Grass
genome evolution – a story with GC bits
I have
long been fascinated by the fact that many eukaryotic organisms have very much
more DNA than they need for coding, and that within some taxa
there is huge variation in the genomic DNA content (the ‘C-value’). Amongst
plants there is a thousand-fold range in C-value. Even within genera there can
be striking differences, as exemplified by Vicia with
its 7-fold range in C-value, the variation being accounted for by differences
in the amount of non-coding DNA. Thus, acquisition of more non-coding DNA, much
of it represented by different types of repetitive sequences, is a major part
of genome evolution. However, changes in noncoding
DNA can also go in the opposite direction, as shown by some very neat work by
the Czech–Italian team, Šmarda et al., Brno, Florence
and Parvia (pp. 421–433). They have
measured C-values and GC (guanidine cytosine) contents within the genus Festuca, in a second genera from which Festuca diverged, and in a third genera
more recently diverged from the Festuca lineage.
In general, with the exception of diploid Vulpia species,
there was a correlation between genome size and GC content. Further, when these
two characters were used to assemble a phylogenetic
tree, there was a very close fit to a tree assembled from more specific
sequence data. This enabled the authors to conclude that the initial divergence
of the Festuca lineage
involved an increase in genome size and GC content, characters still seen in
the basal fescues. Subsequent and still ongoing evolution of both broad- and
fine-leaved fescues and divergence of younger taxa
from the Festuca lineage
has involved reductions in genome size and in GC content. The early
evolutionary increases and subsequent deceases in genome size are most probably
indicators of gains and losses of GCrich mobile DNA
elements known as retro-transposons. Thus, the use of
general quantitative features of plant genomes has provided a good picture of
the dynamic nature of genome evolution.
