Tuesday, May 19, 2015

Sulcatone makes our scent distinctly human, and mosquitoes have figured this out

So, say you're a mosquito (um, yay?) and you've developed a taste for human blood. This can mean you've evolved to make a bunch of Or4, a protein that enables you to detect a substance called sulcatone.

Sulcatone (aka 6-methyl-5-hepten-2-one) is a volatile odorant, meaning that (1) it can fairly easily end up floating about in the air around us, and (2) it has a smell. It's emitted by various plants and animals, and serves a variety of functions. For some reason, relative to other animals, humans produce a lot of it. Consequently, mosquitoes that prefer to consume human blood can use our sulcatone stink to hunt us down. Other blood-feeding insects including bed bugs and highland midges likely use the same approach.

This is sulcatone. It's a heptenone, which means it consists of a chain of 7 carbon atoms (in red) (HEPT-) and also contains a double bond (5-6) (-EN-) and a ketone group (2) (-ONE). Together, these characteristics make it volatile and give it a particular smell, which means it is useful to various organisms as a means of communicating with each other through the air.

Sulcatone is apparently found in citronella oil, which makes me wonder how the heck those citronella candles marketed to ward off mosquitoes work. Maybe they overwhelm the mosquitoes' sense of smell?

Ants of the genus Lasius will emit sulcatone if they are attacked by a predator in order to alert their neighbours and cause them to scatter. Substances used for this purpose are called alarm pheromones. In turn, it is thought that certain beetles residing in ant nests (ants form all sorts of interesting symbiotic relationships with other organisms) will emit sulcatone to avoid being attacked by their hosts.

In other ant species, sulcatone will call forth an army to attack a predator. Wolf spiders and other ant munching arthropods will thus avoid areas where the chemical has been released. Gelis agilis, a small wingless parasitoid wasp (it paralyzes a host and injects it with a single egg, which hatches into a larva that feeds on the host) also releases sulcatone when alarmed. It is thought that it uses the chemical to mimic the ant response, scaring away a predator that might also happen to eat the wasp.


References

Harraca V, Ryne C, Birgersson G, Ignell R. 2012. Smelling your way to food: can bed bugs use our odour? Journal of Experimental Biology 215(Pt 4):623-629. [Full text]

Malcicka M, Bezemer TM, Visser B, Bloemberg M, Snart CJP, Hardy ICW, Harvey JA. 2015. Multi-trait mimicry of ants by a parasitoid wasp. Scientific Reports 5:8043. [Full text]

McBride CS, Baier F, Omondi AB, Spitzer SA, Lutomiah J, Sang R, Ignell R, Vosshall LB. 2014. Evolution of mosquito preference for humans linked to an odorant receptor. Nature 515(7526):222-227. [Full text]

Stoeffler M, Maier TS, Tolasch T, Steidle JL. 2007. Foreign-language skills in rove-beetles? Evidence for chemical mimicry of ant alarm pheromones in myrmecophilous Pella beetles (Coleoptera: Staphylinidae). Journal of Chemical Ecology 33(7):1382-1392.

Stoeffler M, Boettinger L, Tolasch T, Steidle JLM. 2013. The tergal gland secretion of the two rare myrmecophilous species Zyras collaris and Z. haworthi (Coleoptera: Staphylinidae) and the effect on Lasius fuliginosus. Psyche: A Journal of Entomology. [Full text]

http://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI:16310