變態之後殘留的記憶:一隻蛾能記得幼蟲時代所學的事嗎?

Retention of Memory through Metamorphosis: Can a Moth Remember What It Learned As a Caterpillar?

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PLoS ONE 3(3): e1736
Douglas J. Blackiston, Elena Silva Casey, Martha R. Weiss*
Department of Biology, Georgetown University, Washington, D. C., United States of America

究竟在經過激烈的變態以後，蝶或蛾是否能記得幼蟲時期的記憶呢？科學家推測蝶與蛾在變態之後帶有幼蟲時期有關痛苦的記憶。該研究利用綠色的煙草天蛾(Manduca sexta)幼蟲進行實驗，經過三週以菸草葉餵食，幼蟲會進入具有褐色外骨骼的蛹期，約有18-24小時其內部狀似濃湯，一切結構分解並由其消化管重新再組成中央神經系統，最後羽化時已成由內到外都全新的、具有翅、複眼與口器的天蛾成蟲。

在果蠅的研究中，已發現幼蟲期的記憶與神經連結能夠在激烈的身體大翻修後保存下來，但是此現象在天蛾身上尚未明瞭。曾有研究指出天蛾成蟲會偏愛其幼蟲時期取食的寄主植物，然而存疑者認為這並非受記憶的影響、而是因為幼蟲在寄主植物上化蛹且在原處羽化。

本研究以Y型路徑選擇裝置進行行為實驗，實驗組幼蟲先以條件制約方式使五齡幼蟲學習到把乙酸乙酯 (EA, Ethyl Acetate)的氣味與溫和的電極聯想在一起，結果顯示未接受過電極或只單純受過電極的幼蟲，對於EA的氣味並沒有偏好或厭惡的選擇，但是曾同時受過EA+電極八次的五齡幼蟲，則明顯地表現出厭惡EA氣味的選擇，且此選擇直到羽化為成蟲仍然保持。若以三齡幼蟲進行EA+電極配對實驗，結果直到五齡幼蟲時仍能表現出對EA厭惡的選擇，但是經過變態為成蟲之後則對EA沒有顯著的厭惡表現了。至於蛹期所接觸的環境中化學物質則並未改變成蟲的選擇偏好：對未經實驗處理的蛹施以EA氣味，其羽化成蟲對EA或新鮮空氣的選擇沒有顯著差異，而已對幼蟲施予EA+電極配對實驗後的蛹雖經過洗滌，其羽化成蟲仍然厭惡EA氣味。
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 naı¨ve 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.

相關參考:
Homepahe of Dr. Martha R. Weiss:Caterpillar Behavior
Shocked Moth Remembers Past Life as Caterpillar! By Devin Powell ScienceNOW Daily News 5 March 2008