DNA adaptation according to role
When a bee changes its role in the hive, its DNA adapts
Bees each perform a specific but flexible role within the hive. Older individuals may, if necessary, take over tasks usually carried out by younger bees. Until now, it was unknown that such behavioural changes are accompanied by adaptations at the DNA level. Each role is in fact associated with its own distinct methylation pattern.
The fate of bees within their colony is determined from a very early age. The diet received during the larval stage irreversibly defines the role they will play in the hive: queen or worker. Each individual then develops physical and behavioural traits specific to its status. Their genomes, however, are strictly identical.
The environment and life experience of each organism can influence gene expression, notably through the addition of methyl groups (CH3) to certain nucleotides (the basic constituents of DNA). Low levels of methylation tend to promote transcription of genetic information, whereas high levels generally inhibit the expression of the affected gene. The epigenome thus characterises the genome as modified by methylation in each cell. Until now, once new traits had been acquired, this process was considered irreversible in bees.
In the light of new findings, this assumption appears to be incorrect. The methyl groups that accumulate on the DNA of worker bees throughout their lives may in fact be repositioned. This would depend on the behaviour adopted by these pollinating insects at a given age. These results were published in Nature Neuroscience by Brian Herb of the Johns Hopkins University School of Medicine (JHU Medicine; United States).
Each role performed by a bee leaves a mark on its DNA
Worker bees of Apis mellifera perform different functions throughout their lives. From the sixth day after emergence, they take on the role of nurse bees. They feed the larvae using secretions produced by their hypopharyngeal and mandibular glands. At around 21 days of age, after having fulfilled other tasks, they become foragers. Their role then consists of leaving the hive to collect pollen, a nutrient essential to colony survival. All these changes leave detectable traces in their DNA. These two types of workers therefore display distinct methylation patterns.
To test the reversibility of genetic adaptations, all nurse bees from two hives (6,000 to 7,000 bees per unit) were removed during a period when foragers were absent. Upon their return, half of the foragers reverted to the role of nurses to replace the missing individuals. These bees were subsequently sampled. Their epigenome was analysed using whole-genome bisulfite sequencing (WGBS), with methylation patterns investigated using a technique known as comprehensive high-throughput array-based relative methylation (CHARM).
The results were compared with data collected from foragers and young workers responsible for larval care. The findings are unequivocal. The converted nurse bees exhibited the same methylation patterns as young workers. Moreover, the genes that had lost methyl groups corresponded precisely to those that had been modified during the bee’s life. The behaviour of these insects is therefore reversible, as are the adaptations made to their DNA.
According to one of the study’s co-authors, Gro Amdam of Arizona State University, the bee brain may possess “epigenetic roadmaps” specific to each role performed by worker bees. A change in behaviour would then trigger a shift in these programmes, leading to a readjustment of the organism to the role it must fulfil.
Source: www.futura-sciences.com