Trailblazers: Investigating Chemotaxis with C.elegans
In the presence of a repellent, more time is spent chang- ing direction. While this movement may appear slow, it is actually well suited to the nematode’s noisy and turbulent soil environments. As soil-dwelling organisms, C.elegans have adapted to live in a dynamic habitat that is sometimes aquatic and other times terrestrial. Consequently, they can detect and respond to both volatile and water-soluble molecules. The latter (water soluble molecules) are used mainly for short-range navigation and particularly avoidance. While the former (volatile molecules) are used for longer range food searching. In general, C.elegans recognize a wider variety of volatile odorants and are more sensitive to
End
End
Start
Start
Figure 4: Biased Random Walk and Pirouette movement.
volatile signals. Still, in experiments, C.elegans have shown chemotaxis in modest gradients of both types of cues and overall scientists have documented unique responses to at least 40 different chemicals (Table 1). Scientists have also observed changes in chemical preferences that suggest imprinting, associative learning, and long-term memory. All these behaviors are controlled by the 302 neurons that make up the C.elegans nervous system.
TABLE 1: Chemicals recognized by C.elegans
Volatile
Water Soluble
Basic pH, Lysine, Histidine, Cysteine, Serotonin, Biotin, cAMP, cGMP, Cl-, SO4-, NO3-, Br-, I-, Na+, K+, Li+, Ca++, Mg++
Alcohol, Ketones, Diketones, Esters, Pyrazines, Thiazoles, Aldehydes, Aromatics, Ethers.
Attractant
Heptanol, Octanol, Nonanol, Benzaldehyde, Thrimethylthi- azole, Ethyl Hepanoate
Acidic pH, Heavy Metals, SDS, Tryptophan
Repellant
Each neuron within C.elegans has a defined role, shape, and position. Moreover, these features are invariant – they are the same between individuals of different populations and generations. Over the last 30 years scientists have determined the locations of every neurons as well as the roughly 7,000 connections between them. This has been translated into a comprehensive neural map, or connectome (Figure 2). Investigators have also classified each neuron by task: (1) sensory neurons that collect information from the environment, (2) motor neurons that control muscles, and (3) interneurons that connect sensory and motor neurons. While all three neural types are involved in chemotaxis, most research has focused on the biology of sensory neurons. C.elegans has 129 sensory neurons out of which 32 focus primarily on chemical stimuli. These chemosensory (CS) neurons are primarily organized into left/right pairs that cluster around two small openings at the head (the amphid and labial) and one at the tail (the phasmid). Here, mol- ecules from the outside environment attach to receptors in the neuron’s exposed tip. This activates ion channels that in turn change the neuron’s membrane potential and eventually triggers the release of neurotrasmitters. This process is known as signal transduction. The morphologies of CS
4
Visit our website at www.edvotek.com
Made with FlippingBook flipbook maker