Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 250

Nurse Forager

High insulin/JH

Hormones Low insulin/JH

Tr anscription
factors

FoxO USP MET FoxO USP MET

Target genes

Nurse expression profiles in
fat bodies and brain

Forager expression profiles in
fat bodies and brain

Gene expression

Physiology/
behavior

Small lipid stores, foraging

Large lipid stores, brood care

Figure 5.2 Theoretical model for the role of GRNs in the regulation of worker division of
labor. Signaling through Ilps and JH is low in nurses and higher in foragers. These hormones
regulate gene expression through interactions with TFs, some of which have already been
identified in other insect species. Known transcriptional regulators include FoxO, which is
involved in insulin action, as well as ultraspiracle (USP) and methoprene-tolerant (MET), both
of which are associated with JH. Increased insulin signaling in foragers is likely to repress
FoxO target genes by preventing the FoxO protein from binding to their promoters. Increased
JH signaling causes increased USP expression in honeybees, as well as other hypothetical
changes in target gene activation by USP and MET. According to this framework, interactions
among these and other TFs lead to distinct gene expression profiles of nurses and foragers in
the brain and fat bodies. These hormonally controlled GRNs are hypothesized to be causal for
behavioral maturation and stable lipid loss.
Source : Ament et al. (2010) .

The hypothesis further assumes that both the internal (middle portion) subcircuits
and the peripheral (inferior) subcircuits can be redeployed in various parts of the
organism where they produce phenotypic changes, but they fail to show how this can
happen or which mechanism is responsible for the redeployment.
The hypothesis holds GRNs as self-regulated entities, but from a systems biology
view they, and most TFs, are just downstream elements of systemic signal cascades/
networks starting in the CNS (hormones, growth factors, secreted proteins, etc. are
their proximate activators). This is illustrated in Figure 5.2 , which shows how hor-
mones (insulin and juvenile hormone (JH)), secreted under ultimate CNS control in
ants, control the activity of GRNs that determine phenotypic changes (e.g., physiol-
ogy, morphology, and behavior).

The Hypothesis of Punctuated Equilibria—Challenging the neoDarwinian
Hypothesis “from Within”
In 1971, Niles Eldredge observed that the gradualism of evolution propounded by
the neoDarwinian theory was incompatible with paleontological evidence of the