Big Dogs vs. Little Dogs.

Benjamin Lester of Cosmos Online reports:

Much of the amazing variation in size of different breeds of domestic
dogs stems from tiny differences in a single gene, say researchers.

When the team compared the dog DNA samples, they found that all the
small breeds shared the same mutations in a small part of the DNA of
one gene which regulates the production of insulin-like growth factor
(IGF1) - a protein key to promoting growth. . . .

"In the big dogs, the [level] of insulin like growth factor is
larger than in the small dogs," said Alan Wilton, an expert in dog
genetics at Sydney’s University of New South Wales, who was not
involved in the study.

The result, said the researchers, is that dogs with this mutation
stay small. "All dogs under nine kilograms have this – all of them," said Lark. "That’s extraordinary."

Image: Deanne Fitzmaurice

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The insulin-like growth factor-1 is a growth hormone whose fetal form is encoded by a gene with a rather interesting regulation. It is paired with a "receptor" protein that binds to the hormone on fetal and embryonic cells. Normally, a growth hormone-receptor pair works by the hormone activating the receptor, triggering a series of actions resulting in the initiation of cell division and growth.

During embryonic development, IGF-1 has two receptors. One will result in cell growth, while the other seems to bind to IGF-1 without effect. This false receptor is encoded by a gene that is indistinguishable from a sugar binding protein. When the IGF-1 gene is inherited from the father, it is active and produces growth hormone. When the IGF-1 gene is inherited from the mother, it is inactivated. Conversely, when the false receptor is inherited from the father, it is inactive, while the maternally-inherited gene is active.


It can be considered that larger offspring serve the interests of the father, who wants his offspring to be stronger and better adapted to survive. On the other hand, the maternal agenda would be to have more moderately sized offspring that will not be so risky to carry and birth. So, paternally activated growth hormone is countered by maternally activated false growth hormone receptors.

This kind of gene regulation is called "genomic imprinting". It is a strategy used on a number of different genes for different apparent purposes. This particular example is sometimes seen as a 'battle of the sexes', but is illustrative of the forces that can shape the evolution of gene expression strategies.

See this article on the evolution of genomic imprinting at PubMed. PubMed is a repository of research articles at the National Library of Medicine, and access to many of these articles is free (your tax dollars at work).
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