|MadSci Network: Neuroscience|
Hi Micah! You've asked some really good questions, and I don't think that there's enough data out there to unequivocally declare one life form to be the smelling champion. As you suggest, sensitivity to low concentrations of an odorant and to a broad range of molecules are important characteristics. In addition, you would want your smelling champion to be good at discriminating a particular odor from the rest of the smells that are present (or to be able to distinguish among similar molecules). Lab experiments (and some observations outside the lab) have provided insight into some of these questions. Underlying any organism's ability to smell is its repertoire of olfactory receptors. Olfactory receptors are fairly large proteins which reside in the cilia of olfactory neurons; the known olfactory receptors are members of the G-protein coupled receptor family. When an odorant molecule binds the receptor, a conformational change ensues, causing the activation of an associated G-protein (which then goes on to activate other molecules in a cascade of biochemical interactions). If you're familiar with the way light activates the visual pigment rhodopsin, the process in olfactory neurons is somewhat similar. Recent advances in genomics have enabled scientists to look at large numbers of olfactory receptors within and across species. The really groundbreaking work in this field was done by Linda Buck, who was a postdoctoral fellow in Richard Axel's lab at Columbia University. Buck and Axel found a large family of genes for G-protein coupled receptors in rats; these genes were specifically expressed in the olfactory epithelium. Work by other labs (including the Firestein lab at Columbia and the Reed lab at Johns Hopkins) has shown that these genes do, indeed, encode functional olfactory receptors. Until the rat genome is cloned, we won't know exactly how many olfactory receptor genes there are in that species. Still, work on mouse and human olfactory receptor genes has suggested that there are about 1,000 receptor genes in those species, perhaps corresponding to about 1% of the rodent or human genome. That number will be clearer with the completion of the human genome in the next few years. Buck and another former Axel lab postdoctoral fellow, Catherine Dulac, independently discovered a smaller family of ~100 potential pheromone receptors in rodents, although nobody has figured out which pheromones interact with which receptors ... yet. There's limited experimental evidence that pheromones play a part in human behavior, but functional human pheromone receptors have yet to be cloned. For animals with completely sequenced genomes, the picture is a little clearer. The nematode (roundworm) C. elegans has had its genome sequenced in its entirety. Of ~19,600 genes, roughly 700 look like olfactory receptors -- good news for a creature which relies on smell and taste to navigate through its environment. Last year, some olfactory receptors for the fruit fly drosophila were finally discovered by the Carlson and Axel labs. The fly genome is just about complete, and perhaps other families of olfactory receptors will be found, too. So, from all of these studies, it seems that animals devote a large proportion of their DNA to encoding olfactory receptors. How sensitive are they? In vitro studies of rodent olfactory receptors by Buck, Reed, and Firestein show sensitivity in the micromolar to millimolar range. Behavioral studies in living worms show sensitivity to 1:1000 dilutions of an odorant placed on an agar plate. Meanwhile, studies of odorant sensitivity in humans show, for a large battery of smells, sensitivity in the nanomolar range -- although components of mucus may create higher concentrations in the vicinity of olfactory receptors. Some of the more compelling data comes from observations of animals in their daily routines. Certain breeds of dogs are widely revered for their ability to track scents. Turkey vultures can smell prey (or leaks from a gas pipeline!) from miles away, suggesting that their olfactory systems are profoundly sensitive. Humans are pretty good at discriminating enantiomers (left vs. right-handed molecules) such as (d) or (l) carvone, one of which smells like peppermint and the other, caroway. Knowing something about the receptors underlying olfaction, we can ask questions about the ways that the olfactory system works to discriminate among different smells. Can receptors/cells detect more than one odor molecule? Does discrimination happen within one neuron or between neurons? What are the mechanisms for adapting to an odorant (like getting used to a bad smell in a room)? How does the presence of one odor influence your ability to perceive other odors? People in the olfaction field are working on these sorts of questions, which may ultimately enable us to answer your original question, and find the planet's champion smeller. By the way, there's a TON of interesting work out there. I've provided you with information on some widely-used experimental animals, but you may also want to look up information on olfaction in moths (manduca sexta) and salmon for some other perspectives. Cheers, Amanda Kahn firstname.lastname@example.org For information on work by Richard Axel (rodents and flies), Cori Bargmann (worms), Linda Buck (rodents), Randy Reed (linking odors and receptors), and Catherine Dulac (rodents), follow the links on this page: http://www.hhmi.org/science/ neurosci/invest.htm For information on John Carlson's work with flies, click here: http:// pantheon.cis.yale.edu/~jcarlso/index.html For information on Stuart Firestein's work on odors and receptors, click here: http:/ /www.columbia.edu/cu/biology/faculty/firestein/ for information on a bunch of odorants, click here: http://www.nysaes.cornell.edu/ flavornet/index.html Here's an interesting paper on human pheromones: Stern, K; McClintock, MK. Regulation of ovulation by human pheromones. Nature, 1998 Mar 12, 392(6672):177-9. http:/www.nature.com Here are some of the references for the sensitivity data I cited above: Zhao, H; Ivic, L; Otaki, JM; Hashimoto, M; Mikoshiba, K; Firestein, S. Functional expression of a mammalian odorant receptor Science, 1998 Jan 9, 279(5348):237-42. http://www.sciencemag.org/cgi/ content/full/279/5348/237 Krautwurst, D; Yau, KW; Reed, RR. Identification of ligands for olfactory receptors by functional expression of a receptor library. Cell, 1998 Dec 23, 95(7):917-26. Bargmann, CI; Hartwieg, E; Horvitz, HR. Odorant-selective genes and neurons mediate olfaction in C. elegans. Cell, 1993 Aug 13, 74(3):515-27. Malnic, B; Hirono, J; Sato, T; Buck, LB. Combinatorial receptor codes for odors. Cell, 1999 Mar 5, 96(5):713-23. Devos, M, F. Patte, J Rouault, and P Laffort. 1990. Standardized Human Olfactory Thresholds. IRL Press, Oxford UK. For information on turkey vultures and their ability to sniff out gas leaks: http://home.sou.edu/~rible/wildlife/turkeyvulture.html
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