Time was when you tested how hot a chilli pepper was by tasting a teeny bit with your tongue, at least if you were brave. The hotter it tasted, the more capsaicin it contained, and the hotter it was. Then came Wilbur Scoville and his eponymous scale.1 Now, all you need is a well-equipped molecular biology laboratory.
Maria Arnedo-Andrés and her crew have identified a single nucleotide polymorphism, or SNP, associated with pungency in chillies.2 A snip is a single letter difference between the DNA of two different organisms. Sometimes a SNP makes a visible and important difference to the organism. The genetic difference that causes sickle cell anemia is one such SNP. More often, the SNP is just a marker. It is associated with some other difference, but does not actually cause it. Breeders like markers because they allow them to quickly see whether some desired gene has been inherited after a breeding experiment. If the marker is there, chances are the nearby gene is there too. There are gazillions of known SNPs out there, mapped to squillions of differences. But, until now, no SNP that could tell you whether a chilli pepper was hot.
There have been markers before, but they were either unreliable, failing to distinguish hot from sweet. Or they were physically a long way away from the actual genes for hotness, meaning that they were not very useful to breeders.
The researchers grew a wide range of peppers, different species and different varieties. Two people tasted five ripe fruits from each type of plant. If all five were not pungent, the plant was considered non-pungent. But if just one fruit (or more) tasted hot, the plant was considered pungent. Then comes the magic, actually detecting the sequence differences among the different samples.
They found one; in all pungent varieties, and only pungent varieties, there is a letter G at position 253 of an identifiable bit of DNA. In all non-pungent varieties, that space is taken by a T.
This result is just a beginning. Breeders will use the SNP to determine very early on, long before ripe fruits have been produced, whether those fruits will be hot or not. Researchers still don’t fully understand how plants make capsaicin. The SNP will help them home in on the genes responsible. And this blog will have taken the opportunity to use that nifty little icon up there on the right to indicate that we are serious and responsible members of the scientific blogosphere, dealing with peer-reviewed research in a serious and responsible manner.
- An extract of the pepper is diluted with sugar water until a panel of testers can no longer detect any heat. Thus a mild little pepperoncino scores around 500 SHU (Scoville Heat Units), meaning that the extract has to be diluted 500 times to lose all heat, while a decent African birdseye starts at around 100,000 SHU. And Luigi’s little hottie Naga Jolokia is ten times hotter still, at 1,040,000 SHU. [↩]
- Ana Garcés-Claver, Shanna Moore Fellman, Ramiro Gil-Ortega, Molly Jahn and María S. Arnedo-Andrés (2007) Identification, validation and survey of a single nucleotide polymorphism (SNP) associated with pungency in Capsicum spp. Theoretical and Applied Genetics, 115: 907-916. DOI 10.1007/s00122-007-0617-y. [↩]