Small dogs vs big dogs, a one gene characteristic?

[spuriousmonkey]

http://www.nature.com/news/2006/061009/full/061009-12.html

A single gene may explain the vast size difference between that tiny terrier yapping in the park and the massive mastiff ignoring the din.

They then looked for differences in DNA between the large and small water dogs. This is a relatively easy job: a consortium of scientists including Sutter published the DNA sequence of the dog genome last December, and have mapped out the places where there is a lot of variation between individuals in a given breed. There are fewer of these places of variation in purebred dogs than there are in humans.

The team found that one of the few differences in these Portuguese water dogs occurred in a gene called 'insulin-like growth factor 1', or Igf-1.

On a sidenote it is nice to see that genome sequencing does facilitate research.

This is one of many genes already known to influence the size of mice: when Igf-1 is knocked out, the animals grow up to be mini-mice. So the team wondered whether this gene was responsible for dog body size.

To answer this question, scientists closely analysed the Igf-1 genes in 75 Portuguese water dogs and 350 other dogs of very large and very small breeds — from pomeranians and Yorkshire terriers up to great Danes and St Bernards. They also examined the gene in wild dogs, such as wolves and foxes, who are distantly related to domestic dogs.

They found that almost all of the 18 small breeds carried the identical variant of the gene as small Portuguese water dogs. But almost none of the 15 giant breeds carried this gene variant.

The gene seems to work by setting how much of the growth factor dogs make. In Portuguese water dogs, smaller animals make less of the growth factor than big ones.

I thought it was quite a nice story. Let me as a sidetrack refocus your attention on the following quote again.

There are fewer of these places of variation in purebred dogs than there are in humans.

Despite a tremendous amount of apparent differences between different dog breeds the genetic variation seems rather minimal.

Needless to say one could think of several reasons.
1. the differences between dog breeds are skin deep. There only was selection for appearances (and certain behavioural patterns).
2. The dog is a relative newcomer.
3. morphology is regulated by the same set of genes in all structures. One only needs to change the regulation of the regulators. A minimum amount of genetic change.

Well...i guess we just have to wait to see what fraggle will add.

 

[Fraggle Rocker]

And the correct answer is:

2. The dog is a relative newcomer.

The dog self-domesticated in only one place, in what is now China, around 15,000 years ago. They were spread around the world by traveling with humans before any other pack of dogs and pack of humans experimented with forming a two-species community.

All domestic dogs are the descendants of that one pack or group of packs. This does not allow for much genetic diversity.

The DNA of the average mongrel Third World street dog more closely resembles that of a wolf than two strikingly different breeds of domestic dog resemble each other.

And both are closer than a native Australian is to a Norwegian.

 

[valich]

Actually, dogs were first domesticated from ancestral wolves about 100,000 years ago, but the lineage of dogs today dates back to about 15,000 years – and most of the diversity of breeds today only about 300 years ago. All due to the very intensive selective breeding that led to paedomorphism (neoteny & progenisis).

77% of the canine genome is similar to humans and 10% is identical to it, especially in the same accelerated brain evolution.