You'd think they'd look slimmer….

small_sheep.jpgDark sheep, that is. A study in the new issue of Science (subscription required) reports that some dark sheep on a Scottish island are yielding a phenotype difference that is not what you might expect from the selection on the genotype. The darker sheep are larger in size, which was expected to correlate directly with increased fitness based on some previous studies. But it turns out to be more complicated than that. Dark sheep are decreasing in numbers.

The data in this report suggest that there are unexpected factors in play–TYRP1 (tyrosinase-related protein 1) is the gene they focused on, which is known to vary at a single allele in the population as GG and GT for dark coats and TT for light coats (Gratten et al, 2007). They find that there 2 are quantitative trait loci (QTLs) associated with their observations. (QTLs are essentially statistical suggestions that a trait appears to be associated with a genomic region.)

These QTLs appear to have antagonistic effects because the TYRP1 G allele (dominant for dark coat color) is associated with large body size (and hence with increased fitness) but also with decreased lifetime pt(i).

Mostly I started reading this paper because I’m a knitter, and was curious about coat colors. But the story got compelling to me because of the unexpected observation on the fitness. It was one of those reminders that the expected answer based on some known genotypes and observed phenotypes may not be the whole story. A recessive QTL feature has to be considered.

I was also thinking about QTLs lately because we were looking at the Animal QTL Database for information about species that are not as well covered in some of the tools designed more for model organisms. This database has pig, cattle, chicken and sheep QTL data that may be of interest to those interested in livestock species. It is often hard to know exactly which species data can be found in which databases. And farmed species are often given short-shrift despite their importance to our food and our economies. Another place to look for these species is the ArkDB, which has cat, chicken, cow, deer, duck, horse, pig, quail, salmon, sea bass, sheep and turkey data. I have to say this is the first database I have seen that uses Stimpy as a model cat image….

Mouse Genome Informatics has QTL data (search markers for type = QTL) , RGD has some, and Gramene has plant QTLs. On the MGI mailing list people often refer to Manley’s Map Manager software for this analysis as well–but the Manager team refers people to R/qtl and J/qtl now.

By the way–that painting is one that I own, called “I Believed Ewe“. I love it. You can’t really see it here, but she has the most funny skeptical look on her face.

Gratten, J., Wilson, A.J., McRae, A.F., Beraldi, D., Visscher, P.M., Pemberton, J.M., Slate, J. (2008). A Localized Negative Genetic Correlation Constrains Microevolution of Coat Color in Wild Sheep. Science, 319(5861), 318-320. DOI: 10.1126/science.1151182

EDIT: I was just looking for something else and found a web server that deals with QTLs in outbred strains: Happy,