Hundreds of genetic mutations accumulated over thousands of years have transformed the red jungle fowl of South Asia into the domesticated chickens that are a fixture on farms -- and dining tables -- worldwide, according to a scientific analysis of poultry DNA published this week in the journal Nature.

Swedish and American scientists identified about 7.5 million genetic variations between domesticated chickens and the jungle fowl, their primary wild ancestor. Then the scientists zeroed in on a few dozen differences that seemed particularly important based on their frequent prevalence in eight distinct populations of birds raised for meat or eggs.

One of the key genetic changes was in the DNA that carries the code for the thyroid-stimulating hormone receptor.

Among other things, this receptor helps regulate reproduction in birds and mammals based on changes in the length of day. It normally restricts reproduction to certain times of the year. But the researchers hypothesize that the mutation in chickens -- which changed a single amino acid in the structure of the receptor -- may have enabled them to breed year-round, said Leif Andersson, a geneticist at Uppsala University in Sweden and the study’s senior author.

Andersson and his colleagues also found genetic changes that were unique to broiler chickens, which are raised for their meat instead of their egg-laying abilities.

One of them involves a gene called TBC1D1. In humans, a version of this gene appears to increase the risk of becoming obese; in mice, a mutation that renders the gene inactive causes animals to be lean.

In chickens, the variant might help birds grow meatier breasts and legs.

The broiler chickens also shared a deletion in another gene that caused it to lose its normal function. The gene, called SH3RF2, is known to affect body weight. The researchers conducted breeding experiments and discovered that 10-week-old birds that inherited the deletion from both parents were 20% bigger than birds that didn’t inherit the deletion from either parent.

karen.kaplan@latimes.com