified two gene clusters (Clusters I and II) resulting from an ancestral duplication in a late ancestor of winged insects, which remained conserved in most lineages, only in some of them being subject to further duplications or losses. One remarkable yet neglected feature of InR evolution is the loss of the tyrosine kinase catalytic domain, giving rise to decoys of InR in both clusters. Within the Cluster I, we confirmed the presence of the secreted decoy of insulin receptor in all studied Muscomorpha. More importantly, we described a new tyrosine kinase-less gene (DR2) in the Cluster II, conserved in apical Holometabola for ∼300 My. We differentially silenced the three P. apterus InRs and confirmed their participation in wing polymorphism control. We observed a pattern of Cluster I and Cluster II InRs impact on wing development, which differed from that postulated in planthoppers, suggesting an independent establishment of insulin/insulin-like growth factor signaling control over wing development, leading to idiosyncrasies in the co-option of multiple InRs in polyphenism control in different taxa.

Comparative transcriptomic analysis of a wing-dimorphic stonefly reveals candidate wing loss genes

Background

The genetic basis of wing development has been well characterised for model insect species, but remains poorly understood in phylogenetically divergent, non-model taxa. Wing-polymorphic insect species potentially provide ideal systems for unravelling the genetic basis of secondary wing reduction. Stoneflies (Plecoptera) represent an anciently derived insect assemblage for which the genetic basis of wing polymorphism remains unclear. We undertake quantitative RNA-seq of sympatric full-winged versus vestigial-winged nymphs of a widespread wing-dimorphic New Zealand stonefly, Zelandoperla fenestrata, to identify genes potentially involved in wing development and secondary wing loss.

Results

Our analysis reveals substantial differential expression of wing-development genes between full-winged versus vestigial-winged stonefly ecotypes. Specifically, of 23 clusters showing significant similarity to Drosophila wing development-related genes and their pea aphid orthologues, nine were significantly upregulated in full-winged stonefly ecotypes, whereas only one cluster (teashirt) was substantially upregulated in the vestigial-winged ecotype.

Conclusions

These findings suggest remarkable conservation of key wing-development pathways throughout 400 Ma of insect evolution. The finding that two Juvenile Hormone pathway clusters were significantly upregulated in vestigial-winged Zelandoperla supports the hypothesis that Juvenile Hormone may play a key role in modulating insect wing polymorphism, as has previously been suggested for other insect lineages.

Behavioural thermoregulation hastens spring mating activity in Pyrrhocoris apterus (Heteroptera: Pyrrhocoridae)

Post-diapause, overwintered adults of the true bug Pyrrhocoris apterus L. (Heteroptera, Pyrrhocoridae) form conspicuous aggregations at warm spots in early spring. Using a combination of laboratory experiments and field observations, we assessed the influence of this behaviour on the seasonal timing of reproduction. In the laboratory, post-diapause pairs mated after an accumulation of 80° days (dd), and females started to lay eggs after an additional 80 dd (considering 10.3 °C as a lower temperature threshold for post-diapause development). In the field, however, females mated as early as late March (based on data from four seasons), which corresponded to the accumulation of <15 dd (based on summing the temperatures recorded at a meteorology station). Such a 'discrepancy' between laboratory and field data is explained by thermoregulation. Aggregated adults bask in sunny spots, thereby increasing their body temperatures to ~25 °C, which is 7-16 °C above the temperature of the surrounding ground. This thermal excess speeds up their post-diapause development and enables early mating and oviposition. As a result, behavioural thermoregulation hastens the seasonal start of reproduction in P. apterus by > 1 month.

Genotyping-by-sequencing supports a genetic basis for wing reduction in an alpine New Zealand stonefly

Wing polymorphism is a prominent feature of numerous insect groups, but the genomic basis for this diversity remains poorly understood. Wing reduction is a commonly observed trait in many species of stoneflies, particularly in cold or alpine environments. The widespread New Zealand stonefly Zelandoperla fenestrata species group (Z. fenestrata, Z. tillyardi, Z. pennulata) contains populations ranging from fully winged (macropterous) to vestigial-winged (micropterous), with the latter phenotype typically associated with high altitudes. The presence of flightless forms on numerous mountain ranges, separated by lowland fully winged populations, suggests wing reduction has occurred multiple times. We use Genotyping by Sequencing (GBS) to test for genetic differentiation between fully winged (n = 62) and vestigial-winged (n = 34) individuals, sampled from a sympatric population of distinct wing morphotypes, to test for a genetic basis for wing morphology. While we found no population genetic differentiation between these two morphotypes across 6,843 SNP loci, we did detect several outlier loci that strongly differentiated morphotypes across independent tests. These findings indicate that small regions of the genome are likely to be highly differentiated between morphotypes, suggesting a genetic basis for wing reduction. Our results provide a clear basis for ongoing genomic analysis to elucidate critical regulatory pathways for wing development in Pterygota.

Molecular phylogeny informs generic and subgeneric concepts in the Schizoptera Fieber genus group (Heteroptera : Schizopteridae) and reveals multiple origins of female-specific elytra

Wing dimorphism occurs in many genera of Schizopteridae Reuter, 1891 and other litter bugs (Heteroptera:Dipsocoromorpha), in both males and females. In the largest litter bug genus, Schizoptera Fieber, and closely related taxa, sexual wing dimorphism is observed in several species whereby males are macropterous, but females possess elytra, or hardened forewings – a feature that is rare outside of beetles and that we here refer to as female-specific elytra. Phylogenetic hypotheses for Schizoptera are unavailable, but are essential to reveal if female-specific elytra evolved once or multiple times within the genus and to test if the presence of elytra can reverse states to macropterous wings. In addition, generic and subgeneric concepts of this speciose genus-group have not been tested in a phylogenetic framework, and relationships with other schizopterid genera remain largely unknown. Our molecular phylogeny of Schizoptera and related genera documents that this genus is currently polyphyletic, and we raise the subgenus Kophaegis to generic rank to render Schizoptera monophyletic (Orthorhagus was recently elevated to genus). Relationships within Schizoptera reveal several well supported clades, some of them corresponding to currently recognised subgenera. To examine the value of previously used diagnostic features, we optimise 11 morphological characters on the molecular phylogeny and update generic and subgeneric diagnoses. Tracing transitions between macropterous and elytrous wing types, we show that female-specific elytra evolved at least seven times within Schizopteridae, four of those times within the Schizoptera genus-group, and that elytra reversed to macropterous wings at least twice. We propose that Schizopteridae may be an excellent model to study the selective pressures that have given rise to sexually dimorphic traits.

Wing morph-specific differences in the metabolism and endocrine control of reserve mobilization in adult males of a flightless bug, Pyrrhocoris apterus (L.) (Heteroptera)

The differences in the metabolism and endocrine control of reserve mobilization in long-winged (macropterous) and short-winged (brachypterous) males of a flightless firebug (Pyrrhocoris apterus) were studied. We found that protein content in the gut was significantly lower in 5-10 day-old macropterous males due to their fasting and higher in 28 day-old ones than in the same aged brachypterous counterparts as the result of renewed food intake. Overall protease activity was significantly lower in 10-14 day-old macropters, while an abrupt increase in the activity starting on day 21 after adult ecdysis was also associated with renewal of the food intake. The levels of carbohydrates in haemolymph were only slightly lower in 1-10 day-old macropterous males than in the same aged brachypters. However, more than twofold higher lipid content in haemolymph of 7-10 day-old macropterous males than in the same aged brachypterous males was found. Higher mobilization of lipid reserves from the fat bodies in macropterous males was accompanied by more intensive adipokinetic response and higher levels of adipokinetic hormone in the body. It is the first report of endocrine regulation of wing morph-related differences in the lipid mobilization in males of wing-polymorphic insects.

Physiology and ecology of dispersal polymorphism in insects

Studies of dispersal polymorphism in insects have played a pivotal role in advancing our understanding of population dynamics, life history evolution, and the physiological basis of adaptation. Comparative data on wing-dimorphic insects provide the most definitive evidence to date that habitat persistence selects for reduced dispersal capability. The increased fecundity of flightless females documents that a fitness trade-off exists between flight capability and reproduction. However, only recently have studies of nutrient consumption and allocation provided unequivocal evidence that this fitness trade-off results from a trade-off of internal resources. Recent studies involving wing-dimorphic insects document that flight capability imposes reproductive penalties in males as well as females. Direct information on hormone titers and their regulation implicates juvenile hormone and ecdysone in the control of wing-morph determination. However, detailed information is available for only one species, and the physiological regulation of wing-morph production remains poorly understood. Establishing a link between the ecological factors that influence dispersal and the proximate physiological mechanisms regulating dispersal ability in the same taxon remains as a key challenge for future research.