Stretch-induced moulting in Oncopeltus fasciatus

Fifth instar nymphs of Oncopeltus fasciatus initiate a moulting cycle when they reach a critical weight of 27.9±1.1 (±S.D.) mg for females or 23.7±1.2 mg for males. This critical weight is not absolute. Its precise value depends on the somatic size of the individual. Large-bodied individuals have a higher critical weight than small-bodied ones. Sexual size-dimorphism accounts largely for sex differences in critical weights. Animals of a subcritical weight can be induced to moult by means of a saline injection, in the absence of further food intake or growth. Since a saline injection only stretches the abdominal wall it is suggested that moulting in Oncopeltus is triggered by stimulation of abdominal stretch receptors, as in bloodsucking Reduviidae. The critical weight is thus an index of the size at which a critical degree of stretch is achieved.     

Hormonal control of vitellogenin synthesis in Oncopeltus fasciatus

Previous work indicated the existence of two vitellogenins (A and B) in the haemolymph of Oncopeltus fasciatus, and that vitellogenin B was juvenile hormone (JH)-dependent whereas A was not (Kelly and Telfer, 1977). We have extended these results using several electrophoretic techniques in combination with limited proteolysis of key proteins to show that (1) vitellogenin B is present in eggs in a modified form while vitellogenin A cannot be detected in eggs. (2) Vitellogenin A may be a precursor of B since it has a molecular weight of 200,000D, approximately three times that of vitellogenin B (68,000D) and analysis by limited proteolysis shows that two proteins to be nearly identical. (3) Neither ovariectomy nor treatment with the anti-allatotropin, precocene II prohibits the appearance of vitellogenins A and B in the haemolymph. (4) Injection of ecdysone or 20-hydroxyecdysone into adult, male Oncopeltus fasciatus induces the appearance of both vitellogenin A and B in the haemolymph, suggesting the possible involvement of ecdysteroids in the control of vitellogenin synthesis in this species. (5) We have no evidence for JH control of the synthesis of vitellogenin, however, the ratio of vitellogenin A to B in the haemolymph is higher in the precocene-treated females.     

Juvenile hormone induced ovarian uptake of a female-specific blood protein in Oncopeltus fasciatus

An adult female-specific blood protein was demonstrated in Oncopeltus by gel electrophoresis. This protein is the major band in soluble yolk fractions. It is also present at substantial concentrations in the haemolymph of starved and diapausing adult females. Thus, the failure of the ovaries to form yolk under these conditions is characterized by an inability to remove vitellogenin from the blood. Application of a juvenile hormone analog (JHA) restored protein yolk deposition in starved and diapausing adult females. Whereas other blood proteins decreased no more than two-fold upon JHA treatment, the vitellogenin concentration decreased 20-fold in starved females. The vitellogenin concentration in the blood of diapausing females was not significantly affected by JHA, apparently because synthesis kept pace with ovarian uptake in this case.     

Effects of a juvenile hormone analogue on the duration of the fifth instar in the milkweed bug Oncopeltus fasciatus

Topical application of JHA to fifth instar nymphs of Oncopeltus fasciatus, immediately following ecdysis from the fourth instar, decreases the duration of the fifth instar by approximately 36 hr in addition to inducing a supernumerary larval moult. JHA appears to accelerate the time of subsequent ecdysis in two ways: first, the onset of ecdysone secretion is accelerated, and is accompanied by a similarly premature initiation of mitotic activity in epidermal cells. This is the classical prothoracicotropic action of JH. Second, the period between the onset of mitotic activity and the time of ecdysis itself is shortened. That is, once cellular activities associated with the moulting cycle are triggered by ecdysone, such activities are completed more rapidly in the presence of JHA. It appears that the larval-larval moult induced by JHA requires intrinsically less time to accomplish than a normal metamorphic moult.     

Precocene-induced degeneration of the corpus allatum of adult females of the bug Oncopeltus fasciatus

Application of precocene II, either topically or by contact method, to newly emerged adult females of O. fasciatus inhibits egg maturation and induces degeneration of the corpus allatum (CA). Seven days after precocene-treatment the CA were very small. Ultrastructural studies revealed a number of disintegrating nuclei, degenerating mitochondria, lysosome-like bodies and autophagic vacuoles. Fifteen days after precocene treatment the CA showed further reduction in the size and number of cells, and the degenerative changes were much more pronounced. It appears that the effect of prococene on the CA is irreversible.     

Synchrony of juvenile hormone-sensitive periods for internal and external development in last-instar larvae of Oncopeltus fasciatus

Breakdown of the moulting glands in Oncopeltus can be completely inhibited by topical application of a juvenile hormone analogue prior to day 2 of the fifth instar, and partially inhibited by application prior to day 4. The analogue-sensitive period for the inhibition of external metamorphosis is very similar to that for the inhibition of cell death in the moulting glands. A decline in response to the analogue between days 2 and 4 is correlated with rising ecdysteroid levels in the haemolymph. That this rise in ecdysteroids may be responsible for termination of the juvenile hormone-sensitive period is suggested by premature loss of sensitivity to the analogue by the moulting glands following premature exposure to 20-hydroxyecdysone.     

The accessory gland and metathoracic scent gland function in Oncopeltus fasciatus

The paper provides further clues as to the physiological function and biological significance of the ‘accessory gland’ in the metathoracic scent apparatus of the lygaeid Oncopeltus fasciatus. From various lines of evidence (gas chromatographic, cytochemical) it is concluded as probable that the accessory gland secretes small quantities of a mucopolysaccharide secretory product together with water. The difficulty of reconciling these data with other data indicating that the accessory gland is involved in the biosynthesis of the scent aldehydes is discussed. It is suggested that the water secreted into the median scent reservoir by the accessory gland provides O. fasciatus with a means of volumetric compensation for a diminished output of scent repellent.     

Endocrine influence upon the development of vitellogenic competency in Oncopeltus fasciatus

Immunoelectrophoretic analysis of the blood of precocious adult females of Oncopeltus demonstrated the presence of the A form of vitellogenin but no detectable AB form, the form present in mature adult-female haemolymph. Although the appearance of the AB form could be induced by topical treatment of precocious adult females with juvenile hormone, no yolk deposition was induced in these females. Histological examination revealed that the ovaries of precocious adult females reached only a previtellogenic stage of development with or without treatment with juvenile hormone. Topical treatment of normal larvae and adults with the hormone demonstrated that vitellogenin synthesis could not be induced with juvenile hormone treatment alone until after adult emergence. Since the precocious adult females are chronologically younger than normal last-stage larval instars, we suggest that a transition period during which juvenile hormone is absent (i.e. the precocious moult in treated animals; the final moult in control animals) must occur before some tissue of the precocious or normal adult females, presumably the fat body, becomes competent to respond to further exposure to the hormone by making the AB form of vitellogenin. The ovary requires a similar transition period prior to becoming capable of depositing vitellogenin; however, the times when the fat body and the ovary become competent to respond to the transition period differ.     

Effect of food limitation on food consumption and reproductive allocation by adult milkweed bugs, Oncopeltus fasciatus

Male/female pairs of adult milkweed bugs, Oncopeltus faciatus were offered reduced food rations (100 mg milkweed seeds/week, 50 mg seeds/week, and 25 mg seeds/week) at 232C, 14L : 10D photoperiod. The high rate of food consumption normally occurring during the teneral period (first 8 days posteclosion) and subsequent food consumption were reduced on both reduced rations. Initial fresh weight was lost by bugs on both reduced rations and their weekly fresh body weight tended to remain less than bugs on the 100 mg seeds/week ration.Normal onset of egg production (about 3 weeks posteclosion) was delayed 6 weeks and normal length of a female's life (13–14 weeks) was increased 7–8 weeks on the 25 mg seeds/week ration. Rate of egg production exhibited a linear decrease with decrease in food ration on both reduced rations due to an increased interclutch interval, while fresh egg weight, duration of egg stage, number of eggs/clutch and percentage of clutches containing fertile eggs were little affected.Gross egg production efficiency (efficiency of conversion of ingested food into eggs = dry mg egg production × 100/dry mg food ingested) was high on both the 100 and 50 mg seeds/week rations (27.9% and 21.1%, respectively) and low (7.7%) on the 25 mg seeds/week ration. These data are discussed in the context of the reciprocal interaction between nutrient intake and egg production.     

DNA-dependent RNA polymerase activities during embryogenesis in the bug, Oncopeltus fasciatus

Using α-amanitin to inhibit polymerase II activity in intact nuclei from Oncopeltus embryos, it is demonstrated that there is no difference in relative amounts of α-amanitin-resistant (Form I) and α-amanitin-sensitive (Form II) polymerases at two stages of embryonic development (70 and 140 hr), although the total polymerase activity is considerably higher at the earlier stage. However the RNA made under these circumstances (presumably due to Form I activity) appears to be, as expected, largely ribosomal.When the RNA polymerase activities are solubilized and separated, there is a substantially higher level of Form I activity in 70-hr embryos over that in 140-hr embryos. It is suggested that this high level of polymerase activity is correlated directly with the high level of ribosomal RNA synthesis at this stage.     

Composition of scents from the larva of the milkweed bug Oncopeltus fasciatus

The dorsal abdominal scent glands of the larva of Oncopeltus fasciatus may have different, although not necessarily unrelated, functions. It has been found that oct-2-en-1-al is the major component of the scent from the anterior gland, 4-oxo-oct-2-en-1-al the major component of the scent from the posterior gland, and that the larva releases the secretion from the posterior gland more readily than it does from the anterior gland. Minor components of the scents include hex-2-en-1-al, 4-oxohex-2-en-1-al, and possibly hept-2-en-1-al. The secretions, after release onto the dorsal surface of the abdomen, are subsequently brushed off with the hind tarsi onto the surface on which the larva is standing. This behaviour can be elicited by stimulating the dorsal abdominal surface of the larva with the vapour of octenal and similar aldehydes.     

A carrier lipoprotein for juvenile hormone in the haemolymph of Locusta migratoria

In the haemolymph of adult female locusts six different lipoprotein fractions have been demonstrated by means of isoelectric focusing. One of these binds injected ³H-Cecropia juvenile hormone. The carrier protein is a yellow lipoprotein with a molecular weight of approximately 220,000 daltons and an isoelectric point of pH 6·8. The binding of the hormone to the protein is stable during gel filtration over Sephadex G-25 and during dialysis for 24 hr against phosphate buffer pH 7·0.The hormone is quickly metabolized in the locusts. In the haemolymph were found more polar compounds such as 10-epoxy-7-ethyl-3,11 dimethyl-2,6-tridecadienoic acid and the corresponding dioldienoic acid.Both compounds were not bound by the pH 6·8 carrier lipoprotein under in vivo conditions.     

In vitro stimulation of cell death in the moulting glands of Oncopeltus fasciatus by 20-hydroxyecdysone

The moulting glands of the milkweed bug, Oncopeltus fasciatus, normally degenerate just before the time of ecdysis to an adult (day 7 of the fifth instar). Morphologically normal cell death can be prematurely stimulated in vitro by 20-hydroxyecdysone. Breakdown is triggered by a 24-hr period of exposure to 20-hydroxyecdysone, but an additional incubation period is required before clear signs of degeneration are manifested. Glands removed after the onset of endogenous ecdysteroid secretion degenerate in vitro in the absence of added hormones. Thus, in the moulting glands of Oncopeltus, ecdysteroids appear to act as an important trigger for metamorphic cell death.     

Synthesis and intercellular transport of ribosomal RNA in the ovary of the milkweed bug Oncopeltus fasciatus

Analysis of cellular and subcellular fractions obtained by dissection and subsequent purification of the main components of the Oncopeltus fasciatus ovary reveals that the uptake and incorporation of tritiated uridine into high molecular weight RNA differs markedly between the trophic syncytium and the oöcytes. The pattern of labelling indicates that the trophic syncytium is actively engaged in the synthesis of ribosomal RNA, but that the oöcytes do not synthesize detectable amounts of ribosomes. The ribosomal RNA which eventually appears in the cytoplasm of the oöcytes is postulated to come from the trophic syncytium via the nutritive cords. There is no other fraction of non-4 S RNA detectable in the oöcytes by these methods.     

The relationship of age,sex, and reproductive state to assimilation efficiency in the adult milkweed bug, Oncopeltus fasciatus

The influence of age, sex, and reproductive state on efficiency of assimilation was examined in the milkweed bug Oncopeltus fasciatus. Amounts of food ingested and excreted decline with age in virgin males and females. Mated females laying eggs ingest significantly more food than virgin animals of either sex and also maintain a consistently higher assimilation efficiency. A qualitative analysis of the level of assimilation required for reproductive effort is discussed.     

Dietary influence of cardenolides on larval growth and development of the milkweed bug Oncopeltus fasciatus

Larvae of the milkweed bug Oncopeltus fasciatus were reared on the seeds of eight different species of milkweed (Asclepias), representing a wide range of cardenolide concentrations in the diet. There were few significant differences in larval developmental period, wet body weight of teneral adults, dry weight of adults, and pronotal width of adults reared on the different diets. However, the data indicate no significant correlations between cardenolide content, and body weight or size of the adult insects.There was no evidence in this study of a physiological cost or adverse effect on the larval growth and development of insects which sequestered and stored differing quantities of cardenolides. Instead, the data support a recently-proposed model of cardenolide sequestration which may be energy-independent.The validity of evidence supporting a physiological cost hypothesis for sequestration of cardenolides by the monarch butterfly is discussed in the light of these findings.     

Sex-determined differential mortality of milkweed bugs, Oncopeltus fasciatus (hemiptera), to aflatoxins

Studies on the aflatoxins, toxic metabolites of Aspergillus flavus and A. parasiticus, have involved test systems ranging from cell cultures to laboratory animals. This work reports on the differential response by sex of Oncopeltus fasciatus to aflatoxin B₁ (AFB₁). Young adult milkweed bugs were chosen randomly from our stock colony and housed in glass culture jars. Triplicate sets of experimental animals were fed 5 μg/ml of AFB₁ in their liquid diet. The first death for the experimental females occurred at day 4, and at 10 days for the experimental males. A 50% lethality level for experimental females developed by day 8. Males subjected to the same concentration achieved a 50% lethality level at day 24. For the females the LD₅₀ occurred after consuming 0.49 μg/ml of AFB₁. The results indicate that adult female milkweed bugs were hypersensitive to AFB₁ as compared to adult males. This organism is more sensitive than the American cockroach and less sensitive than the fruitfly, housefly, and honeybee to toxic aflatoxicosis. Even the female is not sufficiently sensitive to rate highly as a bioassay organism for AFB₁. The extreme difference in mortality between the sexes is significant, unusual, and unexplained.     

Diverging functions of Scr between embryonic and post-embryonic development in a hemimetabolous insect, Oncopeltus fasciatus

Hemimetabolous insects undergo an ancestral mode of development in which embryos hatch into first nymphs that resemble miniature adults. While recent studies have shown that homeotic (hox) genes establish segmental identity of first nymphs during embryogenesis, no information exists on the function of these genes during post-embryogenesis. To determine whether and to what degree hox genes influence the formation of adult morphologies, we performed a functional analysis of Sex combs reduced (Scr) during post-embryonic development in Oncopeltus fasciatus. The main effect was observed in prothorax of Scr-RNAi adults, and ranged from significant alterations in its size and shape to a near complete transformation of its posterior half toward a T2-like identity. Furthermore, while the consecutive application of Scr-RNAi at both of the final two post-embryonic stages (fourth and fifth) did result in formation of ectopic wings on T1, the individual applications at each of these stages did not. These experiments provide two new insights into evolution of wings. First, the role of Scr in wing repression appears to be conserved in both holo- and hemimetabolous insects. Second, the prolonged Scr-depletion (spanning at least two nymphal stages) is both necessary and sufficient to restart wing program. At the same time, other structures that were previously established during embryogenesis are either unaffected (T1 legs) or display only minor changes (labium) in adults. These observations reveal a temporal and spatial divergence of Scr roles during embryonic (main effect in labium) and post-embryonic (main effect in prothorax) development.     

Effects of precocene I and II on flight behaviour in Oncopeltus fasciatus, the migratory milkweed bug

Treatment of newly emerged adult Oncopeltus fasciatus with the corpus allatum inhibitors. 7-methoxy-2,2-dimethyl chromene or 6,7-dimethoxy-2,2-dimethyl chromene (preocene I and II) results in an inhibition of long-term flight (presumed migratory) behaviour in both males and females and inhibition of oögenesis in females. Treatment of reproductive females with precocene briefly stimulates flight behaviour (which ceases in such females as oviposition begins) and then subsequently inhibits it. Oviposition also ceases in such females and oöcyte resorption occurs. Topical application or injections of JH III to precocenetreated animals results in immediate restoration of the tendency to make long tethered flights while corpora cardiaca extract injections, corpora cardiaca implantation, sham implantation, sham injections and sham topical applications were ineffective in restoring prolonged flight behaviour to precocene-treated animals.Restoration of flight by JH III injection was observed within 1 hr after treatment with the hormone. These results indicate that JH is necessary for prolonged flight and presumably migratory behaviour in this species and its stimulatory effect on flight behaviour is immediate. Possible mechanisms of action of the hormone on flight behaviour are discussed.