Role of starch food digestion in dog domestication

Published in The Hindu on January 24, 2013

The adaptation which played a crucial role in transforming wolves into domestic dogs has been finally found. Genetic changes that facilitated the digestion of a starch-rich diet and not the morphological or behavioural helped this transformation.

According to a paper published in Nature today (Jan 24), selection at just three genes with key roles in starch digestion was sufficient to bring about this adaptation.

It is not clear why and how the domestication was achieved. Even the identification of genetic changes that helped dogs digest starch better cannot explain the “driving force” behind this process. What is clear is that permanent settlement of humans and development of agriculture “catalysed” the process of domestication.

A change in the ecological niche was responsible for carnivorous wolves to turn into starch-eating dogs. The authors of the paper imagine that “scavenging in waste dumps” near human settlements would have started this process.

With increased dependence on agriculture produce for food requirements, both humans and dogs had to undergo adaptive responses to be able to digest starch-rich diet. According to Nature , the breakdown of starch takes place through three important stages, and domesticated dogs show all these genetic changes compared with wolves.

The stages are: 1) conversion of starch into maltose and other oligosaccharides by the action of a particular enzyme (alpha-amylase) found in the intestine. In the case of humans, it protein is found in the saliva and that is the reason why digestion of starch starts in the mouth itself. But in the case of dogs, the protein is expressed only in the intestine 2) production of glucose from oligosaccharides, sucrose and isomaltase, and finally 3) transportation of glucose across the biological barrier called cell membrane that separates the inside of the cell from the outside.

The scientists from the Science for Life Laboratory, Uppsala University and MIT, among other institutions, studied the whole-genome of 12 wolves and 60 dogs from 14 diverse breeds for identifying the genetic differences. Thirty-six candidate domesticated regions containing 122 genes were identified for scrutiny.

They studied changes in all the three stages of starch digestion in both dogs and wolves before arriving at a conclusion. In the case of amylase enzyme, they found the copy number was much higher in the case of dogs. While wolves had just two copy numbers, domesticated dogs had an average 7.4-fold increase in copy number.

Increase in copy number refers to a particular gene or chromosomal region (containing many genes) getting duplicated. The more the copy number, the more will be the expression level and, in turn, greater will be the activity caused by that gene(s). When a gene is expressed, it refers to the production of an appropriate protein. So the more the expression level, the more will be the production of the protein. The expression level will be more when the copy number is more. When the expression level is more, the activity of the protein gets enhanced.

In the case of amylase, the expression level was 28-fold higher in dogs than wolves. And the activity was 4.7-fold higher in dogs.

In the second stage which involves the production of glucose, the scientists found a nearly two-fold increase in glucose production in dogs when compared with wolves.

Once the starch is digested to glucose, it has to be transported through the plasma membrane where the glucose can be utilised to produce energy. “To benefit from an increased capacity to digest starch, dogs would therefore be expected to show a parallel increase in glucose uptake,” they write. They did find evidence in favour of such an enhanced uptake of glucose produced.