Diapause: Delaying the Developmental Clock in Response to a Changing Environment - Current Topics in Developmental Biology - page 226

that morphs which are adapted to the expected adverse conditions are
produced.
In Pieris napi (Pieridae), diapausing pupae develop into spring venosa
morpha that have a fat, dark, and hairy body, and dark scales on the veins
of the wings (on all hind wings and on forewing apex). Conversely, nondia-
pausing pupae produce the summer castoria form which has a slender, light,
and smooth body with light wings ( Oliver, 1970 ). This pathway decision is
linked to modifications of larval growth and it is mostly state-independent
since “diapause” larvae develop slower than those set for direct development
only after the pathway decision has been made in the fourth larval instar
( Friberg, Dahlerus, & Wiklund, 2012; Shapiro, 1975, 1977 ). Indeed,
direct-developing larvae grow slower than “diapausing” larvae during the
first two instars, whereas they develop at the same rate during the third
and fourth instars. In the fifth (last) instar, when the pathway decision is
made, larvae set to be “nondiapausing” grow faster than those set for dia-
pause, revealing once again that many species modify their larval growth
rates in relation to their developmental pathway (diapause vs. direct devel-
opment) ( Friberg et al., 2012 ).
Developmentally, diapause-bound polyphenisms are induced by the dif-
ferential timing of ECD pulses during a “hormone-sensitive” period. The
strength of ECD pulses during such a sensitive period (during either larval
or pupal life) determines the induction, or the inhibition, of the alternative
developmental pathway. It appears that the timing of the ECD pulse and a
threshold in hormonal sensitivity ( Hartfelder & Emlen, 2012; Nijhout,
2003; Saunders et al., 2002 ) are key to determining outcome. A well docu-
mented case is that of Araschnia levana (Nymphalidae) in which the last larval
instar can develop into two butterfly morphs: levana and prorsa ( Koch &
B¨ckmann 1987 ). Under short days, larvae develop into dark diapausing
pupae from which the levana morph (orange/dark with black spots) emerges
in spring, once dormancy is ended. In contrast, larvae grown under long days
develop into light pupae (direct development) from which the prorsa form
(white and black) ecloses. This phenomenon depends by the timing of the
ECD pulse. In “ prorsa ” pupae, it pulses during the “hormone-sensitive”
period (between 2 and 10 days after pupation), while it is delayed in “ levana ”
pupae because of diapause. Injection of 20-ECD into 3-day-old diapausing
pupae ( levana ) induces them to promptly break dormancy and develops into
prorsa morphs instead of the expected levana form. However, injection of
20-ECD into 14-day-old levana pupae, while still able to promptly break
dormancy, is not able to reverse the morph identity since the “ levana ”