Injury response checkpoint and developmental timing in insects

In insects, localized tissue injury often leads to global (organism-wide) delays in development and retarded metamorphosis. In Drosophila, for example, injuries to the larval imaginal discs can retard pupariation and prolong metamorphosis. Injuries induced by treatments such as radiation, mechanical damage and induction of localized cell death can trigger similar delays. In most cases, the duration of the developmental delay appears to be correlated with the extent of damage, but the effect is also sensitive to the developmental stage of the treated animal. The proximate cause of the delays is likely a disruption of the ecdysone signaling pathway, but the intermediate steps leading from tissue injury and/or regeneration to that disruption remain unknown. Here, we review the evidence for injury-induced developmental delays, and for a checkpoint or checkpoints associated with the temporal progression of development and the on-going efforts to define the mechanisms involved.     

Injury response checkpoint and developmental timing in insects

In insects, localized tissue injury often leads to global (organism-wide) delays in development and retarded metamorphosis. In Drosophila, for example, injuries to the larval imaginal discs can retard pupariation and prolong metamorphosis. Injuries induced by treatments such as radiation, mechanical damage and induction of localized cell death can trigger similar delays. In most cases, the duration of the developmental delay appears to be correlated with the extent of damage, but the effect is also sensitive to the developmental stage of the treated animal. The proximate cause of the delays is likely a disruption of the ecdysone signaling pathway, but the intermediate steps leading from tissue injury and/or regeneration to that disruption remain unknown. Here, we review the evidence for injury-induced developmental delays, and for a checkpoint or checkpoints associated with the temporal progression of development and the on-going efforts to define the mechanisms involved.     

Mechanisms and Consequences of Developmental Acceleration in Tadpoles Responding to Pond Drying

Many amphibian species exploit temporary or even ephemeral aquatic habitats for reproduction by maximising larval growth under benign conditions but accelerating development to rapidly undergo metamorphosis when at risk of desiccation from pond drying. Here we determine mechanisms enabling developmental acceleration in response to decreased water levels in western spadefoot toad tadpoles (Pelobates cultripes), a species with long larval periods and large size at metamorphosis but with a high degree of developmental plasticity. We found that P. cultripes tadpoles can shorten their larval period by an average of 30% in response to reduced water levels. We show that such developmental acceleration was achieved via increased endogenous levels of corticosterone and thyroid hormone, which act synergistically to achieve metamorphosis, and also by increased expression of the thyroid hormone receptor TRΒ, which increases tissue sensitivity and responsivity to thyroid hormone. However, developmental acceleration had morphological and physiological consequences. In addition to resulting in smaller juveniles with proportionately shorter limbs, tadpoles exposed to decreased water levels incurred oxidative stress, indicated by increased activity of the antioxidant enzymes catalase, superoxide dismutase, and gluthatione peroxidase. Such increases were apparently sufficient to neutralise the oxidative damage caused by presumed increased metabolic activity. Thus, developmental acceleration allows spadefoot toad tadpoles to evade drying ponds, but it comes at the expense of reduced size at metamorphosis and increased oxidative stress.     

Temporal expression and steroidal regulation of piRNA pathway genes (mael, piwi, vasa) during Silurana (Xenopus) tropicalis embryogenesis and early larval development

It has been extensively documented that exposure of amphibians and teleost fish to exogenous steroid hormones like estrogen, androgen, xenoestrogen or steroid biosynthesis inhibitors can impair their gonadal development or induce sex reversal against genotypic sex. However, the molecular pathways underlying sexual development and the effects of sex steroids or other exogenous hormones in these aquatic vertebrates remain elusive. Recently, a germ plasm-associated piRNA (piwi-interacting RNA) pathway has been shown to be a determinant in the development of animal gonadal germline cells. In the current study, we examined whether this piRNA pathway is involved in the regulation of sex steroid hormones in gonadal development. We firstly established developmental expression patterns of three key piRNA pathway genes (mael, piwi and vasa), during Silurana (Xenopus) tropicalis embryogenesis and early larval development. All three genes exhibit high expression at early developmental stages and have significantly decreased expression thereafter, indicating a very active involvement of piRNA pathway at the beginning of embryogenesis. We further examined gene expression changes of those genes in frog larvae exposed to two sex steroid biosynthesis inhibitors, fadrozole and finasteride, both of which are known to result in male-biased or female-biased phenotypes, respectively. We found that fadrozole and finasteride exposures increased the expression of piRNA pathway genes such as mael and vasa at the larval stage when the expression of piRNA pathway genes is programmed to be very low. Therefore, our results indicate that the piRNA pathway is likely a common pathway by which different sex steroid hormones regulate gonadal sex differentiation.     

The dare gene: steroid hormone production, olfactory behavior, and neural degeneration in Drosophila.

Steroid hormones mediate a wide variety of developmental and physiological events in insects, yet little is known about the genetics of insect steroid hormone biosynthesis. Here we describe the Drosophila dare gene, which encodes adrenodoxin reductase (AR). In mammals, AR plays a key role in the synthesis of all steroid hormones. Null mutants of dare undergo developmental arrest during the second larval instar or at the second larval molt, and dare mutants of intermediate severity are delayed in pupariation. These defects are rescued to a high degree by feeding mutant larvae the insect steroid hormone 20-hydroxyecdysone. These data, together with the abundant expression of dare in the two principal steroid biosynthetic tissues, the ring gland and the ovary, argue strongly for a role of dare in steroid hormone production. dare is the first Drosophila gene shown to encode a defined component of the steroid hormone biosynthetic cascade and therefore provides a new tool for the analysis of steroid hormone function. We have explored its role in the adult nervous system and found two striking phenotypes not previously described in mutants affected in steroid hormone signaling. First, we show that mild reductions of dare expression cause abnormal behavioral responses to olfactory stimuli, indicating a requirement for dare in sensory behavior. Then we show that dare mutations of intermediate strength result in rapid, widespread degeneration of the adult nervous system.     

The expression of the let-7 small regulatory RNA is controlled by ecdysone during metamorphosis in Drosophila melanogaster

In Caenorhabditis elegans, the heterochronic pathway controls the timing of developmental events during the larval stages. A component of this pathway, the let-7 small regulatory RNA, is expressed at the late stages of development and promotes the transition from larval to adult (L/A) stages. The stage-specificity of let-7 expression, which is crucial for the proper timing of the worm L/A transition, is conserved in Drosophila melanogaster and other invertebrates. In Drosophila, pulses of the steroid hormone 20-hydroxyecdysone (ecdysone) control the timing of the transition from larval to pupal to adult stages. To test whether let-7 expression is regulated by ecdysone in Drosophila, we used Northern blot analysis to examine the effect of altered ecdysone levels on let-7 expression in mutant animals, organ cultures, and S2 cultured cells. Experiments were conducted to test the role of Broad-Complex (BR-C), an essential component in the ecdysone pathway, in let-7 expression. We show that ecdysone and BR-C are required for let-7 expression, indicating that the ecdysone pathway regulates the temporal expression of let-7 in Drosophila. These results demonstrate an interaction between steroid hormone signaling and the heterochronic pathway in insects.     

Nitric Oxide Synthase Regulates Growth Coordination During Drosophila melanogaster Imaginal Disc Regeneration

Mechanisms that coordinate growth during development are essential for producing animals with proper organ proportion. Here we describe a pathway through which tissues communicate to coordinate growth. During Drosophila melanogaster larval development, damage to imaginal discs activates a regeneration checkpoint through expression of Dilp8. This both produces a delay in developmental timing and slows the growth of undamaged tissues, coordinating regeneration of the damaged tissue with developmental progression and overall growth. Here we demonstrate that Dilp8-dependent growth coordination between regenerating and undamaged tissues, but not developmental delay, requires the activity of nitric oxide synthase (NOS) in the prothoracic gland. NOS limits the growth of undamaged tissues by reducing ecdysone biosynthesis, a requirement for imaginal disc growth during both the regenerative checkpoint and normal development. Therefore, NOS activity in the prothoracic gland coordinates tissue growth through regulation of endocrine signals.     

The Interaction between a Sexually Transferred Steroid Hormone and a Female Protein Regulates Oogenesis in the Malaria Mosquito Anopheles gambiae

Molecular interactions between male and female factors during mating profoundly affect the reproductive behavior and physiology of female insects. In natural populations of the malaria mosquito Anopheles gambiae, blood-fed females direct nutritional resources towards oogenesis only when inseminated. Here we show that the mating-dependent pathway of egg development in these mosquitoes is regulated by the interaction between the steroid hormone 20-hydroxy-ecdysone (20E) transferred by males during copulation and a female Mating-Induced Stimulator of Oogenesis (MISO) protein. RNAi silencing of MISO abolishes the increase in oogenesis caused by mating in blood-fed females, causes a delay in oocyte development, and impairs the function of male-transferred 20E. Co-immunoprecipitation experiments show that MISO and 20E interact in the female reproductive tract. Moreover MISO expression after mating is induced by 20E via the Ecdysone Receptor, demonstrating a close cooperation between the two factors. Male-transferred 20E therefore acts as a mating signal that females translate into an increased investment in egg development via a MISO-dependent pathway. The identification of this male–female reproductive interaction offers novel opportunities for the control of mosquito populations that transmit malaria.     

Spiny lobster development: mechanisms inducing metamorphosis to the puerulus: a review

This review outlines current knowledge of mechanisms effecting metamorphosis in decapod crustaceans and insects. The comparative approach demonstrates some of the complexities that need resolving to find an answer to the question raised frequently by ecologists: “What triggers metamorphosis in spiny lobsters?” It is evident that crustacean moulting and metamorphosis are genetically controlled through endocrine systems that mediate gene expression. The molecular mechanisms underlying these developmental processes have been studied intensively in insects, particularly in the fruitfly, Drosophila melanogaster (Diptera), and some lepidopteran species. Comparatively, there is minimal information available for a few decapod crustacean species, but none for spiny lobsters (Palinuridae). Nothing was known of hormone signalling transduction pathways, via nuclear receptors (NRs) and gene activation during larval moults in palinurids—until a recent, ground-breaking study of early phyllosomal development of Panulirus ornatus by Wilson et al. (Rock Lobster Enhancement and Aquaculture Subprogram. FRDC Project 2000/263, Australian Govt, Fisheries Research and Development Corporation and Australian Institute of Marine Science, Nov 2005). Their study not only identified homologues of five hormone NRs of D. melanogaster, but also patterns of gene regulation showing strong similarities to those of gene expression found in insect larval development. Their results indicated that control of moulting and metamorphosis in palinurids closely parallels that in insects, suggesting that insects can serve as model systems for elucidating molecular mechanisms in larval decapods. In insects and crustaceans, the steroid hormone, ecdysone, (20E) initiates moulting. In insects, juvenile hormone (JH) mediates the type of larval moult that occurs, either anamorphic or metamorphic. The latter results when the level of JH in the haemolymph drops in the final larval instar. High levels of JH inhibit the metamorphic moult during insect larval development. The interaction of 20E and JH is not fully understood, and the operative molecular mechanisms are still being elucidated. No nuclear receptor for JH has been identified, and alternative JH signalling pathways await identification. In decapod crustaceans, methyl farnesoate (MF), a precursor of JH, replaces the latter in other functions mediated by JH in insects; but there is little evidence indicating that MF plays a similar ‘antimetamorphic’ role in decapod larval moults.     

Microarray and RNAi Analysis of P450s in Anopheles gambiae Male and Female Steroidogenic Tissues: CYP307A1 Is Required for Ecdysteroid Synthesis

In insects, the steroid hormone 20-hydroxyecdysone (20E) coordinates major developmental transitions. While the first and the final steps of 20E biosynthesis are characterized, the pathway from 7-dehydrocholesterol to 5β-ketodiol, commonly referred as the “black box”, remains hypothetical and whether there are still unidentified enzymes is unknown. The black box would include some oxidative steps, which are believed to be mediated by P450 enzymes. To identify new enzyme(s) involved in steroid synthesis, we analyzed by small-scale microarray the expression of all the genes encoding P450 enzymes of the malaria mosquito Anopheles gambiae in active steroidogenic organs of adults, ovaries from blood-fed females and male reproductive tracts, compared to inactive steroidogenic organs, ovaries from non-blood-fed females. Some genes encoding P450 enzymes were specifically overexpressed in female ovaries after a blood-meal or in male reproductive tracts but only three genes were found to be overexpressed in active steroidogenic organs of both females and males: cyp307a1, cyp4g16 and cyp6n1. Among these genes, only cyp307a1 has an expression pattern similar to other mosquito steroidogenic genes. Moreover, loss-of-function by transient RNAi targeting cyp307a1 disrupted ecdysteroid production demonstrating that this gene is required for ecdysteroid biosynthesis in Anopheles gambiae.     

Physiological effects of compensatory growth during the larval stage of the ladybird,Cryptolaemus montrouzieri

The growth rate of insects may vary in response to shifty environments. They may achieve compensatory growth after a period of food restriction followed by ad libitum food, which may further affect the reproductive performance and lifespan of the resulting phenotypes. However, little is known about the physiological mechanisms associated with such growth acceleration in insects. The present study examined the metabolic rate, the antioxidant enzyme activity and the gene expression of adult Cryptolaemus montrouzieri (Coleoptera: Coccinellidae) after experiencing compensatory growth during its larval stages. Starved C. montrouzieri individuals achieved a similar developmental time and adult body mass as those supplied with ad libitum food during their entire larval stage, indicating that compensatory growth occurred as a result of the switch in larval food regime. Further, the compensatory growth was found to exert effects on the physiological functions of C. montrouzieri, in terms of its metabolic rates and enzyme activities. The adults undergoing compensatory growth were characterized by a higher metabolic rate, a lower activity of the antioxidant enzymes glutathione reductase, catalase, and superoxide dismutase and a lower gene expression of P450 and trehalase. Taken together, the results indicate that although compensatory growth following food restriction in early larval life prevents developmental delay and body mass loss, the resulting adults may encounter physiological challenges affecting their fitness.     

Epithelial cell-turnover ensures robust coordination of tissue growth in Drosophila ribosomal protein mutants

Highly reproducible tissue development is achieved by robust, time-dependent coordination of cell proliferation and cell death. To study the mechanisms underlying robust tissue growth, we analyzed the developmental process of wing imaginal discs in Drosophila Minute mutants, a series of heterozygous mutants for a ribosomal protein gene. Minute animals show significant developmental delay during the larval period but develop into essentially normal flies, suggesting there exists a mechanism ensuring robust tissue growth during abnormally prolonged developmental time. Surprisingly, we found that both cell death and compensatory cell proliferation were dramatically increased in developing wing pouches of Minute animals. Blocking the cell-turnover by inhibiting cell death resulted in morphological defects, indicating the essential role of cell-turnover in Minute wing morphogenesis. Our analyses showed that Minute wing discs elevate Wg expression and JNK-mediated Dilp8 expression that causes developmental delay, both of which are necessary for the induction of cell-turnover. Furthermore, forced increase in Wg expression together with developmental delay caused by ecdysone depletion induced cell-turnover in the wing pouches of non-Minute animals. Our findings suggest a novel paradigm for robust coordination of tissue growth by cell-turnover, which is induced when developmental time axis is distorted.     

Effects of starvation,chilling, and injury on endocrine gland function in Galleria mellonella

Starvation, chilling, and injury of last instar Galleria mellonella larvae typically elicit extra larval molts or a delay in pupation. The primary sites of action and the nature of the signals by which these treatments affect development are not known. However, since the connections of the brain to the nerve cord are crucial for the effects of starvation and chilling, these signals apparently affect the brain-centered program of developmental regulation via the nerve cord. Chilling, and occasionally starvation, cause extra larval molts in last instar larvae treated prior to the nervous inhibition of their corpora allata; release of a cerebral allatotropin, which stimulates the production of juvenile hormone, appears to be involved in this effect. After this time, a delay in pupation is the principal effect of starvation and chilling, and is apparently due to a temporal inhibition of the release of the prothoracicotropic hormone. Chilling also appears to inhibit unstimulated ecdysteroid production by the prothoracic glands. The effect of injury is not mediated by the nerve cord, but appears to involve an inhibitory humoral factor that affects either the brain or the prothoracic glands themselves. Injury also stimulates juvenile hormone production, an effect which is enhanced when the brain is separated from the nerve cord and which is evidenced by a delay of ecdysis and the occasional retention of some larval features in the ecdysed insects. None of the effects of these various treatments on the brain and the endocrine glands persist when the brains or glands are implanted into untreated hosts.     

Severe developmental timing defects in the prothoracicotropic hormone (PTTH)-deficient silkworm,Bombyx mori

The insect neuropeptide prothoracicotropic hormone (PTTH) triggers the biosynthesis and release of the molting hormone ecdysone in the prothoracic gland (PG), thereby controlling the timing of molting and metamorphosis. Despite the well-documented physiological role of PTTH and its signaling pathway in the PG, it is not clear whether PTTH is an essential hormone for ecdysone biosynthesis and development. To address this question, we established and characterized a PTTH knockout line in the silkworm, Bombyx mori. We found that PTTH knockouts showed a severe developmental delay in both the larval and pupal stages. Larval phenotypes of PTTH knockouts can be classified into three major classes: (i) developmental arrest during the second larval instar, (ii) precocious metamorphosis after the fourth larval instar (one instar earlier in comparison to the control strain), and (iii) metamorphosis to normal-sized pupae after completing the five larval instar stages. In PTTH knockout larvae, peak levels of ecdysone titers in the hemolymph were dramatically reduced and the timing of peaks was delayed, suggesting that protracted larval development is a result of the reduced and delayed synthesis of ecdysone in the PG. Despite these defects, low basal levels of ecdysone were maintained in PTTH knockout larvae, suggesting that the primary role of PTTH is to upregulate ecdysone biosynthesis in the PG during molting stages, and low basal levels of ecdysone can be maintained in the absence of PTTH. We also found that mRNA levels of genes involved in ecdysone biosynthesis and ecdysteroid signaling pathways were significantly reduced in PTTH knockouts. Our results provide genetic evidence that PTTH is not essential for development, but is required to coordinate growth and developmental timing.     

Can larval epidermis omit the secretion of a pupal cuticle in a saturniid moth, Samia cynthia ricini?

When the larval tissue is exposed to the hormonal milieu lacking juvenile hormone, adult characters appear directly omitting the pupal stage in some insects but not in others. In Samia cynthia ricini, a species belonging to the latter group, a possible omission of pupal characters was tested by previously untried experiments. Firstly, the possibility that the larval epidermis of only some stages is capable of responding so as to omit to secrete the pupal cuticle was tested. Pieces of larval integument taken from various developmental stages were implanted into developing (pharate) adults. None of these failed to secrete the pupal cuticle. Secondly, pieces of larval integument were first implanted into brainless pupae and left there for a month to eliminate the effect of a trace of juvenile hormone which might have been carried over by the implants. They were then caused to develop, and they again secreted pupal cuticle. It is concluded that the larval epidermis cannot omit secreting pupal cuticle in this species.