Whenever a memory is formed, thousands of new connections – synapses – form in the brain, but what underlies these changes? For example, do the expression patterns of brain genes differ between memories? This is the question that Gene Robinson and his colleagues from University of Illinois Urbana-Champaign and East Tennessee University asked. Robinson explains that honeybees are an ideal species for answering this question because so many elements of their environment can be controlled with precision, and the impact of these changes on the insects' behaviour and gene expression patterns can be analysed. Curious to find out how the bee's brain gene expression patterns differ when forming two similar, but distinct, memories, Robinson and his colleagues trained groups of bees to go foraging at different times of day and in different places and then looked for differences in the insects' brain gene expression patterns (p. 979).
Robinson's student, Nicholas Naeger, travelled to Darrell Moore's lab in Tennessee where he teamed up with Byron Van Nest, Jennifer Johnson and Sam Boyd to train honeybees to visit one of two feeders. Robinson explains that flowers open and produce nectar and pollen at specific times of day and bees learn to remember this. So, the team trained one group of bees to forage at a lilac-flavoured feeder in the morning and another group of bees to forage at a lavender-flavoured feeder at a different location in the afternoon. Having successfully trained 40 bees to feed at each feeder, the team collected bees from both groups 15 min before their normal departure while they prepared to go foraging. Naeger and his colleagues also collected the trained bees when they were inactive, because the insects' activity levels and time of day can affect brain gene expression patterns and Robinson wished to account for these factors in the gene expression patterns.
Returning to Urbana-Champaign, Naeger extracted mRNA from the insects' brains and then used a microarray analysis where he could simultaneously compare the expression levels of 11,000 honeybee genes between two groups of bees. Comparing different combinations of bees (e.g. morning-trained active bees with morning-trained inactive bees or afternoon-trained active bees with afternoon-trained inactive bees), the team eventually identified 1329 genes (over 10% of the honeybee genome) that showed expression pattern changes in response to the different memories. And when Sandra Rodriguez-Zas statistically analysed the complex microarray gene expression patterns and took out genes involved in time keeping and activity, which are essential to the memory, the team narrowed down the number of genes that were differentially expressed in the two memories to 352.
Identifying groups of genes that respond differently in the two memories, Robinson highlights genes involved in synapse formation and genes involved in other forms of chemosensory behaviour. ‘The specific genes are interesting, but what is more interesting is to consider the kind of memories we are looking at. It is known that changes in synapses are associated with building memories. However, both groups are building the same kind of memory – it's just for a different place and time – and we see still see differences – this hints at previously unknown forms of specificity for synapse formation in memory,’ says Robinson. He adds, ‘The differences are extensive, telling us that no two memories are alike when it comes to the genome.’
Ultimately Robinson hopes to identify specific categories or genes involved in memory formation, and to identify genes that are diagnostic of specific memories and their locations in the brain.