Pheromones synthesis perception and reception in insects

PHEROMONES
SYNTHESIS, PERCEPTION AND
RECEPTION IN INSECTS
RAVINDREN R
TAM/2018-20

INSECT COMMUNICATION:
Communication may be
Visual Olfaction
Tactile Auditory

OLFACTION:
 Chemical signals are more in insects.
 A form of language to mediate interactions between
organisms.
 Semiochemicals are predominent.

SEMIOCHEMICALS:
• Signalling or communication chemicals used to carry
information between living organisms.
• Cause changes in their behaviour or physiology.
• Emitted by one individual and cause a response in
another.
• Semiochemicals are classified into
i. Intraspecific semiochemical - between individuals of the
same species(Pheromones)
ii. Interspecific semiochemical - between different species
(Allelochemicals)

PHEROMONES:
 Peter Karlson and Martin Luscher(1959) - term
“Pheromone”.
 Pheromone – pherin( to transport) + harmone (to
stimulate)
 Pheromones are chemicals secreted into the external
environment by an animal which elicit a specific reaction
in a receiving individual of same species(intraspecific
semiochemicals).

 Pheromones are ectoharmones produced by exocrine
glands and are ectodermal in origin.
 Pheromone production is under the control of harmones
secreted by Carpora allata.
 Pheromones are derivatives of fatty acids or terpenes.
 Pheromones which affect behavior directly through the
nervous system are called releaser pheromones; those that
affect metabolism are known as primer pheromones.

• Many pheromones are perceived as scents by olfactory
receptors and affect the recipient via the central nervous
system.
• In other cases, pheromones are ingested by the recipient.
• These may be perceived by the sense of taste, exerting
their effects via the central nervous system, or the
pheromone, once ingested, may be absorbed and
influence biochemical reactions within the recipient.

Primer
PHEROMONES
Releaser
Sex pheromone
Alarm pheromone
Aggregation pheromone
Trail marking pheromone
Epideictic and territory marking pheromone
Anti-aprodisiac pheromone

Pheromones synthesis perception and reception in insects

TYPE I
TYPE II
C12-C18 carbon
chain with
functional groups -
alcohol, aldehyde
and acetate
C17-C23 carbon
chain Comprising
unsaturated
hydrocarbons and
their epoxy
derivates
Biosynthesised
from de-novo
synthesised fatty
acid.
75% Moths
Orginate from long
chain
hydrocarbons.
Oenocytes or
epidermal cells.
Geometridae,
cockroach
PHEROMONES

PHEROMONES
Mono component – Eg: Silkworm - bombykol
Multi component – Eg: Bark Beetle - ipsenol
and ipsdienol

LOCATION OF PHEROMONE PRODUCTION:
 Pheromone producing cells may be individual cells or
cluster of cells forming gland.
 Located on different parts – antennae, head, thorax, legs
and abdomen.
 Abdomen – most common location in case of Dictyoptera,
Coleoptera and Lepidoptera.

 Hymenoptera – Head and abdominal regions.
 American cockroach – Produced in gut and released from
faecal pellets.
 German cockroach – Produced in 10th abdominal segment
and released through cuticular surface.

CHEMICAL CHARACTERISTICS OF PHEROMONES
 Great variety in the chemicals used as pheromones
depending on species, function and mode of action
 The degree to which these features are required depends
on the particular functions of the pheromone in question.
 Sex attractant pheromones - specific and volatile.
 Alarm pheromones - highly volatile in order to alert
other nearby insects to a source of danger, but also to
minimize the period of disruption.
 Marking pheromones - specific, but less volatile as their
persistence is a requisite.

ALIPHATIC HYDROCARBONS
Aliphatic hydrocarbons are used as pheromone
components by many insects.
The sex attractant pheromones of many female
Lepidoptera are straight chain hydrocarbons, most
commonly with chain lengths of 12, 14 and 16 carbon
atoms
Most moth pheromones are acetates.

AROMATIC HYDROCARBONS
Aromatic hydrocarbons have not been widely reported
as pheromones, but they do occur in the male scent
brushes of a number of noctuid moths.
Phenyl ethanol, benzyl alcohol and benzaldehyde are
examples.

NITROGEN-CONTAINING CYCLIC COMPOUNDS
A number of ants use heterocyclic nitrogen compounds
as trail pheromones,
Odontomachus use them as alarm pheromones.
Some male arctiid moths and danaid butterflies
produce pheromones from pyrrolizidine alkaloids

TERPENOIDS
Mono- and sesquiterpenes are commonly used as
components of alarm pheromones in aphids, termites
and formicine ants.
Some ants also use terpenes as trail pheromones,
They are the marking compounds used by a few
bumblebees.

PHEROMONE BIOSYNTHESIS:
 Most insect pheromones consists of multicomponent
blends.
 Pheromone components are derived from dietary
components or synthesized ??????
 Pheromone specific enzymes are responsible for
producing the myriad of pheromone molecules.
 The products of normal metabolism are modified by
these enzymes.
 Mainly fatty acid pathways and isoprenoid pathways.

 Chain shortening of fatty acids is also involved to
produce queen pheromone(honey bees)
Pletter et al., 1996
 Fatty acid elongation followed by decarboxylation
produce the hydrocarbon pheromones – Lepidoptera and
Diptera
(Blomquist and Vogt, 2003)

ORIGINS OF PHEROMONE CHEMICALS
 Pheromones may be synthesized de novo in the
glandular cells, or derived from precursors present in
the food.
 Many pheromones with ring structures are known to
be derived from such precursors, but there is also
evidence, in some cases, that the insects can synthesize
them.
 In other cases, symbiotic microorganisms may
contribute to synthesis

 Most pheromones of most insects are synthesized de
novo from smaller molecules.
 The hydrocarbon pheromones of many Lepidoptera
and Diptera are synthesized from fatty acids in a series
of steps.
 These involve chain shortening or elongation,
desaturation and, finally, the modification of the
functional group by reduction, acetylation or,
sometimes, oxidation.

de novo BIOSYNTHESIS OF HYDROCARBON
PHEROMONE

ENZYMES INVOLVED IN PHEROMONE BIOSYNTHESIS
• a) Acetyl-CoA carboxylase and fatty acid synthetase to
make 16 and 18 carbon fatty acids
• b) Desaturases to make mono- and di unsaturated fatty
acids
• c) Specific chain-shortening enzymes to make the right
chain length fatty acid
• d) A reductase, an acetyltransferase, or an oxidase is
used, sometimes in combination

PHEROMONE PRECURSORS FROM FOOD
 Many of the compounds in the pheromones of male
 Androconial brushes are derived from chemicals in the
larval food.
 These may be commonly occurring compounds, such
as benzaldehyde, benzyl alcohol and butyric acid used
by a number of Noctuidae.

SYNTHESIS BY MICROORGANISMS
 Bacterial symbionts in the hindgut of some bark
beetles and the beetle, Costelytra, are able to
synthesize some of the compounds used by their host
insects as pheromones.

PHEROMONE BIOSYNTHESIS ACTIVATING
NEUROPEPTIDE (PBAN)
•A polypeptide hormone that controls the synthesis of the
sex pheromone in moths has been named PBAN
• PBAN is produced in the suboesophageal ganglion (SOG)
• Transported to the corpora cardiaca (CC) before its release
into the hemolymph
• PBAN acts directly on pheromone gland cells by using
calcium and cAMP (Cyclic adenosine monophosphate) as
second messengers.
(Barth and Lester, 1973)

CASE STUDY
• In H.zea, ligation of female between head and thorax –
completely eliminated pheromone production.
• Injection of homogenates of brain sub-oesophagial
ganglion into the abdomen of ligated female restored
pheromone production.
• From this experiment, it is concluded that a factor in brain
SOG controlled pheromone production in H.zea .
(Raina and Klun,1984)

PHEROMONE DISPERSAL:
 Dispersal of pheromones into the environment involves
specific behavior and is often enhanced by cuticular
structures.
 Dispersal of volatile pheromones from the surface of the
cuticle is believed to be facilitated by microtrichia.

DISPERSAL IN MALES
Androconial organs :
 Ending in a brush-like row process.
 Androconia is extented to release Pheromones

DISPERSAL IN FEMALES
PHEROMONE GLANDS:
# Pheromone glands are exposed to outside by
-depressing the tip of the abdomen
-extension of the abdomen
-gland is inverted by haemolymph pressure
# Exposure of the gland is accompanied by wing
vibration which facilitate dispersal.

Ants drags the tip of the abdomen over the surface as it
runs.
In bumblebee, labial gland pheromones are transferred
to the vegetation by biting.

PHEROMONE PERCEPTION AND RECEPTION:
 Exocrine glands release variety of chemicals.
 They may be non-volatile or volatile.
 Taste stimuli are sensed mainly in aquatic medium and
the signals are water borne.
 But some aquatic insects use olfactory signals also.
 Airborne signals are received by various
chemoreceptors.

 Chemoreceptors trap odour molecules and transfer
them to specific recognition sites.
 The odour signals depolarizes the neural axon and
generate a nerve impulse.
 Different sources release their volatiles simultaneously
-intermingle to form a complex and fluctuating
olfactory environment.
 Interactions between odour components, or their
resulting neural codes, take place at the different levels
of the sensory system and are intrinsic to olfaction

 Odours are first transformed into a neural signal by the
Odour receptor neurons(ORNs)
 Insect ORNs are not randomly distributed but are
housed in morphofunctional units, the olfactory
sensilla.
 The olfactory sensilla can take different shapes,
appearing as long or short hairs, plates, pegs, or
cavities
 The antennae has several thousands of ORNs.
 Other head appendages, the labial and maxillary palps,
can bear olfactory sensilla with various odor
specificity depending on insect orders

PHEROMONE BINDING PROTEINS:
 Two types of proteins are observed in the extracellular
spaces of sensilla.
 One type specifically binds chemicals that are part of
the insects’ pheromone mixture, and are therefore
called pheromone binding proteins (PBP).
 The other type binds less specifically a variety of
nonpheromone molecules (e.g., food odours) and are
called general odorant binding proteins (GOBP).

 ORs vary in their degree of specificity, from narrowly to very
broadly tuned, so even if ORNs express only one OR type
they can show very different width of chemical tuning
(Hallem and Carlson 2006)
 Binding to the PBP protects the pheromone from enzymatic
degradation
(Vogt and Riddiford 1986).
 Without protection, 93% of the pheromone would be degraded
before binding to the receptor
(Kaissling 2009).

PHEROMONE SPECIFIC ORNs
 The sex pheromone is a relatively simple blend
comprising a small number of components released in
stable ratios.
 Each pheromone - specialist ORNs (Pher-ORNs).
 Pher-ORNs are narrowly tuned to one component, which
means that this component triggers excitatory responses at
very low concentration although higher concentrations of
related compounds also activate them

SIGNAL TRANSDUCTION
Extracellular transducer processes (perireceptor
events) and pheromone-receptor interaction includes
 The adsorption of the pheromone and its diffusion
from the adsorption site on the hair towards neuron
 The solubilization of the mostly lipophilic pheromone
by binding to the Pheromone Binding Proteins in the
sensillum lymph.
 The activation of the receptor molecule in the plasma
membrane of the receptor cell
 The deactivation of the pheromone within the
sensillum lymph

 The transduction of the chemical signal into a neural
code involves sequential steps.
The binding of odorant molecules to their
corresponding type of ORs
Change in the dendritic transmembrane potential
called the receptor potential.
The membrane depolarization opens voltage-gated
ion channels involved in the generation of action
potentials whose firing frequency constitutes the
neural code of the chemical signal.

PERCEPTION AND SIGNAL PROCESSING
Peripheral perception at the antenna where specialized
Sensilla are present
Odorant molecules diffuse into the lumen of the sensilla
Bound to pheromone binding proteins that solublize the
pheromone in the aqueous receptor lymph
Carry them to specialized receptors on the surface of
dendrites in the sensillum.
Electrochemical signal transduction

PHEROMONE DEGRADATION AND DEACTIVATION
 Degradation of pheromone was first found in the silk
moth B. mori by Kasang (1971).
 Living antennae exposed in air for 10 seconds to 3H-
bombykol were subsequently eluted for 10 minutes by
pentane and for another 10 minutes by a chloroform-
methanol mixture, and the amounts of bombykol and its
metabolites in the resulting solutions were checked by
thin-layer chromatography.
 Bombykol had been turned into aldehyde and acid, and
later into esters, with a half-life of 4 to 5 minutes

Pheromone-degrading enzymes (esterases, aldehyde
oxidases) were isolated from moth antennae
(Vogt et al. 1985)
The olfactory tissues also contain odorant degrading
enzymes (ODEs) that catabolize the odour and other volatile
molecules.

FACTORS CONTROLLING PHEROMONE
PRODUCTION
INTRINSIC FACTORS:
1.Male and mating factor:
 In Drosophila melanogaster, the sperm or chemical
factor associated with sperm was responsible for
termination of female receptivity
 Females of Helicoverpa zea mate several times in their
lifetime but not more than once in any given night.
 Females of Gypsy moth mates only once followed by
permanent decline in pheromone titre.
(Giebultowicz et al., 1991)

Mating causes a rapid decline in the pheromone titre
that was attributed to a factor produced in the accessory
glands of the male and transferred to the female at the time
of mating.
(Raina 1989)
2.Female age:
3 to 4 days old Gypsy moth showed mated behavior
including termination of pheromone production.

EXTRINSIC FACTORS:
1.Photoperiod:
 Most moths produce and release pheromone in the
night time (Carde & Webster,1980)
 H.zea, pheromone production starts at the onset of
scotophase and vice versa in Gypsy moth.
 Trichoplusia ni doesnot show any diurnal changes in
pheromone titre.
2.Temperature:
 Most pheromone titre reduces at 35o
C.
 Temperature acts as an exogenous cue to modify the
expression of calling rhythms.
(Charlton & Carde)

Host plant factors:
 Trans-2-hexanol, a volatile component of oak leaves
induced pheromone production in Antheraea
polyphenus (Riddiford & Williams,1967)

BEHAVIORAL RESPONSES TO PHEROMONE
 Pheromone perception triggers a sustained upwind
flight in male moths (positive anemotaxis).
 Fluttering in silk worm
 Wing raising behavior in Blattodea germanica
 Increased locomotion in trail-following ants
 Grooming of antenna in male Periplaneta americana

CONCLUSION:
 Insects have well defined communication system
which rely more on chemical communication in the
form of pheromones for mate finding and aggregation.
 These properties of pheromone can be exploited for
attracting the crop pest by formulating and used for
better crop protection.

REFERENCES:
 Biocommunication in Insects – T.N.Ananthakrishnan,
Alok Sen, 1997.
 A textbook of Insect physiology – K.C.Chitra, 2004.
 Perspectives in Insect behavior – M.L.Agarwal, 2009.
 The Insects – Structure and function – Chapman R F,
2013(5th edition).
 Insect pheromone biochemistry and molecular biology –
The biosynthesis and detection of pheromones and plant
volatiles – Gary J. Blomquist, Richard G. Vogt, 2003.

Pheromones synthesis perception and reception in insects

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