Utricularia Gibba - Genetic Efficiency

Genetic Efficiency

In 2013, the genome of U. gibba was sequenced. At only 82 megabases, the genome is exceptionally small for a multicellular plant. Despite its size, the genome accommodates 28,500 genes – more than plants with much larger genomes. The main difference between other plant genomes and that of U. gibba is a drastic reduction in non-coding DNA. Only 3% of the plant's DNA is not part of a gene or material that controls those genes, in contrast to human DNA which is 98.5% non-coding. Retrotransposons, which dominate the DNA of most flowering plants, make up just 2.5% of U. gibba's DNA. The discovery casts doubt on the idea that repetitive, non-coding DNA, popularly known as junk DNA, is necessary for life. "At least for a plant, junk DNA really is just junk – it's not required," declares study co-author Victor Albert. T. Ryan Gregory who studies the evolution of genome sizes said "The study further challenges simplistic accounts of genome biology that assume functions for most or all DNA sequences, without addressing the enormous variability in genome size among plants and animals."

Utricularia gibba and the tomato split from a common ancestor approximately 87 million years ago. Since that time, both plants have experienced episodes of whole genome duplication (WGD) in which the plants' DNA content doubled in size. Utricularia gibba experienced at least three cycles of increasing genome size. Since then, it has lost most unneeded DNA, unlike the tomato, and now has a genome only a tenth as long as the tomato's.

Compared to Arabidopsis, the introns of Utricularia gibba are somewhat fewer in number per gene, and conserved cis-acting elements of its promoters are compressed. Most critical genes have returned to single copy status. However, the mitochondrial and plastid genomes of U. gibba do not appear to be compressed relative to those of other angiosperms. The compression of its nuclear DNA is thought to have occurred via both numerous microdeletions and some large-scale recombinant deletions. It is hypothesized that a "sloppy" recombination process has caused unused material to be deleted over time. The presence of numerous GC-rich sequences throughout the nuclear genome of U. gibba is considered to have created a molecular mechanistic bias in favor of deletions, but this does not preclude the presence of a selection pressure to preserve such deletions. Selection pressures in favor of conserving energy or conserving phosphorous have been suggested to be operative in the reduction of the nuclear genome size of of U. gibba. Trap formation is induced in U. gibba by low phosphorus but not low nitrogen, indicating that phosphorus availability is more limiting in its environment. It had also been previously proposed that an increased mutation rate due to greater environmental mutagen exposure could have increased natural selection for loss of unneeded DNA, but no evidence for this was found in the relative mutational diversities of U. gibba and Arabidopsis.