Abstract
Diapause is an important escape mechanism from seasonal stress in many insects. A certain minimum amount of time in diapause is generally needed in order for it to terminate. The mechanisms of time-keeping in diapause are poorly understood, but it can be hypothesized that a well-developed neural system is required. However, because neural tissue is metabolically costly to maintain, there might exist conflicting selective pressures on overall brain development during diapause, on the one hand to save energy and on the other hand to provide reliable information processing during diapause. We performed the first ever investigation of neural development during diapause and non-diapause (direct) development in pupae of the butterfly Pieris napi from a population whose diapause duration is known. The brain grew in size similarly in pupae of both pathways up to 3 days after pupation, when development in the diapause brain was arrested. While development in the brain of direct pupae continued steadily after this point, no further development occurred during diapause until temperatures increased far after diapause termination. Interestingly, sensory structures related to vision were remarkably well developed in pupae from both pathways, in contrast with neuropils related to olfaction, which only developed in direct pupae. The results suggest that a well-developed visual system might be important for normal diapause development.
Highlights
Different dormancy stages are common escape mechanisms in animals living in seasonal environments that are characterized by predictable temporal variability in levels of abiotic and biotic stress [1]
We looked for the final age when no change in size is seen at later timepoints. (iv) To see if brain development is similar between the postdiapause and direct pupae, we compared P9 in direct pupae to P158 in diapause pupae. (v) to test whether adults differed depending on developmental pathway, we compared diapause and direct adult brains
Our results show that brain development is arrested at around P3 in diapausing pupae
Summary
Different dormancy stages are common escape mechanisms in animals living in seasonal environments that are characterized by predictable temporal variability in levels of abiotic and biotic stress [1]. Diapause is a deep form of dormancy that occurs at a species-specific developmental stage (egg, larva, pupa or adult). In species having several annual generations, there are often a number of non-diapausing, directly developing generations (generally 1–2), followed by a final generation that initiates winter diapause [1]. Two main phases ensue: the first is diapause maintenance, during which insects are non-responsive to external conditions. At some point during diapause development, usually during mid-winter, the maintenance state is terminated and shifts to the second phase, post-diapause quiescence, during which insects again become responsive to external conditions. After termination, the diapause phenotype is typically maintained by cold winter conditions, and diapause shifts from being internally to externally maintained [3].
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