| The ecology and behaviour of animal learning and memory have been studied extensively Differences in learning between closely related species create excellent opportunities for studying species-specific adaptations of learning. Parasitic wasps are good model systems for exploring learning and memory formation and their underlying mechanisms. We investigated learning and memory formation of two closely related parasitic wasp species, Cotesia. glomerata and Cotesia. rubecula, to reveal natural differences in memory acquisition. We wanted to establish the type of memory that is different between these wasp species. Using a strictly controlled classical conditioning set-up, we defined differences in specific forms of memory between the two species., by measuring the dynamics of memory formation. For this purpose, we applied (1) retrograde amnesia by cooling directly after learning to inhibit anesthesia sensitive memory (ASM) or (2) inhibition of translation or transcription with anisomycin (ANI) and actinomycin D (AcD), respectively, to block long-term memory (LTM). The two species were then tested for memory retention after different conditioning trials. Following these treatments, we studied the memory dynamics in two closely related species. The main results were as follow listed below:C. glomerata memory retention was completely inhibited by actinomycin D (ACD) after one conditioning trial for 24 hrs. When ACD-fed wasps were given a single conditioning trial, 24hrs memory was reduced to a level that was not different from naive ACD-fed wasps, showing that 24h memory was composed entirely of a transcription-dependent memory component. In C. rubecula a single conditioning does not induce 24 hr memory. Memory for nasturtium is formed after a single trial, but lasts only some hours. A massed learning protocol (3 subsequent oviposition experiences on nasturtium without a rest interval) yielded for both species displayed similar result for 24h memory with single trial learning. For C. glomerata, massed learning gave a strong memory compared to naive wasps and a strong reduction of this memory by ACD treatment to a level not different from naive, ACD-fed, but in C. rubecula, 24 hour memory retention was again similar to naive wasps.A spaced learning protocol (3 oviposition experiences on nasturtium with a 10 min interval) yielded a stable memory lasting 5 days in contrast with naive wasps for both species. There was, however, a difference in the temporal dynamics of LTM. In C. glomerata, maximum inhibition of memory performance in ANI- fed and ACD-fed wasps was reached and maintained stable after 4 h a showing that consolidation of a protein synthesis-dependent memory was complete after 4h. In C. rubecula 24h memory was only partially inhibited by ACD and anisomycin (ANI). This shows that a protein synthesis-independent memory coexists with a protein synthesis-dependent memory in C. rubecula at 24 h, while memory is entirely protein synthesis dependent at 72hrs. At 72hrs, the memory retention was completely inhibited by ANI. LTM consolidation took 72h in C. rubecula.In C. glomerata, wasps treated by a cold shock directly after a single trial had a similar memory performance at 1 h after a single trial as naive wasps, showing that memory was completely inhibited by cold shock. 4h memory retention after cold shock was inhibited partially compared to wasps after a single trial, while after 8h the cold shock treated wasps progressively approached the memory performance of untreated wasps after a single trial. When ACD-fed or ANI-fed wasps were given 1 single conditioning trial ,1h memory retention are not affected and 4h memory retention inhibited partially while 8h memory retention was completely inhibited by inhibitor .Thus, the ASM component after a single trial learning in C. glomerata contributes 100% to the observed memory trace at 1h and progressively decays within 8 h. In C. rubecula, the decay rate occurs at a faster rate than in C. glomerata, as cold shock treated wasps had a similar memory performance 20 min. after a single trial as naive wasps, whereas after 1h memory was significantly less affected by cold shock treatment. After 4h the cold shock treated wasps had similar memory retention levels as untreated wasps after a single trial. Thus, the ASM component after single trial learning in C. rubecula is 100% at 20 min and progressively decays within 4 h. Wasps subjected to cold shock 20min, 1h, 2h or 3h after training and then 4h retention were tested in each case. 4h retention was found to be severely disrupted when cold anesthesia was delivered 20min after training, but became increasingly resistant to cold anesthesia, as such treatment was delivered 1h after training.In C. glomerata, the ASM component decays within a similar time window as consolidation phase of LTM, suggesting that ASM consolidates directly into LTM, without an intermediate or coexisting anesthesia-resistant memory (ARM) trace. In C. rubecula, ASM decays several hours before the start of the consolidation phase of LTM, and its memory consolidates first into an ARM trace that later on coexists with an LTM trace and then decays after 48h. A model showed the differences in ARM and LTM consolidation between the two wasp species.cAMP responsive element binding protein (CREB), a transcription factor plays a key role in the LTM formation. To elucidate the role of the CREB in wasp memory formation, we analyzed creb expression in wasp brain by FISH. First results show a positive signal in the kenyon cells around the mushroom bodies, suggesting that the gene is predominantly expressed in kenyon cells around the mushroom bodies, a structure that is involved in learning and memory processes. However, expression can also be found around the lamina and medulla of the optic lobesThe species-specific difference in LTM acquisition and consolidation is adaptive given the extreme differences in the effects of learning on two parasitoid species evolved distinctly different foraging strategies. This is the first demonstration of natural differences in quantity and quality of learning events required for LTM formation between closely related species. The present studies indicate that the development of learning and memory of insects is closely associated with evolution and environment. Our results suggested that different mechanism between the closely related species underlines the formation and maintenance of LTM.
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