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.
Ans:
Hormones are the
chemical messengers of multicellular organisms that allow the cells to
communicate and engage in coordinated responses.
PROTHORACICOTROPIC
HORMONE
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.
ECDYSTEROIDS
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
Ans:
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.
DNA
Transcription
mRNA
Translation
Preprohormone
Signal
transduction
Proteolysis
Prohormone
Proteolysis
Inactive
Fragment Glycosylation
Phosphorylation
Hormone
The synthesis and
processing of peptide hormones
Que 10.
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
2. Urea
3. Ammonia
Que. 9 discuss
energy metabolism during flight in Locusta
migratoria
Ans
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.
Que 11.
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.
