Geographic variation in the level of melanism in common frogs (Rana temporaria) has been repeatedly observed. Vences et al. (2002) described a strong altitudinal cline (increasing melanism with altitude) and we observed a similar cline along a latitudinal gradient. A widely accepted intiutive explanation for these clines is thermal adaptation, as it has been shown for different ectotherm species that more melanistic individuals can heat up faster, which supposed to be beneficial in cold environments (e.g. Forsman 1995; Vences et al. 2002).
As a bold attempt to reveal the genetic background of melanism in common frogs, we (= Cim) went to the lab, and sequenced the coding region of the melanocortin-1 receptor gene (MC1R, it has been shown to regulate melanism in numerous mammals and birds, and even in few reptiles) of a bunch of frogs from two altitudinally divergent populations differing in dorsal melanism (see the pics).
A 'light' frog from Tvedöra
A 'dark' frog from Ammarnäs
Unfortunately, we only found five synonymous substitutions and our analyses did not show any sign of recombination effects with genetic regions linked to MC1R either. Interestingly, a pilot phylogenetic analysis revealed that amphibian MC1R forms a separate clade against all remaining vertebrates (fishes, reptiles, birds and mammals).
So the question remained open: what causes the striking differences in dorsal melanism in common frogs?
This journey into the field of serious genetics also gave a new experience to me; I had to learn how quickly can the coolness of certain methods disappear in this field – we were ca. half year late with this study to be able to publish it in some leading genetic or evolutionary journal. Luckily, AnZF has a tradition of publishing papers on ectotherm melanism, and accepted this paper too.
Herczeg G, Matsuba C, Merilä J. 2010. Sequence variation in the melanocortin-1 receptor gene (MC1R) does not explain variation in the degree of melanism in a widespread amphibian. Annales Zoologici Fennici in press.
Vences M, Galan P, Vieites DR, Puente M, Oetter K, Wanke S. 2002. Field body temperatures and heating rates in a montane frog population: the importance of black dorsal pattern for thermoregulation. Annales Zoologici Fennici 39:209-220.
Forsman A. 1995. Heating rates and body temperature variation in melanistic and zigzag Vipera berus: does colour make a difference? Annales Zoologici Fennici 32:365-374.
4 Comments
Couldn’t the variation be in the regulation, either cis (next to the gene) or trans (somewhere else)? IIRC, one of the loci involved in melanism in mice is a cis-acting sequence, so it seems sensible that somewhere there are trans regulators that will show variation in natural populations.
Yeah, it can be either what you suggest, or any of the other possible candidate genes like ‘agouti’, or regulators of other genes, etc. We cannot even exclude the possibility that melanism is environmentally determined. Some more detailed analyses of this question will come soon.
This project was a sort of gambling. MC1R was found to act as colour regulator in many species, so we gave it a try in amphibians at the first time. However, it wasn’t as bold as it might sound now, even negative results was highly appreciated in the MC1R business at that time.
For another classic negative result for MC1R this is a terribly good read
MC1R variants correlate with thinhorn sheep colour cline but not individual colour.
Canadian Journal of Zoology 86, 147-150. doi: 10.1139/Z07-130
Check it out – it’s in print now:
http://www.sekj.org/PDF/anz47-free/anz47-037.pdf