Chickens reveal new evolutionary surprise

Posted on Wednesday 28 October 2015

Led by Professor Greger Larson at Oxford University’s Research Laboratory for Archaeology, scientists studied individual chickens of a long-term pedigree and found that two mutations had occurred in the mitochondrial genomes of the birds in only 50 years.

It was previously thought the rate of change in the mitochondrial genome was never faster than about two percent per million years. The identification of these mutations shows that the rate of evolution in this pedigree is 15 times faster.

Determining genetic sequences along the pedigree, the team also discovered a single instance of mitochondrial DNA being passed down from a father. This is surprising, proving so-called ‘paternal leakage’ is not as rare as previously believed.

Studying a well-documented 50-year pedigree of a population of White Plymouth Rock chickens, developed by Professor Paul Siegel at Virginia Tech, researchers reconstructed how mitochondrial DNA passed from mothers to daughters within the population.

Selective mating within this breed of chickens started in 1957, now resulting in an over tenfold difference in the size of the chickens in the two groups when weighed at 56 days old.

Knowing that the base population had started from seven partially inbred lines, the team analysed DNA from the blood samples of 12 chickens of the same generation using the most distantly related maternal lines.

The results, published in Biology Letters, show there is now considerable evidence of a disparity between long-term and short-term estimates of mitochondrial changes. There have previously been few studies of short-term mitochondrial evolution, including both mutation rates and paternal leakage.

Dr Michelle Alexander, Lecturer in Bioarchaeology at the University of York and Lead Author, said: “The one thing everyone knew about mitochondria is that it is almost exclusively passed down the maternal line, but we identified chicks who inherited their mitochondria from their father, meaning so-called ‘paternal leakage’ can happen in avian populations. Both of these findings demonstrate the speed and dynamism of evolution when observed over short time periods.”

Professor Larson, Senior Author of the paper, said: “Our observations reveal that evolution is always moving quickly but we tend not to see it because we typically measure it over longer time periods. Our study shows that evolution can move much faster in the short-term than we had believed from fossil-based estimates.

“Previously, estimates put the rate of change in a mitochondrial genome at about two percent per million years.  At this pace, we should not have been able to spot a single mutation in just 50 years, but in fact we spotted two.”