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Link to original content: https://pubmed.ncbi.nlm.nih.gov/18320055
Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar? - PubMed Skip to main page content
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Comparative Study
. 2008 Mar 5;3(3):e1736.
doi: 10.1371/journal.pone.0001736.

Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar?

Affiliations
Comparative Study

Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar?

Douglas J Blackiston et al. PLoS One. .

Abstract

Insects that undergo complete metamorphosis experience enormous changes in both morphology and lifestyle. The current study examines whether larval experience can persist through pupation into adulthood in Lepidoptera, and assesses two possible mechanisms that could underlie such behavior: exposure of emerging adults to chemicals from the larval environment, or associative learning transferred to adulthood via maintenance of intact synaptic connections. Fifth instar Manduca sexta caterpillars received an electrical shock associatively paired with a specific odor in order to create a conditioned odor aversion, and were assayed for learning in a Y choice apparatus as larvae and again as adult moths. We show that larvae learned to avoid the training odor, and that this aversion was still present in the adults. The adult aversion did not result from carryover of chemicals from the larval environment, as neither applying odorants to naïve pupae nor washing the pupae of trained caterpillars resulted in a change in behavior. In addition, we report that larvae trained at third instar still showed odor aversion after two molts, as fifth instars, but did not avoid the odor as adults, consistent with the idea that post-metamorphic recall involves regions of the brain that are not produced until later in larval development. The present study, the first to demonstrate conclusively that associative memory survives metamorphosis in Lepidoptera, provokes intriguing new questions about the organization and persistence of the central nervous system during metamorphosis. Our results have both ecological and evolutionary implications, as retention of memory through metamorphosis could influence host choice by polyphagous insects, shape habitat selection, and lead to eventual sympatric speciation.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Diagram of Y choice apparatus used for larval and adult testing.
Individual M. sexta were placed into a short “loading arm” attached to a 10cm diameter central chamber to which a vacuum was applied. Air was bubbled through 20 ml of EA and pulled through one of the two side arms, while ambient air was pulled through the other. Larvae and adults were allowed to move freely within the apparatus for ten minutes, at which time their position was scored as either the EA arm, ambient air arm, or no choice (defined as the loading arm or any portion of the central chamber).
Figure 2
Figure 2. Larvae conditioned with forward-paired shock+odor avoid EA at fifth instar.
Proportion of M. sexta larvae choosing ambient air rather than EA in the Y choice apparatus after receiving one of five treatments: no exposure to odor or electric shock (N = 46), shock only (N = 43), odor only (N = 29), the forward pairing of shock+odor at fifth instar (N = 41), or forward pairing of shock+odor at third instar (N = 32). Only larvae conditioned with shock+odor demonstrate a significant aversion to EA as larvae, and larvae trained at third instar recall the aversion at fifth instar, indicating retention of memory across molts. *** (p<0.001) indicates values that differ significantly from random choice (dashed horizontal line) by a two-tailed binomial calculation. Values are means±SD.
Figure 3
Figure 3. Larvae conditioned with forward-paired shock+odor at fifth instar retain odor avoidance as adults.
Proportion of adult M. sexta choosing ambient air rather than EA in the Y choice apparatus after receiving one of five treatments: no exposure to odor or electric shock (N = 31), shock only (N = 23), odor only (N = 28), the forward pairing of shock+odor at fifth instar (N = 27), or forward pairing of shock+odor at third instar (N = 15). Only individuals that received the shock+odor pairing as fifth instar caterpillars maintained odor avoidance as adults. While individuals trained at third instar demonstrated odor aversion at fifth insta (Fig 2.), the behavior was lost during pupation. ** (p<0.01) indicates the value that differs significantly from random choice (dashed horizontal line) by a two-tailed binomial calculation. Values are means±SD.
Figure 4
Figure 4. Aversion to EA in adults is not due to exposure to odors from the larval environment.
Proportions of untreated M. sexta larvae (light bars) and adults (dark bars) that had an EA-impregnated gel added to their pupal case (N = 40 adults, 30 larvae), and of shock+odor conditioned larvae whose pupal cases were washed (N = 39 adults, 31 larvae), that chose ambient air over EA in the Y choice apparatus. * (p<0.05) or ** (p<0.01) indicates values that differ significantly from random choice (dashed horizontal line) by a two-tailed binomial calculation. Values are means±SD.

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