Friday, 26 June 2015

Selfish mitochondrial DNA proliferates and diversifies in small, but not large, experimental populations of Caenorhabditis briggsae

Wendy S. Phillips, Anna L. Coleman-Hulbert, Emily S. Weiss, Dana K. Howe, Sita Ping, Riana I. Wernick, Suzanne Estes and Dee R. Denver

Mitochondrial DNA (mtDNA) with deletion mutations can occur during aging in humans. These mutants are energetically compromised, and can accumulate at damaging concentrations in tissues. It is widely believed that these molecules have a replicative advantage, although the mechanism for this remains controversial.

Here, the authors study the nematode worm C. briggsae, and the dynamics of mtDNA deletions with both age and colony size, to probe the effect of competition in large populations of worms. For N = 1, 10, 100 and 1000 colony sizes, the colonies were propagated from generation to generation by transferring N worms to a new plate every 48 hours. The abundance of mtDNA deletions were then measured every 5 generations. This was repeated in nematode strains with low (0-5%), medium (5-20%) and high (40-50%) initial mutant loads.

The authors found that small populations (N = 1, 10) showed an accumulation of mtDNA mutants with time. However, for large populations (N = 100, 1000), deleterious mutations tended to die away with the number of generations. The interpretation is, for large populations, there exists a strong selective pressure against deleterious mutants (and therefore energetic inefficiency),  due to competition with other worms. This is strong enough to overcome any proliferative advantage of the deletion mutations.