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A O Lea

Publications and source records attributed to A O Lea.

At least 19 recordsLinked to original sources
PubMed1991

Control of follicular epithelium development and vitelline envelope formation in the mosquito; role of juvenile hormone and 20-hydroxyecdysone.

Using microsurgical manipulations, hormone applications, and transmission electron microscopy we have investigated the regulation of differentiation of the follicular epithelium and formation of the vitelline envelope (VE) in primary follicles in the ovary of the mosquito, Aedes aegypti. During the first 3 days after eclosion, the primary follicle grows, and cells of the follicular epithelium differentiate, their content of mitochondria, rough endoplasmic reticulum, and Golgi complexes increases significantly. Growth and differentiation of the follicular epithelium appear to be under the control of juvenile hormone (JH), because they are blocked by removal of corpora allata in newly closed adult females and can be restored by either implantation of corpora allata or application of JH III. In insects, including mosquitoes, VE is the first layer of the eggshell to be deposited. It is formed from the secretory products of the follicle cells and its deposition coincides with yolk accumulation by developing oocytes. Only follicle cells adjacent to the oocyte deposit VE. In decapitated females, given a blood meal by enema and injected with picogram doses of 20-hydroxyecdysone (20-HE), follicle cells synthesize the VE precursors and deposit morphologically normal VE, in contrast to saline injected controls which deposit no VE. We conclude that 20-HE, as well as factors originating from the blood meal and the oocyte, are required for the normal formation of VE in the mosquito follicles.

Aedes↗
PubMed1990

Juvenile hormone controls previtellogenic proliferation of ribosomal RNA in the mosquito fat body.

During the previtellogenic development of mosquito fat body cells, the nucleolus, the organelle responsible for producing ribosomes, enlarges threefold, reaching maximal size between 2 and 3 days after eclosion. The granular component of the nucleolus containing ribosomal precursors increases considerably as well. These signs of nucleolar activation correlate with the synthetic rate and accumulation of poly(A)- RNA (predominantly ribosomal RNA) in the fat body cells. The amount of poly(A)- RNA in fat body cells increases during the first 2 days after eclosion and then declines gradually. The rate of RNA synthesis exhibits similar kinetics, but both the rise and the decline are sharper than for the accumulation of RNA. All the characteristics of nucleolar activation, its enlargement, accumulation of poly(A)- RNA, and the increased rate of RNA synthesis, are blocked by removal of the corpora allata (CA) in newly eclosed adult females but could be restored by either implantation of CA or topical application of juvenile hormone III or its analog, 7-S-methoprene, to allatectomized females. Thus, previtellogenic activation of fat body nucleoli for ribosomal RNA production is controlled by juvenile hormone from the corpora allata.

Animals↗
PubMed1989

Ultrastructural localization of phenoloxidase in the midgut of refractory Anopheles gambiae and association of the enzyme with encapsulated Plasmodium cynomolgi.

A melanogenic enzyme, phenoloxidase, was localized ultrastructurally in the midgut epithelia of 2 strains of Anopheles gambiae, a refractory strain that melanotically encapsulates Plasmodium cynomolgi ookinetes on the midgut, and a susceptible strain that does not. Midguts were incubated with either dopa or dopamine, and the resultant electron-dense product of phenoloxidase activity was localized on the basal lamina (BL) and cellular basal membrane labyrinth (BML) in uninfected mosquitoes of both strains. In infected refractory mosquitoes, the reaction products still were observed on the BL and BML but were especially dense in the BML of midgut cells near encapsulated ookinetes and in the capsule itself. In infected susceptible mosquitoes, phenoloxidase localization was reduced or absent in the BL and BML and was not observed near parasites. Phenylthiourea (PTU) inhibited the phenoloxidase reaction, indicating that the reaction product deposited in the absence of PTU resulted from enzyme activity and not autooxidation of the substrates. It is concluded that higher levels of phenoloxidase in the refractory strain following a blood meal may contribute to the ability to encapsulate ookinetes.

Animals↗
PubMed1988

FMRFamide- and adipokinetic hormone-like immunoreactivity in the nervous system of the mosquito, Aedes aegypti.

As demonstrated with immunocytochemistry, specific cells and axons in the nervous system of female Aedes aegypti contain antigens immunologically related to FMRFamide (phenylalanine-methionine-arginine-phenylalanine-amide) and locust adipokinetic hormone I (AKH). In the supra-esophageal ganglion, including some medial neurosecretory cells, and in all ganglia of the ventral nerve cord, there are 100-120 cells immunoreactive to a FMRFamide antiserum. The same cells cross-react with a bovine pancreatic polypeptide antiserum, but when the latter antiserum is preabsorbed with FMRFamide, immunoreactivity is lost. However, immunoreactivity is maintained when FMRFamide antiserum is preabsorbed with pancreatic polypeptide, suggesting that the immunoreactive peptide is more closely related to FMRFamide. There are 6-12 cells in the supra- and subesophageal ganglia immunoreactive to an AKH antiserum, and some of the same cells are reactive to the FMRFamide antiserum. As well, unpaired cells in each of the abdominal ganglia are positive for both AKH and FMRFamide. Although the function of the FMRFamide- and AKH-like peptides in mosquitoes is unknown, this study, combined with previous reports on the localization of FMRFamide-like peptides in midgut endocrine cells, supports the concept of a brain-midgut neuroendocrine axis in this insect.

Aedes↗
PubMed1988

Ultrastructure of the encapsulation of Plasmodium cynomolgi (B strain) on the midgut of a refractory strain of Anopheles gambiae.

Using transmission electron microscopy, we investigated the encapsulation of the simian malaria parasite, Plasmodium cynomolgi, in a refractory strain of the mosquito, Anopheles gambiae. After the ookinete penetrates the mosquito midgut epithelium and lodges between the basal membrane and the basal lamina, an electron-dense, melanin-like substance begins to coalesce around the parasite. Completely encapsulated parasites were found as early as 16 hr after the blood meal. Granules of the melanin-like substance often appeared to condense onto the parasite from the fluid in the extracellular spaces of the basal membrane labyrinth. Melanin granules also appeared to condense from the hemolymph onto the basal lamina underlying the parasite. In addition, groups of tubules, vesicles, and membranous whorls often were found in midgut cells that were located next to or were enclosing parasites. These structures were unusually electron-dense, and may have been associated with melanization. Hemocytes rarely were observed near completed capsules and neither hemocytes nor their remnants were components of the capsules. During later stages of encapsulation, parasites appeared abnormal and often were infiltrated with melanin. Although late-stage capsules were usually located basally, completed capsules enclosed by membranes were occasionally observed near the apical border of the midgut. Other capsules associated with cellular debris, were found in the lumen of the midgut from 1 to 6 days after the blood meal.

Animals↗
PubMed1987

Toxic and antifeeding actions of melittin in the corn earworm, Heliothis zea (Boddie): comparisons to bee venom and the insecticides chlorpyriphos and cyromazine.

The acute and sublethal effects of melittin were compared to whole bee venom, chlorpyriphos and cyromazine injected into the corn earworm, Heliothis zea (Boddie). Melittin had twice the toxicity of crude venom, but only 3% that of chlorpyriphos. Melittin significantly reduced growth rate, diet consumption and food utilization efficiencies of fourth instar larvae. Bee venom and insecticides reduced these parameters to a lesser extent.

Bee Venoms↗
PubMed1986

Mosquito trypsin: immunocytochemical localization in the midgut of blood-fed Aedes aegypti (L.).

A polyclonal antibody was raised against trypsin purified from the midgut of blood-fed Aedes aegypti. Using this antibody and our modification of the peroxidase-antiperoxidase immunocytochemical reaction, strong activity was found in the lumen of the midgut at the light-microscopical level. The activity was localized mainly in the posterior part of the distensible, abdominal midgut, along the periphery of the blood bolus and within the peritrophic membrane. Immunoreactivity appeared 8 h after the blood meal and was most prominent around 24 h, coinciding with our previous spectrophotometric determinations of trypsin. At the electron-microscopical level, secretory granules, immunocytochemically labelled with anti-trypsin antibody and protein A-colloidal gold, were first detected about 12 h after the blood meal. At 18 h, the secretory pathway could be followed immunocytochemically from the formation of granules in the Golgi complex until their release by exocytosis in the midgut lumen. By 24 h, there was a reduction in secretory granules, and large lysosomes appeared. The process of secretion described for this mosquito is comparable to similar events in vertebrate secretory systems and the presence of an intracellular trypsinogen is suggested.

Aedes↗
PubMed1986

Internalized proteins directed into accumulative compartments of mosquito oocytes by the specific ligand, vitellogenin.

We have investigated the internalization pathways for a specific protein, vitellogenin, and a non-specific protein, horseradish peroxidase, in the mosquito oocyte in vivo. The internalized proteins were localized by electron microscopical immunocytochemistry or autoradiography; the relationship of their destination compartments with lysosomes was monitored by visualization of acid phosphatase. Proteins internalized by the oocyte follow either a specific accumulative route or a lysosomal degradative route. Via coated vesicles, both proteins enter the same compartment, the endosome, where they dissociate from membrane-binding sites. The route to their final destination depends on the presence of the specific ligand. In its absence, the degradative route is followed, and the endosome with non-specific protein fuses with lysosomes. In the presence of the specific ligand, the accumulative route is followed, and both specific and non-specific proteins are delivered into an accumulative compartment, the transitional yolk body. During the transformation of the transitional yolk body into the final storage compartment, a mature yolk body, vitellogenin undergoes crystallization, whereas the non-specific protein is concentrated in small vesicular extensions of the compartmental membrane. These vesicles are separated from the yolk bodies and apparently deliver the non-specific protein into the lysosomal system. We concluded that any protein bound to the membrane would be internalized by the oocyte, but only binding of the specific ligand to its receptor serves as a transmembrane signal stimulating the formation of accumulative compartments.

Acid Phosphatase↗
PubMed1986

FMRFamide- and pancreatic polypeptide-like immunoreactivity of endocrine cells in the midgut of a mosquito.

Immunocytochemical surveys of midguts from female mosquitoes, Aedes aegypti, reveal that half of the estimated 500 endocrine cells in a midgut contain a substance recognized by antisera to bovine pancreatic polypeptide and a molluscan peptide, FMRFamide (phenylalanine-methionine-arginine-phenylalanine-amide). With light microscopy the cells resemble an endocrine type because of their basal position in the epithelium, conical shape, and, in some instances, apical extensions to the lumen. At the ultrastructural level, the immunoreactive substance is contained specifically within the secretory granules of such cells. Immunoreactive cells are distributed exclusively in the midgut region where blood is stored, and ingestion of vertebrate blood reduces the number of such cells and the intensity of reaction in others. These two facts suggest that a blood meal stimulates release of the immunoreactive substance from the cells. Since the immunocytochemical localization is supplemented by a demonstrated secretory response, the cells are considered to be peptidergic endocrine cells.

Aedes↗
PubMed1985

Hormone-mediated formation of the endocytic complex in mosquito oocytes.

The developmental events leading to oocyte competence to internalize proteins, and the hormonal control of these events in the mosquito Aedes aegypti have been studied. The oocytes of newly eclosed females have an undifferentiated cortex. During previtellogenic development, a highly specialized endocytic complex, consisting of numerous coated vesicles and uncoated endosomes, microvilli, and, presumably, vitellogenin receptors, forms in the oocyte cortex. Morphometric analysis and probes with a protein tracer, horseradish peroxidase, have shown that only the oocytes with developed endocytic complexes are competent for protein uptake. In vivo experiments have demonstrated that the formation of the endocytic complex is controlled by juvenile hormone from the corpora allata. This developmental event was blocked by ablation of corpora allata at eclosion, but it was restored by either implantation of corpora allata into allatectomized female or the application of juvenile hormone III.

Aedes↗
PubMed1985

Ultrastructure of midgut endocrine cells in the adult mosquito, Aedes aegypti.

The ultrastructure of endocrine cells in the midgut of the adult mosquito, Aedes aegypti, resembled that of endocrine cells in the vertebrate gastro-intestinal tract. Midgut endocrine cells, positioned basally in the epithelium as single cells, were cone-shaped and smaller than the columnar digestive cells. The most distinctive characteristic of endocrine cells was numerous round secretory granules along the lateral and basal plasma membranes where contents of the granules were released by exocytosis. Secretory granules in each individual cell were exclusively of one type, either solid or 'haloed', and for all cells observed, the range in granule diameter was 60-120 nm. The cytoplasm varied in density from clear to dark. Lamellar bodies were prominent in the apical and lateral cellular regions and did not exhibit acid phosphatase activity. The basal plasma membrane was smooth adjacent to the basal lamina, whereas in digestive cells the membrane formed a labyrinth. Some endocrine cells reached the midgut lumen and were capped by microvilli; a system of vesicles and tubules extended from beneath the microvilli to the cell body. An estimated 500 endocrine cells were distributed in both the thoracic and abdominal regions of the adult midgut. In one midgut, we classified a sample of endocrine cells according to cytoplasmic density and granule type and size; endocrine cells with certain types of granules had specific distributions within the midgut.

Aedes↗
PubMed1978

Blood meal size as a factor affecting continued host-seeking by Aedes aegypti (L.).

The effect of ingested blood on the host-seeking response of two strains of Aedes aegypti was examined. Using an olfactometer, females fed partial blood meals were scored for host-seeking behavior within 1 h, and their blood meal sizes were measured chemically immediately afterwards. The suppression of host-seeking within 1 h after a blood meal appears to be caused by abdominal distention from ingested blood. Mosquitoes of either strain were attracted to a host when the blood meal size was less than 2.5 microliter; above this threshold there was a sharp decline in the tendency to respond. Small mosquitoes resulting from a low larval diet had a lower threshold, and were more likely to cease host-seeking after a small blood meal. Multiple feeding within a single gonotrophic cycle may result if mosquitoes take small blood meals which are insufficient to terminate host-seeking. Partial meals and reduced feeding success of mosquitoes can result from defensive host behavior, which in the laboratory rat was shown to increase at high mosquito densities.

Aedes↗