New Phytol. http://doi.org/5ms (2015)

The diversity in plant nuclear genome sizes spans three orders of magnitude. Recently, extremely compact genomes (60–80 Mb) have been sequenced, probably close to the theoretical limit to code all the genes needed for an average plant to function. Laura Kelly and colleagues sought the other extreme, and partially sequenced the 45-Gb genome of two diploid species of the lily relative Fritillaria, in order to understand what drives the expansion in these genomic giants.

The genomes of both species have expanded tremendously since they diverged. The majority of this expansion comes from a heterogeneous mix of low-abundance sequences, and not from a few highly repetitive elements, like in barley or cotton. The mechanism behind this appears to be the amplification of diverse repeats combined with a slower rate of deletion, resulting in the accumulation of ancient, highly diverged decayed DNA, originally derived from repetitive sequences.

This mechanism of DNA accumulation may be shared by other large-genome organisms, such as salamanders and lungfish. In the future it will be interesting to understand the evolutionary costs and benefits of maintaining such large and complex genomes, and whether they are on a trajectory towards extinction.