Involvement of the scalloped gene in morphogenesis of the wing margin via regulating cell growth in a hemimetabolous insect Gryllus bimaculatus.

The acquisition of wings was a key event in insect evolution. As hemimetabolous insects were the first group to acquire functional wings, establishing the mechanisms of wing formation in this group could provide useful insights into their evolution. In this study, we aimed to elucidate the expression and function of the gene scalloped (sd), which is involved in wing formation in Drosophila melanogaster, and in Gryllus bimaculatus mainly during postembryonic development. Expression analysis showed that sd is expressed in the tergal edge, legs, antennae, labrum, and cerci during embryogenesis and in the distal margin of the wing pads from at least the sixth instar in the mid to late stages. Because sd knockout caused early lethality, nymphal RNA interference experiments were performed. Malformations were observed in the wings, ovipositor, and antennae. By analyzing the effects on wing morphology, it was revealed that sd is mainly involved in the formation of the margin, possibly through the regulation of cell proliferation. In conclusion, sd might regulate the local growth of wing pads and influence wing margin morphology in Gryllus.

Embryonic development of the cricket Gryllus bimaculatus.

Extensive research into Drosophila melanogaster embryogenesis has improved our understanding of insect developmental mechanisms. However, Drosophila development is thought to be highly divergent from that of the ancestral insect and arthropod in many respects. We therefore need alternative models for arthopod development that are likely to be more representative of basally-branching clades. The cricket Gryllus bimaculatus is such a model, and currently has the most sophisticated functional genetic toolkit of any hemimetabolous insect. The existing cricket embryonic staging system is fragmentary, and it is based on morphological landmarks that are not easily visible on a live, undissected egg. To address this problem, here we present a complementary set of "egg stages" that serve as a guide for identifying the developmental progress of a cricket embryo from fertilization to hatching, based solely on the external appearance of the egg. These stages were characterized using a combination of brightfield timelapse microscopy, timed brightfield micrographs, confocal microscopy, and measurements of egg dimensions. These egg stages are particularly useful in experiments that involve egg injection (including RNA interference, targeted genome modification, and transgenesis), as injection can alter the speed of development, even in control treatments. We also use 3D reconstructions of fixed embryo preparations to provide a comprehensive description of the morphogenesis and anatomy of the cricket embryo during embryonic rudiment assembly, germ band formation, elongation, segmentation, and appendage formation. Finally, we aggregate and schematize a variety of published developmental gene expression patterns. This work will facilitate further studies on G. bimaculatus development, and serve as a useful point of reference for other studies of wild type and experimentally manipulated insect development in fields from evo-devo to disease vector and pest management.

Notch/Delta signalling is not required for segment generation in the basally branching insect Gryllus bimaculatus.

Arthropods and vertebrates display a segmental body organisation along all or part of the anterior-posterior axis. Whether this reflects a shared, ancestral developmental genetic mechanism for segmentation is uncertain. In vertebrates, segments are formed sequentially by a segmentation 'clock' of oscillating gene expression involving Notch pathway components. Recent studies in spiders and basal insects have suggested that segmentation in these arthropods also involves Notch-based signalling. These observations have been interpreted as evidence for a shared, ancestral gene network for insect, arthropod and bilaterian segmentation. However, because this pathway can play multiple roles in development, elucidating the specific requirements for Notch signalling is important for understanding the ancestry of segmentation. Here we show that Delta, a ligand of the Notch pathway, is not required for segment formation in the cricket Gryllus bimaculatus, which retains ancestral characteristics of arthropod embryogenesis. Segment patterning genes are expressed before Delta in abdominal segments, and Delta expression does not oscillate in the pre-segmental region or in formed segments. Instead, Delta is required for neuroectoderm and mesectoderm formation; embryos missing these tissues are developmentally delayed and show defects in segment morphology but normal segment number. Thus, what initially appear to be 'segmentation phenotypes' can in fact be due to developmental delays and cell specification errors. Our data do not support an essential or ancestral role of Notch signalling in segment generation across the arthropods, and show that the pleiotropy of the Notch pathway can confound speculation on possible segmentation mechanisms in the last common bilaterian ancestor.

Prolonged diapause in Mormon crickets: Embryonic responses to three measures of time.

Mormon cricket eggs can remain diapausing in soil for multiple years without forming an embryo. I investigated whether embryonic development was dependent on the number of annual cycles since the egg was laid, duration of the summer period (forcing), or duration of the winter period (chilling). Male and female Mormon crickets collected in Arizona and Wyoming were paired in the lab. For each mating pair, sibling eggs were incubated 12 weeks, eggs with fully developed embryos removed, and the remaining eggs were split evenly among three treatments: a long cold period and a long warm period; a short cold period and a long warm period; and a short cold period and a short warm period, which respectively completed 2 annual cycles, 3 cycles, and 4 cycles in 60 calendar weeks. In each cycle over nine years, developed eggs and eggs that appeared inviable were counted and removed. For each mating pair, I used survival analyses to test for differences in 1) the number of annual cycles, 2) the warm period duration, and 3) the cold period duration required for the embryos to develop. For eight of 11 mating pairs, one of the three factors was not excluded as a determinant of the phenology of embryonic development. Duration of the warm period was not rejected in seven of 11 cases. Duration of the warm period required for 50 % of the eggs to develop ranged from 84 to 144 weeks. In one case from Arizona, the duration of the cold period was the only factor not rejected. Median chill time was 60 weeks, which is also more than one year. Despite this exception, I conclude that duration of the warm period is typically the factor that determines timing of embryonic development for Mormon crickets. For these two high elevation populations, median forcing or chilling exceeded one year.

Identification and expression analyses of a novel serotonin receptor gene, 5-HT2ß, in the field cricket, Gryllus bimaculatus.

Biogenic amine serotonin (5-HT) modulates various aspects of behaviors such as aggressive behavior and circadian behavior in the cricket. In our previous report, in order to elucidate the molecular basis of the cricket 5-HT system, we identified three genes involved in 5-HT biosynthesis, as well as four 5-HT receptor genes (5-HT1A, 5-HT1B, 5-HT2α, and 5-HT7) expressed in the brain of the field cricket Gryllus bimaculatus DeGeer [7]. In the present study, we identified Gryllus 5-HT2ß gene, an additional 5-HT receptor gene expressed in the cricket brain, and examined its tissue-specific distribution and embryonic stage-dependent expression. Gryllus 5-HT2ß gene was ubiquitously expressed in the all examined adult tissues, and was expressed during early embryonic development, as well as during later stages. This study suggests functional differences between two 5-HT2 receptors in the cricket.

Allocation of maternal- and ejaculate-derived proteins to reproduction in female crickets, Teleogryllus oceanicus.

Female fitness has traditionally been thought to be maximized with one or a few matings. More recent research suggests that polyandry, mating with two or more males, can generate an increase in the viability of offspring females produce. However, the mechanism(s) underlying enhanced offspring viability remain largely unknown. The Australian field cricket Teleogryllus oceanicus has proved a useful model for examining the evolutionary significance of polyandry. Embryo viability appears to be associated with a male's investment in accessory gland tissue, implicating a role for seminal fluid. Here, I used amino acids labelled with different radio isotopes to identify proteins manufactured by males and females before they engaged in reproduction. Males incorporated 95% of the radiolabel into the testes, accessory glands and the ejaculate that was transferred to the female at mating. Male ejaculate compounds were incorporated predominantly into the female's somatic tissue. Relatively more female compounds were incorporated into the ovaries and into laid eggs than ejaculate compounds, and relatively fewer female compounds were sequestered in the somatic tissue than ejaculate compounds. The patterns observed suggest that while ejaculate compounds may be incorporated directly into eggs, they are likely to have a larger effect on maternal allocation to offspring.

Striking parallels between dorsoventral patterning in Drosophila and Gryllus reveal a complex evolutionary history behind a model gene regulatory network.

Dorsoventral pattering relies on Toll and BMP signalling in all insects studied so far, with variations in the relative contributions of both pathways. Drosophila and the beetle Tribolium share extensive dependence on Toll, while representatives of more distantly related lineages like the wasp Nasonia and bug Oncopeltus rely more strongly on BMP signalling. Here, we show that in the cricket Gryllus bimaculatus, an evolutionarily distant outgroup, Toll has, like in Drosophila, a direct patterning role for the ventral half of the embryo. In addition, Toll polarises BMP signalling, although this does not involve the conserved BMP inhibitor Sog/Chordin. Finally, Toll activation relies on ovarian patterning mechanisms with striking similarity to Drosophila. Our data suggest two surprising hypotheses: (1) that Toll's patterning function in Gryllus and Drosophila is the result of convergent evolution or (2) a Drosophila-like system arose early in insect evolution and was extensively altered in multiple independent lineages.

Evolution: lending a helping hand in sperm competition?

Most females mate with many males. This can be costly, but the benefits to females are often unclear. A new study raises the possibility that females could benefit through an unconventional genetic pathway, while also showing that males can inadvertently increase rival males' fitness in surprising ways.

Divergent and conserved roles of extradenticle in body segmentation and appendage formation, respectively, in the cricket Gryllus bimaculatus.

The cricket Gryllus bimaculatus is a typical hemimetabolous intermediate germ insect, in which the processes of segmentation and appendage formation differ from those in Drosophila, a holometabolous long germ insect. In order to compare their developmental mechanisms, we have focused on Gryllus orthologs of the Drosophila developmental regulatory genes and studied their functions. Here, we report a functional analysis of the Gryllus ortholog of extradenticle (Gb'exd) using embryonic and parental RNA interference (RNAi) techniques. We found the following: (1) RNAi suppression of Gb'exd results in the deletion or fusion of body segments. Especially the head was often very severely affected. This gap-like phenotype may be related to reduced expression of the gap genes hunchback and Krüppel in early RNAi germbands. (2) In the appendages, several segments (podomeres) were fused. (3) Head appendages including the antenna were transformed to a leg-like structure consisting of at least one proximal podomere as well as several tarsomeres. The defects in appendages are reminiscent of the phenotype caused by large exd clones in Drosophila antennal discs. These findings led us to the conclusion that (1) Gb'exd is required for segment patterning in the gnathal to abdominal region, acting in a gap gene-like manner in the anterior region. (2) Gb'exd plays important roles in formation of the appendages and the determination of their identities, acting as a regulatory switch that chooses between the fates of head appendages versus the appendage ground state. Although functions of Gb'exd in appendage patterning appear fundamentally conserved between Gryllus and Drosophila, its role in body segmentation may differ from that of Drosophila exd.

EGFR signaling is required for re-establishing the proximodistal axis during distal leg regeneration in the cricket Gryllus bimaculatus nymph.

Nymphs of hemimetabolous insects, such as cockroaches and crickets, possess functional legs with a remarkable capacity for epimorphic regeneration. In this study, we have focused on the role of epidermal growth factor receptor (EGFR) signaling in regeneration of a nymphal leg in the cricket Gryllus bimaculatus. We performed loss-of-function analyses with a Gryllus Egfr homolog (Gb'Egfr) and nymphal RNA interference (RNAi). After injection of double-stranded RNA for Gb'Egfr in the body cavity of the third instar cricket nymph, amputation of the leg at the distal tibia resulted in defects of normal distal regeneration. The regenerated leg lacked the distal tarsus and pretarsus. This result indicates that EGFR signaling is required for distal leg patterning in regeneration during the nymphal stage of the cricket. Furthermore, we demonstrated that EGFR signaling acts downstream of the canonical Wnt/Wg signaling and regulates appendage proximodistal (PD) patterning genes aristaless and dachshund during regeneration. Our results suggest that EGFR signaling influences positional information along the PD axis in distal leg patterning of insects, regardless of the leg formation mode.

Antenna and all gnathal appendages are similarly transformed by homothorax knock-down in the cricket Gryllus bimaculatus.

Our understanding of the developmental mechanisms underlying the vast diversity of arthropod appendages largely rests on the peculiar case of the dipteran Drosophila melanogaster. In this insect, homothorax (hth) and extradenticle (exd) together play a pivotal role in appendage patterning and identity. We investigated the role of the hth homologue in the cricket Gryllus bimaculatus by parental RNA interference. This species has a more generalized morphology than Oncopeltus fasciatus, the one other insect besides Drosophila where homothorax function has been investigated. The Gryllus head appendages represent the morphologically primitive state including insect-typical mandibles, maxillae and labium, structures highly modified or missing in Oncopeltus and Drosophila. We depleted Gb'hth function through parental RNAi to investigate its requirement for proper regulation of other appendage genes (Gb'wingless, Gb'dachshund, Gb'aristaless and Gb'Distalless) and analyzed the terminal phenotype of Gryllus nymphs. Gb'hth RNAi nymphs display homeotic and segmentation defects similar to hth mutants or loss-of-function clones in Drosophila. Intriguingly, however, we find that in Gb'hth RNAi nymphs not only the antennae but also all gnathal appendages are homeotically transformed, such that all head appendages differentiate distally as legs and proximally as antennae. Hence, Gb'hth is not specifically required for antennal fate, but fulfills a similar role in the specification of all head appendages. This suggests that the role of hth in the insect antenna is not fundamentally different from its function as cofactor of segment-specific homeotic genes in more posterior segments.

Early embryonic development and diapause stage in the band-legged ground cricket Dianemobius nigrofasciatus.

The band-legged ground cricket Dianemobius nigrofasciatus enters diapause at an early embryonic stage when adults are reared under short-day conditions or the eggs are exposed to a low temperature. We examined the morphological features of the embryo during early development and determined the exact stage of entry into diapause. In non-diapause eggs, no periplasmic space was observed in the surface region and a small number of nuclei surrounded by cytoplasm (energids) were found among the yolk granules and lipid droplets 12 h after egg laying (AEL) at 25 degrees C. The energids sparsely but evenly populated the surface region at 40 h AEL, but there were some gaps between these energids. A continuous thin layer of nuclei with cytoplasm had completely covered the egg surface at 56 h AEL, suggesting that the blastoderm is formed between 40 and 56 h AEL. At 72 h AEL, we found a germ band at the posterior pole. Electron microscopy revealed clear cell membranes at 40 h AEL. Staining with rhodamine-dextran dye demonstrated that the cell membrane is formed when the nuclei appear on the egg surface at 12-24 h AEL. These results indicate that cellularization occurs before blastoderm formation. In diapause eggs, neither the embryonic rudiment nor germ band was formed, but a continuous layer of cells covered the egg surface. It is concluded that D. nigrofasciatus enters diapause at the cellular blastoderm.

The Cricket Gryllus bimaculatus: Techniques for Quantitative and Functional Genetic Analyses of Cricket Biology.

All extant species are an outcome of nature's "experiments" during evolution, and hence multiple species need to be studied and compared to gain a thorough understanding of evolutionary processes. The field of evolutionary developmental biology (evo-devo) aspires to expand the number of species studied, because most functional genetic studies in animals have been limited to a small number of "traditional" model organisms, many of which belong to the same phylum (Chordata). The phylum Arthropoda, and particularly its component class Insecta, possesses many important characteristics that are considered favorable and attractive for evo-devo research, including an astonishing diversity of extant species and a wide disparity in body plans. The development of the most thoroughly investigated insect genetic model system to date, the fruit fly Drosophila melanogaster (a holometabolous insect), appears highly derived with respect to other insects and indeed with respect to most arthropods. In comparison, crickets (a basally branching hemimetabolous insect lineage compared to the Holometabola) are thought to embody many developmental features that make them more representative of insects. Here we focus on crickets as emerging models to study problems in a wide range of biological areas and summarize the currently available molecular, genomic, forward and reverse genetic, imaging and computational tool kit that has been established or adapted for cricket research. With an emphasis on the cricket species Gryllus bimaculatus, we highlight recent efforts made by the scientific community in establishing this species as a laboratory model for cellular biology and developmental genetics. This broad toolkit has the potential to accelerate many traditional areas of cricket research, including studies of adaptation, evolution, neuroethology, physiology, endocrinology, regeneration, and reproductive behavior. It may also help to establish newer areas, for example, the use of crickets as animal infection model systems and human food sources.

Cell cycle regulator, small silencing RNA, and segmentation patterning gene expression in relation to embryonic diapause in the band-legged ground cricket.

Insects enter diapause to synchronize their life cycle with biotic and abiotic conditions favorable for their development, reproduction, and survival. Adult females of the band-legged ground cricket Dianemobius nigrofasciatus (Orthoptera, Glyllidae) respond to environmental factors in autumn and lay diapause-destined eggs. The eggs arrest their development and enter diapause at a very early embryonic stage, specifically the cellular blastoderm. To elucidate the physiological mechanisms underlying this very early stage programmed developmental arrest, we investigated the cell division cycle as well as the expression of cell cycle regulators, small silencing RNAs, and segment patterning genes. The diapause embryo arrests its cell cycle predominantly at the G0/G1 phase. The proportion of cells in the S phase of the cell cycle abruptly decreased at the time of developmental arrest, but further changes of the G0/G1 and G2/M were later observed. Thus, cell cycle arrest in the diapause embryo is not an immediate event, but it takes longer to reach the steady state. We further elucidated molecular events possibly involved in diapause preparation and entry. Downregulation of Proliferating cellular antigen (PCNA; a cell cycle regulator), caudal and pumilio (cad and pum; early segmentation genes) as well as P-element induced wimpy testis (piwi) (a small silencing RNA) prior to the onset of developmental arrest was notable. The downregulation of PCNA, cad and pum continued even after entry into developmental arrest. In contrast to upregulation in non-diapause eggs, Cyclin D (another cell cycle regulator) and hunchback, Krüppel, and runt (gap and pair-rule genes) were downregulated in diapause eggs. These molecular events may contribute to embryonic diapause of D. nigrofasciatus.

Developmental and adult expression of semaphorin 2a in the cricket Gryllus bimaculatus.

Developmental guidance cues act to direct growth cones to their correct targets in the nervous system. Recent experiments also demonstrate that developmental cues are expressed in the adult mammalian nervous system, although their function in the brain is not yet clear. The semaphorin gene family has been implicated in the growth of dendrites and axons in a number of different species. While the expression of semaphorin and its influence on tibial pioneer neurons in the developing limb bud have been well characterized in the grasshopper, the expression of semaphorin 2a (sema2a) has not been explored in the adult insect. In this study we used polymerase chain reaction (PCR) with degenerate and gene-specific primers to clone part of the secreted form of sema2a from Gryllus bimaculatus. Using in situ hybridization and immunohistochemistry, we confirmed that sema2a mRNA and protein expression patterns in the embryonic cricket were similar to that seen in the grasshopper. We also showed that tibial neuron development in crickets was comparable to that described in grasshopper. An examination of both developing and adult cricket brains showed that sema2a mRNA and protein were expressed in the Kenyon cells in mushroom bodies, an area involved in learning and memory. Sema2a expression was most obvious near the apex of the mushroom body in a region surrounding the neurogenic tip, which produces neurons throughout the life of the cricket. We discuss the role of neurogenesis in learning and memory and the potential involvement of semaphorin in this process.

Expression patterns of the homeotic genes Scr, Antp, Ubx, and abd-A during embryogenesis of the cricket Gryllus bimaculatus.

We have studied embryogenesis of the two-spotted cricket Gryllus bimaculatus as an example of a hemimetabolous, intermediate germ insect, which is a phylogenetically basal insect and may retain primitive features. We observed expression patterns of the orthologs of the Drosophila homeotic genes, Sex combs reduced (Scr), Antennapedia (Antp), Ultrabithorax (Ubx) and abdominal-A (abd-A) during embryogenesis and compared the expression patterns of these genes with the more basal thysanuran insect, Thermobia domestica (the firebrat), and the derived higher dipteran insect, Drosophila melanogaster. Although Scr is expressed commonly in the presumptive posterior maxillary and labial segment in all three insects, the thoracic expression domains vary. Antp is expressed similarly in the three thoracic segments, the limbs, and the anterior abdominal region among these three insects. The early Antp expression in the firebrat and cricket obeys a segmental register in all three thoracic segments, while in Drosophila its initial expression appears in parasegments 4 and 6. Ubx is expressed in the metathoracic (T3) and abdominal segments similarly in the three insects, whereas the expression pattern in the T3 leg differs among them. abd-A is expressed in the posterior compartment of the first abdominal segment and the remaining abdominal segments in all three insects, although its posterior border varies among them.

Involvement of Wingless/Armadillo signaling in the posterior sequential segmentation in the cricket, Gryllus bimaculatus (Orthoptera), as revealed by RNAi analysis.

In insects, there are two different modes of segmentation. In the higher dipteran insects (like Drosophila), their segmentation takes place almost simultaneously in the syncytial blastoderm. By contrast, in the orthopteran insects (like Schistocerca (grasshopper)), the anterior segments form almost simultaneously in the cellular blastoderm and then the remaining posterior part elongates to form segments sequentially from the posterior proliferative zone. Although most of their orthologues of the Drosophila segmentation genes may be involved in their segmentation, little is known about their roles. We have investigated segmentation processes of Gryllus bimaculatus, focusing on its orthologues of the Drosophila segment-polarity genes, G. bimaculatus wingless (Gbwg), armadillo (Gbarm) and hedgehog (Gbhh). Gbhh and Gbwg were observed to be expressed in the each anterior segment and the posterior proliferative zone. In order to know their roles, we used RNA interference (RNAi). We could not observed any significant effects of RNAi for Gbwg and Gbhh on segmentation, probably due to functional replacement by another member of the corresponding gene families. Embryos obtained by RNAi for Gbarm exhibited abnormal anterior segments and lack of the abdomen. Our results suggest that GbWg/GbArm signaling is involved in the posterior sequential segmentation in the G. bimaculatus embryos, while Gbwg, Gbarm and Gbhh are likely to act as the segment-polarity genes in the anterior segmentation similarly as in Drosophila.

Expression patterns of hedgehog, wingless, and decapentaplegic during gut formation of Gryllus bimaculatus (cricket).

We observed expression patterns of hedgehog (hh), wingless (wg), and decapentaplegic (dpp) during gut development of Gryllus bimaculatus (the cricket), a typical hemimetabolous insect, and compared with those observed in Drosophila, a typical holometabolous insect. Gryllus hh(Gbhh) and Gbwg are expressed in both foregut and hindgut, while Gbdpp is expressed only in the hindgut: at the boundaries between the small and large intestine, and between the large intestine and rectum. Although the expression patterns of Gbhh and Gbwg are essentially comparable to those observed in Drosophila, the expression pattern of Gbdpp differs from those of the Drosophila dpp.

Circadian clock controlling egg hatching in the cricket (Gryllus bimaculatus).

Adult crickets (Gryllus bimaculatus) were maintained under a 12-h light:12-h dark cycle (LD 12:12). After oviposition, their eggs were incubated under different lighting regimens at 23 degrees C, and temporal profiles of egg hatching were examined. When the eggs were incubated in LD 12:12 or in DL 12:12 with a phase difference of 12h from LD 12:12, throughout embryogenesis, 88% to 97% of hatching occurred within 3 h of the dark-light transition on days 17 and 18 of embryogenesis; the phases of the egg-hatching rhythms in the LD 12:12 and DL 12:12 groups differed by about 12 h. In eggs incubated in constant darkness (DD) throughout embryogenesis, a circadian (about 24 h) rhythm of hatching was found, and the phase of the rhythm was similar to that seen in eggs incubated in LD 12:12, but not DL 12:12, throughout embryogenesis. When eggs that had been incubated in DD after oviposition were transferred to DL 12:12 in the middle or later stages of embryogenesis and were returned to DD after three cycles of DL 12:12, the rhythm of hatching synchronized (entrained) to DL 12:12. However, when eggs in the earlier stages of embryogenesis were transferred from DD to DL 12:12 and returned to DD after three cycles, 52% to 94% of hatching did not entrain to DL 12:12. To determine whether photoperiodic conditions to which the parents had been exposed influenced the timing of egg hatching, adult crickets were maintained in DL 12:12, and their eggs were incubated in LD 12:12, DL 12:12, or DD throughout embryogenesis. The egg-hatching rhythm was also found in the eggs incubated under these three lighting regimens. In DD, the phase of the rhythm was similar to that seen in eggs incubated in DL 12:12, not LD 12:12, throughout embryogenesis. The results indicate that in the cricket, the timing of egg hatching is under circadian control and that the circadian rhythm of hatching entrains to 24-h light:dark cycles, but only if the light:dark cycles are imposed midway through embryogenesis. Therefore, by midembryogenesis, a circadian clock has been formed in the cricket, and this is entrainable to light:dark cycles. In addition, the photoperiodic conditions to which the parents (probably the mothers) have been exposed influence the timing of hatching, suggesting that maternal factors may regulate the timing of egg hatching.