Leicester lab learns from locust limbs

Posted by mjs76 at Sep 16, 2011 01:37 PM |
University of Leicester biologist awarded prestigious Fellowship for neurobiological study of extraordinary insects.
Leicester lab learns from locust limbs

Schistocerca gregaria in flight

Dr Tom Matheson from our Department of Biology has recently been awarded a Research Development Fellowship by the Biotechnology and Biological Sciences Research Council (BBSRC) to undertake work on ‘computational approaches to an understanding of limb sensory-motor control’. His fellowship – the only such one awarded nationally in 2011 - will allow him to devote the next 18 months to research.

Dr Matheson’s work focuses on the neuronal control of aimed limb movements, which his lab studies in insects, specifically the desert locust Schistocerca gregaria.

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Dr Tom Matheson

Giant swarms of this notorious species cause destruction across Africa, but in the lab S. gregaria can also be a force for good, providing a particularly useful model for neurobiological studies. (Not least among the desert locust's advantages is its large size: males can be up to 7.5cm long and females can reach 9cm. This makes them a lot less fiddly than, say, the fruitfly Drosophila.)

Work in Dr Matheson’s neurobiology lab, investigating how nervous systems process sensory information and generate behaviours, involves two main projects:

Neuronal control of aimed limb movements

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Locusts are marked with stick-on reflective disks (1) and filmed from the side. Image processing software detects the markers (2), then reconstructs the body posture (3) and limb movements (4). Analysis software extracts relevant information, such as a probability distribution for the location of the foot(5).

The first project analyses the control of aimed limb movements like scratching. When a locust moves its leg in a particular way, what is going on within the nervous system?

Gently brushing a locust’s wing or abdomen causes it to move a leg to scratch the stimulated point. These aimed movements are not just stereotyped reflexes: they can be constantly adapted as the stimulus moves, and the two hind legs can also be co-ordinated in their responses. Yet previous work in the Matheson lab has demonstrated that these aimed movements are co-ordinated by neuronal activity within a restricted part of the insect’s ventral nerve cord (being invertebrates, there is no spinal cord) without the signals going anywhere near the creature’s brain.

To study locust leg movements, the team attach reflective discs to strategic points on the insect which can be tracked by computer; essentially the same technology used to capture actors’ movements in films like Avatar and The Polar Express. Electrophysiological activity within specific nerve cells is recorded simultaneously and this is related to the animal’s leg movements.

Work in the lab asks how the animals adapt to limb damage, and seeks to determine how groups of nerve cells are coordinated. An advantage of working in insects like the locust is that some cells are individually identifiable, so their roles in producing movements can be studied in great detail.

During his BBSRC Fellowship, Matheson will be developing techniques to record the activity of many nerve cells simultaneously. The analysis techniques required to process these complex signals are developed in the lab of Matheson's collaborator, Rodrigo Quian Quiroga in our Department of Engineering. The advances in signal processing developed in the locust will enhance the analysis of electrophysiological data from human subjects.

Phase change in desert locusts

A second project in the Matheson lab investigates processes associated with swarming. Locusts are famous for their swarming behaviour but what is not so widely known is that they undergo a physical and behavioural transformation before they swarm.

When vegetation is relatively sparse and population densities are low, desert locusts are cryptically coloured and shy, moving little, often at night, and avoiding other locusts. In times of plenty, however, when the population rises so much that the insects are constantly bumping into one another, the locusts undergo an extraordinary transformation.

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Hopper in solitary phase. Before...

First to change is behaviour: the animals move more, generally during the day, and crucially become attracted to other locusts. Some of these changes can occur in just a few hours. Over weeks or several generations (the locust lifecycle is a couple of months long) the animals take on bright yellow and black colouration as juveniles, and they become larger. All of these changes are part of the switch from the ‘solitary phase’ to the ‘gregarious phase’ - and then the swarming starts.

The wingless juvenile nymphs (called hoppers for obvious reasons) change from green in their solitary phase to a black and yellow gregarious phase, and already show signs of swarming by forming into marching bands. This sort of change in behaviour and morphology is called ‘density dependent phase polyphenism’* and Dr Matheson and his colleagues are investigating the neuronal changes involved in this process.

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...and after. Hopper in gregarious phase.

They have found, for example, that solitary phase locusts have 30% more hairs on the femurs of their hind legs, where repeated tactile stimulation for as little as four hours is enough to trigger the phase change to the gregarious form.

Another difference between the phases is in a visual nerve cell called the DCMD which stands rather wonderfully for ‘descending contralateral movement detector’ which is used in detecting approaching objects. In gregarious locusts the cell remains very responsive in the face of repeated stimulation, whereas in solitary locusts the response quickly dwindles. The persistent responses in gregarious animals are thought to help locusts avoid colliding with each other when flying in dense swarms. The Leicester team are also investigating how phase change affects both ageing and circadian rhythms in behaviour.

During his BBSRC Fellowship, Tom Matheson will be developing specialised recording techniques, updating his programming skills, working with collaborators in the UK and Germany, and completing a number of research papers. And his locusts will be scratching themselves without using their brains, changing their physical appearance and carrying on with all the other behaviours which make this insect such a fascinating and important experimental subject.

*Today’s word is ‘polyphenism’, the situation whereby two different physical states (phenotype) of a creature can result from a single genetic state (genotype), depending on non-genetic factors such as environment, food supply or, in this case, population density. Good luck using it in a sentence...