Watching the days go by: asymmetric regulation of caterpillar
development by changes in photoperiod
Abstract
Many insects possess the plastic ability to either develop directly to
adulthood, or enter diapause and postpone reproduction until the next
year, depending on environmental cues (primarily photoperiod) that
signal the amount of time remaining until the end of the growth season.
These two developmental pathways often differ in co-adapted life history
traits, e.g. with slower development and larger size in individuals
headed for diapause. The developmental timing of these differences may
be of adaptive importance: if pathways diverge late, the scope for
phenotypic differences is smaller, whereas if pathways diverge early,
the risk is higher of expressing a maladaptive phenotype if the
selective environment changes. Here we explore the effects of changes in
photoperiodic information during life on pupal diapause and associated
life history traits in the butterfly Pararge aegeria. We find that both
pupal diapause and larval development rate are asymmetrically regulated:
while exposure to long days late in life (regardless of earlier
experiences) was sufficient to produce nondiapause development and
accelerate larval development accordingly, more prolonged exposure to
short days was required to induce diapause and slow down pre-diapause
larval development. While the two developmental pathways diverged early
in development, development rates could be partially reversed by altered
environmental cues. Meanwhile, pathway differences in body size were
more inflexible, despite emerging late in development. Hence, in P.
aegeria several traits are regulated by photoperiod, along subtly
different ontogenies, into an integrated phenotype that strikes a
balance between flexibility and phenotype-environment matching.