Relaxation of selective constraint on dog mitochondrial DNA following domestication

Abstract

The domestication of dogs caused a dramatic change in their way of life compared with that of their ancestor, the gray wolf. We hypothesize that this new life style changed the selective forces that acted upon the species, which in turn had an effect on the dog's genome. We sequenced the complete mitochondrial DNA genome in 14 dogs, six wolves, and three coyotes. Here we show that dogs have accumulated nonsynonymous changes in mitochondrial genes at a faster rate than wolves, leading to elevated levels of variation in their proteins. This suggests that a major consequence of domestication in dogs was a general relaxation of selective constraint on their mitochondrial genome. If this change also affected other parts of the dog genome, it could have facilitated the generation of novel functional genetic diversity. This diversity could thus have contributed raw material upon which artificial selection has shaped modern breeds and may therefore be an important source of the extreme phenotypic variation present in modern-day dogs.

Figure 1.

Phylogenetic tree of wolf (W), dog (D), and coyote (C) mtDNA sequences. The tree was constructed using a Bayesian approach. The same topology was obtained with a neighbor-joining approach. Support is indicated at the nodes as percent bootstrap support for 1000 neighbor-joining replicates and Bayesian posterior probabilities. Four clades of dog sequences (I to IV) are indicated as in Vilà et al. (1997). Internal dog branches are marked in orange, and internal wolf branches are marked in light blue. The branch leading to wolf haplotype W1 was basal to the rest of the tree and it was also considered internal. Internal branches that could not be conclusively associated to dogs or to wolves are indicated in discontinuous green.

Figure 2.

Maximum-likelihood estimates of d_N/d_S. Values for wolves are indicated in blue, for dogs in yellow, and for the divergence between a random wolf and coyote sequence in red. Ninety-five percent confidence intervals were bootstrap-derived.