Que 1. Hormones are particularly pervasive in insect systems, affecting a wide variety of physiological processes including, embryogenesis, postembryonic development, behavior, water, balance, metabolism, caste determination, polymorphisms, mating, reproduction and diapauses. Discuss the types of hormones in insects. Include in your essay, the various types of insect hormones, factors that affect their activity, their synthesis, release sites and their modes of action.
Hormones are the chemical messengers of multicellular organisms that allow the cells to communicate and engage in coordinated responses.
PTTH is produced in the lateral neurosecretory cells of the brain and is released in the corpus cardiacum that terminates in the wall of the aorta or, in some insects, is released by the corpus allatum. PTTH acts on the prothoracic gland to regulate the synthesis of ecdysteroid
Mode of Action
The Corpus Cardium is the major neurohemal organ in insects and releases a large number of neuropeptides. It is the release of PTTH that determines the occurrence of the molt by activating the prothoracic glands to produce the ecdysteroid molting hormone. Because PTTH is a peptide hormone, it is unable to enter the cells of the prothoracic gland and must exert its influence from the outside through a G protein coupled receptor. This G-protein coupled receptor activation increases intracellular Ca2+ as a second messenger, which then activates protein kinases that can phosphorylate and activate enzymes in the biosynthetic pathways that lead to a cellular response.
Ø Ecdysone is a steroid hormone belonging to the class of substances known as terpenoids.
Ø The precursors for ecdysteroid synthesis by the prothoracic gland of insects are sterols, such as cholesterol.
Ø The primary site of ecdysteroid synthesis is the prothoracic gland.
Ø In adults, the site of ecdysteroid synthesis has been shifted to the ovaries and the testes.
Ø In many female insects, ecdysteroids are produced by the follicle cells of the ovaries.
Mode of Action of Ecdysteroids
The ecdysteroid receptor (EcR) and the product of the ultraspiracle gene (USP) bind to the hormone. The ecdysteroid receptor complex binds to early genes, stimulating their transcription but inhibiting the transcription of the late genes. The early gene product that is produced subsequently inhibits the early genes but stimulates the late genes, demonstrating the cascade of gene activity that is involved in salivary gland morphogenesis.
THE JUVENILE HORMONES
The corpus allatum (CA) is the major organ of JH synthesis and release.
Mode of action
Ø The major role of JH in insects is to modify the action of ecdysteroids and prevent the switch in the commitment of epidermal cells.
Ø In the presence of ecdysteroids, JH preserves the current program of gene expression.
Ø JH both influences the stage-specific expression of the genome that is initiated by ecdysteroids and also acts by itself to modulate the expression of certain specific genes.
Factors affecting hormonal activity
Hormonal activity in the circulatory system is regulated by
Ø its rate of synthesis by the endocrine glands,
Ø the rate of release into the blood,
Ø its degradation in the blood,
Ø development and presence of hormone receptors on target cell.
Que.2. Outline the synthesis and processing of insect peptide hormones
Ø Peptide hormones are usually synthesized as larger precursor preprohormones and prohormones and then processed by proteolytic enzymes into the smaller final hormone.
Ø The peptide must be inserted into the cisterna of the endoplasmic reticulum, and a signal peptide portion must be attached in order for this to occur.
Ø The pre- and pro-portions are cleaved, and the peptide hormone is then released from the cell by exocytosis.
Signal transduction Proteolysis
Inactive Fragment Glycosylation
The synthesis and processing of peptide hormones
a) Explain the phrase insecticide resistance
It is a shift in the genetics of a pest population that allows individuals within a previously susceptible population to survive
b) Mention two types of biochemical resistance mechanisms employed by insects
Target-site resistance mechanism and Detoxification enzyme-based resistance mechanism.
Que. 4. Discuss cold hardiness
Ø Cold hardiness describes the ability of insects to survive exposure to low temperatures.
Ø Insects tolerate winter temperatures if they first undergo a physiological preparedness that may take several weeks to develop.
Ø The acclimatization process occurs when insects are first exposed to low temperatures, which trigger the accumulation of the cryoprotectants.
Ø A short-term exposure to cold temperatures can also protect some insects from subsequent lethal temperatures.
Ø Insects that are characterized as being freeze tolerant are able to withstand the formation of extracellular ice crystals.
Ø They synthesize ice nucleating proteins that raise the supercooling point of body fluids and serve as catalysts for the nucleation of ice in safe extracellular areas. Water moves from the cells to these extracellular areas, preventing intracellular freezing from occurring.
Ø This gives the cells time to adjust to the osmotic changes that result from the formation of ice crystals and reduces the likelihood of intracellular freezing.
Ø Freeze-avoiding species produce hemolymph cryoprotectants that allow the insect to supercool and remain in a liquid state without the formation of ice crystals.
Ø These species can often supercool to as low as −35°C. The cryoprotectants that are produced include glycerol, sorbitol, trehalose, and mannitol often in concentrations approaching 25% of the insect’s total body weight.
Ø In addition to preventing the formation of ice, these components may stabilize enzymes and cell membranes.
Que.5 State the major excretory products of insects.
1. Carbon dioxide
Que. 9 discuss energy metabolism during flight in Locusta migratoria
In locusts, energy metabolism during flight is initiated by octopamine and regulated by adipokinetic hormone (AKH). Trehalose serves as the major fuel at the onset of flight, but as the hemolymph trehalose levels decline with activity, octopaminergic neurons within the corpus cardiacum stimulate the release of AKH. The AKH activates an adenylate cyclase that increases cAMP levels and subsequently activates a protein kinase. The protein kinase then phosphorylates and activates a lipase that induces the release of diacylglycerols from the triacylglycerols stored in the fat body. AKH also induces the production of a lipoprotein carrier from the fat body that transports these diacylglycerols through the hemolymph to the flight muscles. The metabolism of carbohydrates that are stored in flight muscle during this lipid mobilization is also inhibited by AKH so that the lipid reserves are used exclusively. Octopamine has several other effects on flight behavior, stimulating the interneurons involved in maintaining flight, the power output of the flight muscles themselves, and the proprioreceptors on the wing that monitor flight behavior. This amine may be the functional equivalent of flight-or-fight hormones in vertebrates, released during stress and causing an increase in the insect’s arousal levels.
A) Explain the term synergy as applied to insecticides.
Synergists are added to insecticides e.g. pyrethrins to evade the insect detoxification mechanisms.
b) using examples, explain how it is applied in the use of insecticides by explaining mechanisms of action of synergists.
In the absence of PBO, Pyrethrins are detoxified by the insect detoxification enzymes
In the presence of PBO, the insect detoxification enzyme sites are blocked by PBO
A higher concentration of pyrethrins reaches the binding site on the Na+ channel.
INSECT FLIGHT METABOLISM
Small amounts of substances are located in the flight muscles themselves to power the initiation of flight, but the initial store of ATP in muscle cells is only sufficient for a few
seconds of flight. The transfer of a phosphate group to ADP from arginine phosphate, providing an additional brief period of flight, replenishes it. The muscle may also store small amounts of other fuels including proline, glycogen, and triacylglycerol that are drawn upon during flight.
Flight muscles draw the next most immediate source of energy from substrates in the hemolymph. The disaccharide trehalose is present in high concentrations as a circulating energy source that is used during the early phases of flight.
Hemolymph diacylglycerol also bathes muscle cells, and the amino acid proline is utilized in some insects for flight.
These are mobilized from fat body reserves to maintain their levels in the hemolymph. The fuel for longer flights is stored in the fat body and transported to flight muscles through the hemolymph. This fuel use varies among insect orders.
Flight muscles completely oxidize carbohydrates to carbon dioxide and water
in the absence of any anaerobic metabolism. Glycolysis in insect flight muscle occurs much like that in other animals, with a few additions.
In general, insects that engage in long-range flights oxidize lipid, whereas those that use carbohydrate fly for only short periods. Insects with high wing-beat frequencies and asynchronous muscles tend to utilize carbohydrates, whereas those with synchronous flight muscles are more likely to utilize lipid. Lipid is the most concentrated form of energy storage.