Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 217

Discrete Phenotypic Changes Are Designed by the Brain
Diapause—the Predictive Form of Insect Larval Dormancy
An interesting form of adaptation in plants and animals is dormancy, the suspen-
sion of embryonic growth and development accompanied by a dramatic decline in
metabolic activity during or just before the winter, until better environmental condi-
tions arrive. In the animal world, embryonic dormancy is a characteristic aspect of
the arthropod life cycle, especially insects. It is also observed in various vertebrate
species of fish, reptiles, and mammals. The main form of dormancy in insects is the
diapause that can occur in all the stages of insect development. Larval diapause in
insects is controlled by the brain's circannual calendar, which is based primarily on
the perception of the shortening of the day length and the decline in the environ-
mental temperature. The insect diapause is a predictive form of dormancy, unlike
aestivation , the consequential dormancy of insects, that is arrested by the develop-
ment in response to the unpredictable adverse conditions of hot weather, especially
desiccation—by using physiological and morphological adaptations to prevent
dehydration.
The photoperiod is the main diapause-inducing cue in diapausing insects. The
decline in environmental temperature is another diapause-inducing cue in insects, not
only in concert with the photoperiod in temperate latitudes, but also in itself in equa-
torial regions ( Denlinger et al., 2011 ).
The effect of photoperiod as a diapause-inducing cue in insects implies that they
measure the day length and count the number of shortening days ( Denlinger et al.,
2011 ). It is believed that the neurons in which photoperiodic information is pro-
cessed and the neurons that secrete prothoracicotropic hormone (PTTH) are in the
same brain region ( Denlinger et al., 2011 ).
While diapause-inducing cues are generally common in insects, the molecular
mechanisms of diapause are widely diverse. A common feature of all mechanisms
is that they are neurohormonal mechanisms involving ecdysteroids and JH, as well
as neurohormones pheromone biosynthesis activation neuropeptide (PBAN) and dia-
pause hormone (DH)/DH-like hormones. With ecdysteroids and JH being products
of the lower limbs of the brain/prothoracic gland and brain/corpora cardiaca, respec-
tively, it is reasonable to suggest that insect diapause is cerebrally regulated.
It is noteworthy that these same hormones—ecdysteroids and JH—in different
species can play different and even opposing roles in diapause. If at first this seems
paradoxical, it is worth remembering that the nervous system can relate any exter-
nal or internal stimulus to any gene as shown earlier in this chapter (see page 197,
“Making Environmental Signals Intelligible to Genes”).
Needless to say, growth inhibiting ecdysteroids are not produced in larval stages.
During diapause, the prothoracic gland, in response to brain PTTH, secretes ecdys-
teroids to maintain a specific level of the hormone in the hemolymph.
In general, it is to be expected that ecdysteroids would act as inducers of diapause
and this seems to be the case in many insect species, including the gypsy moth,
Lymnatria dispar , an obligatory diapausing insect species. The insect's prothoracic