Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 220

reduces the probability of diapause in flesh flies kept in strong diapause-inducing
environments (long-day 12:12h at 20°C) and thus expected to diapause ( Rockey
et al., 1991 ). Diapause induction depends on the activation of a dopaminergic circuit
and inhibition of diapause starts with the activation of the GABAergic system in the
brains of SP flesh fly mothers ( Webb and Denlinger, 1998 ).
Switching to the new life history character without affecting the genetic informa-
tion (genes/regulatory sequences) implies that some form of epigenetic information,
transmitted to the offspring via the egg, is responsible for the switch. It is believed
that the mother provides this information to the egg in the form of a maternal cyto-
plasmic factor. The nature of the factor as of yet is not known, but the GABA sig-
nal may transmit the diapause-suppressing information from the brain by inducing
the secretion of a relevant factor in the egg. Investigators believe that “the informa-
tion transfer from mother's brain (the site of the photoperiodic reception) to her ova-
ries occurs sometime after pupariation but before the second day of the adult life”
( Henrich and Denlinger, 1982 ).
Most recently, studies on the endoparasitoids of insect eggs Trichogramma spp.
have shown that short-day-induced diapause is passed on beyond the first generation,
to the second and third generations and, weaker, to the fourth and fifth generations
( Reznik et al., 2012 ).
A Quick Change of the Phenotype—Phase Transition in Locusts
Locusts of the species Locusta migratoria (Linnaeus, 1758) and Schistocerca gre-
garia (Forskål) have a unique and fascinating ability to switch from the solitari-
ous to the gregarious phases involving discrete changes in multiple morphological,
physiological, behavioral, and life history traits. The changes are so extensive that
biologists initially believed they were different species ( DeLoof et al., 2006 ). It goes
without saying that these are discrete changes, not related to the norm of reaction.
Phase transition in locusts is induced by two distinct sensory neural pathways:
1. The visual-olfactory, or cerebral pathway, which is activated by perceiving other conspe-
cific locusts in the vicinity.
2. The tactile or thoracic pathway, which is activated by the stimulation of hind leg mechano-
receptors, as occurs in nature, by jostling with other locusts.
The earliest phenotypic change is the gregarious and migrating behavior, which
locusts display within 2h of perceiving the phase transition-inducing stimuli. The
change in body coloration takes ~24 h, whereas morphological changes in wings, hind
legs, and so on may take several generations to be fully expressed ( Burrows et al., 2011 ).
Morphological changes in transiting from the solitarious to gregarious phase
include alterations in head and brain size, in the number of sensilla, morphometry,
and cuticle color (from green to dark), shorter wings and hind legs, smaller eyes and
antennae. Although long-term gregarious locusts are smaller in size, their brains are
30% larger than the brain of the long-term solitarious locusts. Differences are also
observed in the proportions of various regions of the brain during the two phases.
The increase in the brain size and changed proportions of brain parts in gregarious