Biology Reference
In-Depth Information
the embryos
trigger diapause I
in the presence of adults
(
Inglima,
Perlmutter, & Markofsky, 1981
).
Diapause occurs also in
Austrolebias
(diapauses I and II are facultative and
diapause III is obligate), in
Pterolebias
and
Rachovia
killifish (similar to
Austrofundulus
, diapause II is obligate) and in African
Nothobranchius
killifish
where diapause has evolved independently (
Arezo, Pereiro, & Berois, 2005;
Inglima et al., 1981; Levels, Van Tits, & Denuc´, 1986; Murphy & Collier,
1997; Pri-Tal et al., 2011; Wourms, 1972a, 1972b, 1972c
). Annual fish dia-
pauses are an evolutionary innovation in the Cyprinodontiformes (
Berois
et al., 2012
) and they might result by environmentally sensitizing a plastic
preexisting developmental system or by modifications in the timing of
developmental hormones. Some nonannual Cyprinodontiformes can delay
the hatching in intertidal environments, suggesting that diapause III could
originate from this feature (
Berois et al., 2012; Taylor, 1999
). In addition,
the embryo of nonannual Cyprinodontiformes
Fundulus heteroclitus
can
avoid dehydratation when exposed to air (
Tingaud-Sequeira, Zapater,
Chauvigne, Otero, & Cerda, 2009
). Interestingly, zebrafish embryos (
Danio
rerio
) lack diapause but, when exposed to chronic hypoxia, they slow down
developmental growth (at about 30 hpf stage) without any developmental
aberration. “Hypoxic” embryos inhibit insulin growth factor 1 (IGF1) sig-
naling by enhancing IGF binding protein 1 (IGFBP1) expression. Upon
reoxygenation, embryos accelerate development and growth rate (“catch-
up” growth) by restoring IGF1 signaling (
Kamei et al., 2011
). Despite the
fact that loss of IGF1 signaling produces developmental delayed dwarf
embryos that die at a stage corresponding to the wild-type 18 hpf stage
(
Schlueter et al., 2006
), the transient and timed loss of IGF signaling can
enhance developmental plasticity during hypoxia (
Kamei et al., 2011
),
and resembles the outcome observed in killifish diapausing embryos.
8.1. Do insulin/IGFs play an evolutionary conserved function
in diapause?
Similar to the hypoxia-induced response of the killifish, insulin/IGF-type
factors likely also influence diapause trajectory in Lepidoptera, suggesting
that this signaling pathway has been evolutionary recruited for diapause in
both vertebrates and invertebrates. In support of this scenario, insect insulin/
IGFs like ligands have been identified in several lepidopteran species
(
Antonova, Arik, Moore, &Riehle, 2012
) and are important developmental
hormones that play pivotal roles in modulating holometabolous growth. In
the case of the monarch butterfly,
Danaus plexippus
, which exhibits seasonal
