In contrast to general odours, a chemical emitted by individuals
leading to physiological and/or behavioural changes in individuals
of the same species is called a pheromone（Karlson
and Luscher, 1959). According to their effect, sex pheromones,
alarm pheromones, and aggregation pheromones of insects are
classified as trail pheromones. Sex pheromones are essential in
many insects in order to allow mate finding and reproduction
ensuring the survivial of the species.
Bombykol（(E,Z)-10,12-hexadecadien-1-ol）emitted
by female silkmoths (Bombyx mori L.) was the first pheromone to be
isolated and chemically identified（Butenandt
et al., 1959）.
Currently, about 1500 chemicals are known as pheromones of
different species, in particular agricultural pests（The
Pherobase、The
Pherolist, Byers 2002）.
The chemical structure of sex pheromones is diverse, however there
are similarities among the pheromones used by some moth
species（Byers,
2005）.
These similarities are thought to be due to two biochemical
synthesis pathways from which the pheromones are derived （Ando
et al., 2004）.
In one pathway fatty acids produced de novo by moths are used as a
substrate (Type I), the other is a pathway starting from linoleic
acid, from plant origin (Type II). While substances like bombykol,
i.e. alcohols, aldehydes, and carboxylic acids derive from the
former, linear chain hydrocarbon pheromones are obtained by the
latter（Ando
et al., 2004).
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Silkmoth
Adult
silkmoth (imago) behaviour is very limited besides hatching, in
particular females hardly move afterwards. The male behave
similarly but display clear odour-source localisation behaviour in
response to female phermone, including well-defined
stimulus-dependent responses. Due to the absence of “noise” at
the behavioural level, the silkmoth is an excellent model system to
elucidate the neural bases of odour-source
localisation behaviour.
In many moth species, males
orient towards females using pheromones released by them. In
general, the effective chemical stimulus to initiate male behaviour
is a mixture of several, often chemically diverse compounds: a
blend. Both composition and concentration ratios can be important
to render a blend effective.
The silkmoth uses a
comparatively simple pheromone system. The female releases a blend
with only two components: bombykol [ (E,Z)-10,12-hexadecadien-1-ol]
and bombykal [ (E,Z)-10,12-hexadecadien-1-al]（Kaissling
et al., 1978 Figure
1). Stimulation of the antennae
with bombykol alone is sufficient to release odour-source
localisation behaviour in male moths. Bombykal has a modulating
effect, being somewhat inhibitory or deterrent（Kaissling
et al., 1978）and
its biological function is still poorly understood. The sensitivity
of the male to bombykol is surprisingly high: It has been
calculated that about 170 molecules are sufficient to induce a
behavioural response（Kaissling,
1987).
While odour-source localisation behaviour in the
silkmoth has long thought to be an immutable behavioural routine,
recent research has shown that the internal state of the brain as
well as experience can have modulatory effects (Gatellier et al.,
2004; Gatellier, 2005). For instance, the behavioural sensitivity
to bombykol is correlated with serotonin levels in the brain. In
the intact brain, serotoning levels are high in the daytime and low
at night and this correlates with te behavioural sensitivity
(Gatellier et al., 2004). This implies serotonin plays an important
role in controlling male sensitivity to female pheromone. More
recently, it has also been shown that the presence of host-plant
odours increases behavioural sensitivity to pheromone in male
silkmoths (Namiki et al., 2008).
Figure 1. Chemical structure of female silkmoth sex pheromones (A). Male silkmoth behavioural response to pheromone release by a female (B).
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References
Kaissling,
K.-E. R. H. Wright Lectures on Insect Olfaction, ed. Colbow, K.
(Simon Fraser Univ., Burnaby, 1987).
Kaissling, K.-E,
Kasang, G., Bestmann, H.J., Stransky, W. and Vostrowsky, O. A new
pheromone of the silkworm moth Bombyx mori: sensory pathway and
behavioral effect. Naturwissenschaften 65, 382-384
(1978).
Gatellier, L., Nagao, T. and Kanzaki, R. Serotonin
modifies the sensitivity of the male silkmoth to pheromone. J. Exp.
Biol. 207, 2487-2496 (2004).
Gatellier, L. Neuroethological
studies on neuromodulation of the silkmoth brain. The graduate
school of Life and Environmental Sciences, the University of
Tsukuba. doctor thesis (2005).
Namiki, S., Iwabuchi, S. and
Kanzaki, R. Representation of a mixture of pheromone and host plant
odor by antennal lobe projection neurons of the silkmoth Bombyx
mori. J. Comp. Physiol. A (2008).
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