By Sally Burn
2014 may be the Year of the Horse, but dogs have started the year as a scientist’s best friend, giving paws for thought in several recent papers. Freedman and colleagues barked up the right evolutionary trees to investigate canine evolutionary history, while Waller et al. report that puppy dog eyes give dogs a selective advantage when soliciting human care. Finally, a paper just published in Science sniffs out the genome secrets of an ancient transmissible dog cancer.
A Dog’s Tale Part I: The evolutionary history of dogs
Dogs and wolves share many traits but their exact evolutionary connection is unclear. A new paper out this month in PLoS Genetics attempts to address their phylogenetic relationship and reconstruct the early evolutionary history of man’s best friend. Adam Freedman and colleagues doggedly sequenced the genomes of three region-specific gray wolves, two basal dog breeds historically isolated from wolves, and a jackal outlier. Comparisons of these genomes (plus a boxer dog genome) revealed that modern wolves and dogs arose from a now-extinct common ancestor, contradicting the common notion that dogs simply descended from wolves that cozied up with humans. After the initial divergence, both the dog and wolf lineages went through severe population bottlenecks, resulting in increased disparity between their gene pools. The dog lineage was subsequently domesticated by hunter-gatherers, around 11-16 thousand years ago according to this new paper. Analysis was complicated by the fact that the genomes have not been in total isolation from one other, as extensive wolf-dog interbreeding has permitted further gene flow between the species. Such admixture and the extinction of the common ancestor have rendered the evolutionary history of dogs particularly hard to dissect, leading to vastly different conclusions from different research groups. Indeed, a study published last year concluded that population bottlenecks were not that significant during dog evolution. Another bone of contention has been the link between dietary adaptation and domestication. Grab yourself some kibble as we move on to that shaggy dog tale next…
A Dog’s Tale Part II: A dog’s dinner
The domestic dog is particularly fond of scraps from human tables. However, the human diet changed dramatically when we transitioned from hunter-gatherers to agriculturalists and so therefore did the digestive abilities of dogs. This was the conclusion of a study published in Nature in 2013, in which Erik Axelsson and colleagues discovered that dogs possess a number of gene variants associated with starch digestion. Compared to wolves, dogs have a seven-fold increase in copy number of AMY2B, a gene involved in the breakdown of starch. This was a necessary adaptation to share the starch-rich food of humans. The advent of agriculture was, they argued, a catalytic event in domestication as humans now had attractive scrapheaps, from which genetically-equipped wolves could steal tasty morsels. All of a sudden hanging with the humans was advantageous for survival and so the ancestor of modern dogs was born. In contrast, this month’s Freedman et al. study found that AMY2B copy number varies between dog breeds and is also high in some wolves, discrediting the notion of high AMY2B copy number being an explicitly dog trait. More specifically, AMY2B copy number is low in dog breeds not associated with agricultural societies, reaffirming their conclusion that domestication predated the onset of agriculture. The contrasting conclusions of the two papers demonstrate once again the difficulties in tracing canine evolution.
A Dog’s Tale Part III: Puppy dog eyes
Regardless of how they came to be able to digest our food, one thing we can be sure of is that dogs have a guaranteed mechanism for obtaining it: puppy dog eyes. Now researchers at the University of Portsmouth, UK, have found evidence that puppy dog eyes provide a selective advantage when soliciting human care. The team proposed that a key factor in dog domestication was human selection against aggression; they hypothesized that, in a process of co-evolution, dogs displaying pedomorphic (puppy-like) facial characteristics were preferentially selected by humans desiring increasingly tame canine companions. To test this hypothesis they used the speed of rehoming from shelters as a proxy for artificial selection. Humans stood in front of shelter pens and the facial expressions of the dog inmates were analyzed using a novel system called DogFACS (Dog Facial Action Coding System). As predicted, dogs who displayed puppy-like facial expressions were rehomed faster than those who did not. A key facial movement was the raising of the inner brow to make their eyes look bigger and more puppy-like. So next time you acquiesce and give doe-eyed Fido a superfluous treat, take solace in the fact that your weakness is just part of your DNA.
A Dog’s Tale Part IV: Transmissible dog cancer genome
Our final dog bulletin concerns the world’s oldest known cancer. Canine transmissible venereal tumor (CTVT) spreads when cancer cells pass between dogs during mating. Researchers at the Wellcome Trust Sanger Institute in Cambridge, UK, sequenced the cancer cells’ genome and published their findings last week in Science. They found that the cancer originated in a single dog around 11,000 years ago; the cancerous cells have been passed on ever since as a clonal lineage, long outliving the body from which they came and making CTVT the oldest known living cancer in the world. The cancerous cells still contain the genome of the dog in which the cancer arose, allowing the team to build up a genetic “identikit” of the first infected animal. The canine patient zero was a medium to large husky-like dog, with black or agouti fur. Mutation analysis pinpointed the origin to approximately 11,368 years ago. The cancer was initially contained within an isolated dog population but it became a worldwide problem around 500 years ago, possibly as a result of humans traveling the earth and taking four-legged companions with them. Some of these voyages may have been to sunny locales as the cancer’s genome bears hallmarks of exposure to ultraviolet light. The cancer cells have also undergone many other changes during their evolution, losing 646 genes and acquiring an estimated 1.9 million somatic substitution mutations – several hundred times the number found in most human cancers. Despite this accumulation of mutations the cancer cells have survived, illustrating just how robust mammalian somatic cell lines can be. Indeed, many of the mutations may have allowed the cancer to adapt to niche changes and thrive. While the cancer itself is rare, this study is of note as it chronicles the evolutionary history of a transmissible cancer. Further analysis of the cancer’s genome may therefore provide insights into the processes underlying cancer transmissibility.