Abuzz About Bee Genomics
Graeme O'Neill
Prof. Robert Page, founding director of the'School of Life Sciences at Arizona State University,
has spent 20 years investigating bee behaviour and its genetic underpinnings. He was intrigued
that honeybee 'sisters', who share, live an.d. work together, exhibit such contrasting but
complementary behaviours late info their lives.
Page, a plenary speaker at the 2006 Lorne Genome Conference, says the division of
labour is a hallmark of complex social systems, along with altruism - worker bees, which are
all female, cede their reproductive privileges to their mother, the queen, and will lay down their
lives to protect their sisters. He has been interested in how the division of labour during foraging
evolved - and what light it can throw on the evolution of the honeybee social system.
Page says bees exhibit temporal polyethism - they change behaviours and specialisations
as they age. The final transition involves a move from performing specialised tasks within the
hive to foraging outside the hive. 'It's a very dramatic transition, involving lots of physiological
changes, and 'requiring many genes to be upregulated or downregulated,' he says. Once they initiate foraging, they then tend to specialise either in collecting nectar or pollen. So first, they
had to establish a division of labour between foraging and non-foraging bees, followed by a
division of labour in the type of foraging.
'To a lesser extent, I'm interested in the evolution of the worker caste itself, because the
females stay home to work for mother,' Page says. 'It's a black-box feature of complex insect
societies - instead of going through a normal pattern of adult development, where they leave
the nest, fly around and disperse, they stay home and engage in maternal behaviour.'
What has emerged from his research, says Page, is that bees that begin foraging slightly
earlier in life specialise in pollen, while late starters tend to forage for nectar. Pollen foragers
are more responsive to low sugar concentrations than nectar-foragers, and are also more
responsive to light. 'If certain QTLS are common to different sets of traits, something very
fundamental must be going on,' Page says. 'If the same genes are affecting whole sets of traits,
why are they linked?'
Page says he remembered his first course in insect physiology, where he learned about
the gonotrophic cycle in the female mosquito, involving major changes in foraging behaviour.
'She goes from foraging for nectar to foraging for blood meals high in protein. Her behaviour
is related to the state of her ovaries. As the ovaries change, and begin to produce eggs, her
behaviour changes too. When she has a blood meal, she tunes into a different set of stimuli -
she seeks body heat, avoids light, avoids contact with the ground and seeks out low, dark
places where she just sits while her eggs mature. Now her behaviour changes again - she
seeks out water vapour, and lays her eggs on the water.'
Page says he wondered if bees had co-opted the ancient gonotrophic cycle into a system
featuring specialised behaviours and division of labour. 'We looked at genes associated with the
reproductive state, and knocked some out using RNA-induced silencing, and we were able to
predict the resulting behavioural changes,' he says. 'For example, we found we could predict
behavioural changes due to the gene knockouts, and we showed that the preference for pollen
or nectar is related to the ancient gonotrophic cycle. So is the age of onset of foraging - bees
performing different tasks get locked into them.'
By the time of the Lorne Genome Conference, Page and his colleagues will have published
their latest findings in
Nature.
'Going back to the solitary insect mode, we think what is happening is that the insect
emerges from the cell without its ovaries activated, flies around and then disperses and mates,'
he says.
'The ovaries are then activated by a hormonal signal involving ecdysone and juvenile
hormone, and the ovaries become vitellogenic - they are ready to receive proteins produced
from specialised fat-body cells and convert them into eggs.'
Page says this normally occurs after winter diapause, or a period of reproductive latency.
The mosquito and bee have contrasting life histories - honeybees have pre-reproductive ovary
activation, mosquitoes exhibit post-reproductive activation. 'In the honeybee, the hormonal
signal to activate the ovaries occurs in the pupal stage, not in adulthood,' Page says. 'So when
the honeybee emerges from its cell into the nest, it's not tuned for dispersing and mating. It has
already undergone the equivalent of winter diapause or reproductive latency, and it's already in
a maternal behaviour pattern.'
'In the maternal. nest it's already responding to stimuli that would cause it to exhibit
maternal behaviours - it has cells to clean, food to process, larvae to feed,' Page says. 'It's all
fundamental reproductive behaviour, but with the timing of the activation signal changed. 'In
honeybees, it denies the worker the opportunity to have all those pre-maternal behaviours we
see in the mosquito and other insects that lead solitary lives.'
The genes that set the· switches for these reproductive behaviours . have not yet been cloned
and studied in bees. Page says some likely candidate genes produce insulin-like signalling
molecules similar to those found in humans. 'Now we have the complete genome sequenced
from the honeybee, we can begin to identify candidate genes,' he says. He says natural
selection has co-opted ancient patterns of behaviour in solitary insects and shaped them into
unique patterns of social behaviour in bees.
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