Discussion
The present results reinforce earlier findings from both P.
aegeria and other species (Friberg et al., 2011) suggesting that
photoperiodic control of the diapause/nondiapause developmental switch
is asymmetric: activating diapause development required a consistent
photoperiod signal for a longer time than activating nondiapause
development (Fig. 2). In addition, we here demonstrate a similar
asymmetry in pre-diapause development rate, a trait that has likely been
under selection to match the diapause phenotype (Kivelä et al., 2013).
An increase in daylength led to an immediate increase in development
rate (further accelerated by prolonged exposure to long days), favorable
for producing an additional generation; in contrast, the slow
development typically seen in diapause-destined larvae was here only
engendered by sustained exposure to short days (Fig. 3, Fig. 4). The
results are largely consistent with P. aegeria possessing two
alternative, overall modes of larval development, cued by photoperiod:
slow, “diapause-track” development and fast, “nondiapause-track”
development. It is evident that, while these two modes diverge early in
life, they are not irreversibly locked states; instead, photoperiodic
information is continually used throughout the larval period to update
developmental plasticity. Switching between developmental modes does not
appear to be instantaneous, but shows a degree of inertia, as the
phenotype matching the new photoperiodic environment did not fully
manifest until the next larval instar after the change in photoperiod
had taken place (Fig. 3). This inertia may represent a delay in
perceiving and acting on the changed photoperiod signal, and/or a delay
in “resetting” the hormonal machinery that controls development and
growth.
Meanwhile, a rather different result was obtained for another life
history trait associated with the diapause switch: body size. Pupal
weight has previously been shown to be larger for diapausing individuals
at least in bivoltine P. aegeria populations, and especially in
the Öland population (Aalberg Haugen et al., 2012; Aalberg Haugen &
Gotthard, 2015). Here, differences between individuals reared under
short versus long days were not visible until the pupal stage,
suggesting that these inter-pathway differences, unlike those seen for
development rate, do not emerge until late in larval development.