Ginormous Genome

Researchers find organisms with huge genomes with high mutation rates, overturning a common expectation in evolutionary biology.

May 1, 2012
Sabrina Richards

MEGA MITOCHONDRIA: Researchers find that two flowering plants in the Silene genus (above) have comparatively enormous genomes in their mitochondria, an organelle with a high mutation rate. YALE UNIVERSITY

EDITOR'S CHOICE IN PLANT BIOLOGY

The paper

D.B. Sloan et al., “Rapid evolution of enormous, multichromosomal genomes in flowering plant mitochondria with exceptionally high mutation rates,” PLoS Biology, 10 (1) e:1001241, 2012.

The finding

Based on high mutation rates seen in small mitochondrial genomes, researchers hypothesized that elevated mutation rates would result in smaller streamlined genomes with less noncoding DNA. But Daniel Sloan at the University of Virginia found that mutation rates aren’t always linked to size. Sloan identified two enormous, rapidly mutating plant mitochondrial genomes, suggesting that other mechanisms may regulate genome size.

The paradigm

Mutations within coding regions are risky, but adding more noncoding DNA compounds the risk. Mutations in noncoding sequences can create false start-sequences and other nonsense code that can impede the normal function of the cell as much as mutations in proteins can.

The flowers

Sloan sequenced the mitochondrial genomes of two carnation species with unexpectedly high mutation rates, and discovered two behemoths at 6.7 and 11.3 Mb , more than ten times the size of the largest known plant mitochondrial genomes.

The complication

Sloan’s work doesn’t completely subvert the old paradigm, however, says Benoit Nabholz of Université Montpellier II in France. Though sequences between the genes expanded, the average intron lengths decreased, hinting at a more complex relationship between mutation rate and genome size.