Long-term effects of diet on the neuroendocrine system of the sheep blowfly, Lucilia sericata

ABSTRACT. The medial neurosecretory cells (MNC), corpus allatum (CA) and ovaries of Lucilia sericata Mg. are quiescent in mature flies fed on a diet of sugar and water, but these tissues rapidly respond to the ingestion of a protein meal. The present study revealed that after oogenesis, virgin flies synthesized and stored neurosecretory material, but this activity declined as they grew older. At the same time, the CA, following a decrease in size during the latter stages of oogenesis, became enlarged again. This is interpreted as denoting synthesis but not release of CA hormone. In meat-fed mated flies, cycles of oviposition and oogenesis were reflected in the rise and fall in activity of the MNC and CA. The MNC and CA of male flies also responded to the ingestion of a protein meal, and the activity of these tissues persisted for several days before it declined. The similar response of the MNC and CA in both males and females demonstrates the importance of a meat meal to reproduction in this species: in females it is required for oogenesis, but its ingestion by both sexes is necessary for overall reproductive success.     

Effects of electrocoagulation of cerebral neurosecretory cells and implantation of corpora allata on oöcyte development in Locusta migratoria

The implantation of active corpora allata into intact Locusta females during growth accelerates pre-vitellogenic oöcyte growth and vitellogenesis. Localised stimulation of yolk deposition follows the implantation of active corpora allata between the ovarioles demonstrating a gonadotrophic rôle for the corpus allatum hormone. Electrocoagulation of the median neurosecretory cells of the brain prevents vitellogenesis whilst pre-vitellogenic oöcyte growth occurs normally. Implantation of active corpora allata into females with ablated cerebral neurosecretory cells promotes vitellogenesis in a proportion of test animals although mature oöcytes are never produced.It is suggested that the rôle of the median neurosecretory cells during egg development in Locusta is primarily concerned with the activation and maintenance of activity of the corpora allata. The corpus allatum hormone acts both metabolically and gonadotrophically.     

Actions of the juvenile hormone, 20-hydroxy-ecdysone,and the oöstatic hormone during oögenesis in the flies Phormia regina and Sarcophaga bullata

Application of juvenile hormone (JH) to sugar-fed Phormia flies leads to full ovarian development, i.e. the flies become autogenous. Application of JH to sugar + liver-fed Phormia leads to the simultaneous development of primary, secondary and tertiary oöcytes, suggesting the role of the oöstatic hormone is to shut off the action of the JH. This is consistent with the notion that the oöstatic hormone may act on the corpus allatum either directly or through the neurosecretory system. Application of 20-hydroxy-ecdysone to Phormia or Sarcophaga, when primary oöcytes have started to develop (stage 4A), causes the primary oöcytes to degenerate, with concomitant development of the secondary oöcytes.     

Evidence that the ovaries of Musca domestica do not maintain cyclicity by regulating the corpus allatum.

When the female housefly retains eggs, vitellogenesis in the penultimate oöcytes is suppressed during continued protein feeding. Allatectomy of gravid females or of females with developing oöcytes did not prevent maturation of a second batch of eggs. This result does not support the claim of Adams (1970) that an oöstatic hormone, produced by ovaries with retained eggs, inactivates the corpus allatum (CA) and thereby prevents development of the next batch of eggs.The report of Adams and Hintz (1969) that the CA regulates egg maturation depends on their removal of ring gland, which they incorrectly refer to as ‘allatectomy’. In the present report, removal of the ring gland from 1 day old females suppressed egg development, whereas removal of the CA alone did not. Therefore, sufficient CA hormone for reproduction was secreted within 24 hr of emergence, and it was the removal of the corpus cardiacum, and not the CA, that had prevented egg development in the experiments of Adams and Hintz.     

Enhancement of juvenile hormone biosynthesis in locusts following electrostimulation of cerebral neurosecretory cells

Electrostimulation of the medial neurosecretory cells of day-1 adult female Locusta migratoria resulted in a significant enhancement of juvenile hormone biosynthesis by the corpora allata within 2–3 days of the operation, as determined by a radiochemical assay for juvenile hormone biosynthesis. This elevation in the rate of juvenile hormone biosynthesis was also reflected in basal oöcyte length, with the oöcytes of stimulated animals significantly larger than the sham-operated animals. Radio-frequency cautery of the cerebral axonal tracts of the medial neurosecretory cells prevented this enhancement in juvenile hormone biosynthesis and in basal oöcyte growth in both stimulated and sham-operated animals.Stimulation of the lateral neurosecretory cells resulted in a slight elevation in rates of juvenile hormone biosynthesis 2 days after the operation. However, after cautery of the medial cell tracts, a significant elevation in juvenile hormone biosynthesis was observed 1 and 2 days after stimulation. Basal oöcyte length in stimulated animals differed significantly from sham-operated animals only on day 6. Cautery of the medial cell tracts again attenuated oöcyte growth. Our results suggest that the medial neurosecretory cells are the source of an allatotropin that can be released by electrostimulation. This substance appears to operate directly on the corpus allatum, causing a change in the juvenile hormone biosynthetic machinery.     

Hormonal aspects of oögenesis in the flies Phormia regina and Sarcophaga bullata

The corpora allata (CA) and median neurosecretory cells (MNC) of Phormia regina and Sarcophaga bullata become active with increasing age of the fly, on a diet of sugar alone. To prevent or retard oögenesis the CA or MNCs must be removed shortly after emergence, with subsequent protein meals. Topical JH application partially compensates for CA or MNC removal. This shows that the MNC activate the CA, and not vice versa. The trauma of either operation slightly depresses egg development.Injection of ecdysone into both species in the stage of initial yolk deposition causes the primary oöcytes to degenerate. This leads to development of the penultimate oöcytes. Older and younger egg stages are not sensitive to ecdysone. In P. regina the application of JH to females with developing primary oöcytes stimulates yolk deposition in the penultimate oöcytes.     

Endocrine control of oögenesis in the housefly,Musca domestica vicina

The endocrine system involved in the control of oögenesis in the housefly, Musca domestica vicina, was investigated. Allatectomy, decapitation, and starvation of newly emerged females resulted in inhibition of oögenesis, showing a close relationship between enlargement of the corpus allatum and growth of follicles during the first oögenesis. Histological observation of sexually matured females showed active secretion of the corpus allatum and the medial neurosecretory cells of pars intercerebralis. Topical application of juvenile hormone analogues (JHA) to the allatectomized fly induced the growth of ovary, and critical doses of methoprene and methyl-7, 11-diethyl-juvenate for the maturation of the ovary were determined. JHA stimulated initiationof oögenesis in the starved or decapitated flies as well as vitellogenesis in the sugar-fed one; subsequently it was found that juvenile hormone acted not only as a gonadotropin but also as a regulator of vitellogenesis. Furthermore, JHA stimulated cell lysis in pupal fat body of female flies, indicating a possible influence of juvenile hormone upon the process of releasing vitellogenin.     

Precocene I and II inhibition of vitellogenic oöcyte development in Drosophila melanogaster

Adult female Drosophila melanogaster were exposed to precocene I and II, antiallatropin compounds which result in juvenile hormone deficiency in many insects. The presence of juvenile hormone in Drosophila adults was evaluated by examining vitellogenic oöcyte development, a process regulated by juvenile hormone in these flies. Both precocenes reduced the number of vitellogenic oöcytes present 43 hr after exposure in a dose-dependent manner. Precocene I was effective when applied to either newly eclosed females prior to vitellogenic oöcyte development or to gravid females. Precocene I was also effective in decapitated females, indicating that the action of the compound is not mediated by the brain. Corpus allatum volume, presumably a reflection of secretory activity, increased between 0 and 24 hr after eclosion in control females but not in precocene-treated females even after 48 hr. However, when females were removed from precocene medium, gland volumes increased within 48 hr to approximately those of control flies. This result is consistent with the reversibility of the precocene effect on Drosophila adults. These results suggest that precocene acts on the corpus allatum of Drosophila adult females to produce juvenile hormone deficiency.     

Experimental evidence for a neuroendocrine control of ecdysone biosynthesis in adult females of Locusta migratoria

The considerable increase in ecdysteroid concentration which occurs in normal Locusta ovaries at the end of each cycle of oöcyte maturation is prevented if the median neurosecretory cells of the pars intercerebralis are cauterized, or if the corpora cardiaca are excised 24 hr before the onset of ecdysone synthesis in normal females. Implantation of additional brain-corpora cardiaca complexes into young vitellogenic females advances the time of ecdysone synthesis by some 12 hr. Oöcyte growth itself is not affected in these different types of experiments.It is inferred from the data of the present study that ecdysone synthesis in the follicle cell epithelium of maturing Locusta ovaries is stimulated by a neurohormone produced in the median neurosecretory cells of the pars intercerebralis and secreted into the blood via the corpora cardiaca.     

Effect of extirpation of median neurosecretory cells on reproduction in the female red cotton bug, Dysdercus cingulatus

Extirpation of the cerebral neurosecretory cells of the pars intercerebralis of the red cotton bug, Dysdercus cingulatus, does not prevent maturation of the first batch of eggs in the female. However, vitellogenesis is visibly inhibited by the operation. There is considerable individual variation in the degree of inhibition of vitellogenesis in experimental animals. But in these experimental animals ovaries are shorter, their oöcytes smaller, the number of oöcytes undergoing vitellogenesis fewer: also the follicle cells and their nuclei are smaller. On the contrary, the protein concentration in the blood of experimental animals is higher but is subject to considerable variation and it appears to have no significance. It is concluded that the median neurosecretory cells stimulate vitellogenesis in this animal to a considerable extent.     

Haemolymph ecdysteroid in the housefly,Musca domestica,during oögenesis and its relationship with vitellogenin levels

Ecdysteroid titre in the haemolymph of the housefly, Musca domestica, cycled during oögenesis and peaked at ～50 pg/μl during stages 5, 6 and 7. Levels of 10–20 pg/μl were found in houseflies with pre- and post-vitellogenic ovaries. Removal of the corpus allatum and corpus cardiacum complex resulted in low ecdysteroid levels (10 pg/μl). Ovariectomized flies also had lower ecdysteroid levels than the controls at 2 days (5 pg/μl) after emergence but not at 6 days (22 pg/μl). It is possible that the ecdysteroid peak that occurred during stages 5, 6 and 7 was produced by the ovaries because ovaries secreted and synthesized ecdysteroid in vitro. Endogenous haemolymph ecdysteroid levels had a linear correlation with the amount of vitellogenin that held for hormone concentrations of 5–43 pg/μl. Furthermore, the injection of 20-hydroxyecdysone at doses of 10 ng?1.0 μg/fly increased the amount of vitellogenin from 6 h to 12 h after injection; by 24 h, the vitellogenin returned to control levels. When 20-hydroxyecdysone was injected into ovariectomized flies, it was rapidly degraded and 96% was cleared from the haemolymph within 1 h.     

Factors responsible for the initiation of a second oöcyte maturation cycle in the ovoviviparous cockroach Nauphoeta cinerea

The factors responsible for the initiation of a second oöcyte maturation cycle were investigated by measuring oöcyte growth, vitellogenin titre, and corpus allatum activity after injection of juvenile hormone and/or removal of the egg-case from pregnant females and by performing ovary and corpus allatum transplant experiments.Egg-case removal in late pregnancy results in immediate oöcyte growth, whereas in early pregnancy oöcyte growth is resumed only after a lapse of time, even after injection of juvenile hormone. This, however, induces an immediate increase in the haemolymph vitellogenin titre. A single injection of 2 or 10 μg of juvenile hormone II first stimulates some oöcyte growth after this lapse of time and later activates the corpora allata, which in turn leads to completion of oöcyte maturation. A repeat injection of 10 μg stimulates continuous oöcyte growth without activating the corpora allata. In the presence of an egg case, activation of the corpora allata is suppressed, even after injection of 2 μg of juvenile hormone III, and the oöcytes do not grow. Injection of higher doses stimulates oöcyte growth and leads to expulsion of the egg case in up to 95% of the females. This, however, is not a direct consequence of the increase in size of the ovaries. Ovary transplant experiment show that in young pregnant females the second generation of oöcyte is not yet competent for growth and that ovaries which are competent can mature in young pregnant females, treated with juvenile hormone, whose egg case has been removed.The results are summarized in a model demonstrating the various factors involved in regulating corpus allatum activity in oöcyte maturation and pregnancy and after application of juvenile hormone. We prepose that the corpus allatum activating effect of exogenous juvenile hormone is mediated by the growing oöcyte and that this activation can be suppressed by the continuous presence of exogenous juvenile hormone.     

Ovarian dynamics in Heliconius butterflies: Correlations among daily oviposition rates,egg weights,and quantitative aspects of oögenesis

Both oviposition and ovarian morphology were studied in individual butterflies of the neotropical genus Heliconius, which as adults ingest amino acids from pollen, live up to 6 months, and have continuous oögenesis. Among female Heliconius charitonius, daily oviposition correlates directly with the total number of oöcytes developing in the ovaries. The calculated time required for complete growth of one oöcyte is, however, reasonably constant among individuals of a species with widely varying oviposition rates. Thus, within a species, butterflies laying more eggs per day are not necessarily maturing each at a faster rate, but are processing more oöcytes simultaneously in their ovaries. A further correlation between oviposition rate and adult size suggests that in H. charitonius both ovarian capacity and daily egg production are determined ultimately by extent and/or quality of larval nutrition. Among other Heliconius species, those producing larger eggs generally take longer to make them, but also may develop more oöcytes at the same time in their ovaries. Finally, maximal volume attained by the cap of seven nurse cells associated with each growing oöcyte appears constant within a species and among species is directly proportional to average mature egg weight.     

Juvenile hormone induces ovarian development in diapausing cave-dwelling Drosophila species

Drosophila grisea and macroptera were collected in caves overwintering as adults. The females remained in a state of reproductive diapause which extended until May for macroptera and until July for grisea, whereas the males of both species had mature sperm at all times. Termination of the reproductive diapause under laboratory conditions was accomplished in grisea by exposing them to 14 hr of illumination daily and in macroptera by increasing the temperature to 20°C. Topical application of juvenile hormone (JH) on diapausing grisea caused a prompt termination of diapause and maturation of oöcytes within 10 days. Yolk proteins were found in the haemolymph of diapausing flies but not in their ovaries. In the JH-treated flies, yolk proteins were found in both the haemolymph and the ovaries, suggesting that in this species JH regulates the uptake of yolk proteins.     

Sterol accumulation in developing ovaries of the silkworm Bombyx mori in relation to the diapause hormone

Ovary sterol of the silkworm Bombyx mori during pupal-adult development was investigated using ‘diapause’ and ‘non-diapause’ ovaries. It is composed of cholesterol with high relative concentration of more than 80%, β-sitosterol less than 20% and a trace of campesterol. Cholesterol and β-sitosterol contents per pair of ovaries increased strikingly as ovary development proceeded. In sterol concentration per wet weight of ovaries, free forms of cholesterol and β-sitosterol did not increase, or rather inclined to decrease in the former and were almost constant in the latter. However, the ester forms, especially cholesterol ester, increased remarkably during oöcyte maturation. This suggests that this ester may preferably be deposited in oöcyte yolk.Furthermore, the accumulation of sterol esters especially cholesterol ester was higher in ‘diapause’ than ‘non-diapause’ ovaries. It is highly probable that the diapause hormone stimulates the accumulation of cholesterol ester in oöcytes undergoing oögenesis.     

Ultrastructural aspects of the earwig ovary

Ultrastructural studies of the pre-vitellogenic ovaries of Euborellia annulipes indicate an overall structure basically similar to that of other insect ovaries. The membrane surrounding the pore connecting the nurse cell and oöcyte is coated with a thick electron dense material, whereas the cytoplasmic bridge contains few microtubules. All evidence indicates a unidirectional flow of materials synthesized or assembled in the nurse cell into the developing oöcyte.     

The effects of starvation and subsequent feeding on juvenile hormone synthesis and oöcyte growth in Schistocerca americana gregaria

The effect of starvation on the synthesis of C₁₆ juvenile hormone (JH) and the growth of terminal oöcytes was assessed in Schistocerca americana gregaria at two times during adult life: before activation of the corpora allata and during the first gonotrophic cycle. In both groups, starvation resulted in a decline in JH synthesis within 2–3 days and rates of synthesis remained low throughout the experimental period. The growth rate of oöcytes which were not vitellogenic at the time of starvation was depressed whereas the percentage of resorption of vitellogenic oöcytes increased dramatically with starvation. Although the percentage of resorption increased in animals with vitellogenic oöcytes, some mature oöcytes were produced, particularly in animals in which the oöcytes were greater than 5 mm in length at the time of starvation. This suggests that oöcyte maturation can be divided into two distinct phases—an early phase of vitellogenesis associated with high rates of JH synthesis and a late phase, in oöcytes greater than 5 mm, associated with much lower rates of JH synthesis.Stimulation of JH synthesis by farnesenic acid in 5-day starved animals resulted in high rates of JH synthesis, indicating that starvation did not appreciably alter the enzymic activities of the final two stages in JH synthesis. Thus rate limitation did not occur at these stages.Feeding of 5-day starved animals resulted in a transient increase in the rate of JH synthesis. However, rates of JH synthesis and oöcyte growth remained subnormal throughout the observation period, suggesting that the effects of starvation cannot be entirely reversed by feeding. Thus starvation may decrease the reproductive potential of the females.     

Effect of diet on the neuroendocrine system and egg development in the sheep blowfly, Lucilia sericata

ABSTRACT. A light microscope study of the endocrine and ovarian systems of Lucilia sericata under two diets revealed that in young females fed on sugar and water, medial neurosecretory cells (MNC) synthesized and stored neurosecretory material (NSM) as the flies matured. The MNC remained filled with NSM as long as the diet was maintained. Following a small increase immediately after emergence, the size of the corpora allata (CA) showed little further change, and the nuclei of nurse cells remained small. However, rapid changes occurred in these tissues soon after a meat meal: NSM was discharged from the MNC, and the CA increased in size. These changes were at a maximum 20 h after a meat meal. 4h later, vitellogenesis was well established and the nurse cell nuclei had increased in size 20-fold. Growth of the nurse cell nuclei continued until approximately 6 h before the completion of vitellogenesis when they are resorbed. Oögenesis took about 48 h at 25°C. When 100 μg of each of three different juvenoids were applied topically to different sugar-fed flies, the nuclei of both MNC and nurse cells became enlarged, whereas the CA were somewhat reduced in size. The relationship between protein ingestion and oogenesis is discussed, and the results obtained with L. sericata are compared with those of other species, especially the blowfly Calliphora erythrocephala.     

Phase polymorphism in Schistocerca gregaria: Reproductive parameters

Solitarious phase Schistocerca gregaria were selected according to the criterion of the occurrence of an extra stadium during larval development. The proportion of locusts undergoing an extra ecdysis increased within one generation of rearing under isolated conditions but it failed to reach 100% throughout an observation period encompassing six generations. A higher proportion of female locusts exhibited an extra stadium indicating sexual dimorphism in this phase character.The number of eggs per pod produced by solitarious females was larger than their gregarious counterparts. This was shown to be due to an increase in the number of ovarioles and a relative decrease in the proportion of non-functional oöcytes in solitary females, confirming earlier work.The inverse relationship between ovariole numbers and oöcyte size characterizing locust phase polymorphism was examined. Vitellin content of ovaries containing mature oöcytes was similar in gregarious and solitarious females. However, vitellin content per oöcyte was less in the latter suggesting that the increase in ovariole number may occur at the expense of oöcyte size and vitellin content.     

Ecdysone stimulates and juvenile hormone inhibits follicle formation in a gall midge ovary in vitro

The larval ovaries of the paedogenetic gall midge Heteropeza pygmaea can be cultured in vitro throughout oögenesis. Addition of farnesol or C₁₆ juvenile hormone to the culture medium inhibits follicle formation in the ovaries; addition of ecdysterone, on the other hand, strongly accelerates the formation of follicles. It is suggested that ecdysone triggers or stimulates follicle formation in the ovaries also in vivo, and that juvenile hormone acts on oögenesis in a later stage of development.