by Hennie JJ van Vuuren, Wine Research Centre, University of British Columbia
Transposable elements in corn (maize) were first discovered during the 1940s by Dr. Barbara McClintock. The presence of these transposable elements, also called jumping genes, leads to mosaic color patterns of maize seed. Scientists were initially skeptical and even hostile of this discovery since the genomes of living cells were thought to be stable and, as a result, McClintock stopped publishing papers on this topic for a while. However, she continued to make major discoveries in this field, and in 1983 Barbara McClintock received the Nobel Prize for Physiology/Medicine. It was fascinating to meet this “grand dame” of science when I visited Cold Spring Harbor Laboratory in the eighties.
What are jumping genes? These transposable genetic elements are DNA sequences that move from one location on the genome to another. Over a number of years following this discovery, it became clear that jumping genes are present in large numbers in almost all living cells. For example, jumping genes make up approximately 50% of the human genome, 76% of the rice genome and up to 90% of the maize genome.
What happens in living cells when these jumping genes integrate into different loci in the genomes of cells? Much of what a jumping gene does depends on where it lands in the DNA. Jumping genes are mutagens that can damage the genome of their host cell in different ways; when inserted into a gene or the promoter (genetic switch) it can disrupt the sequence of the specific gene and mutate the gene or disable transcription (reading) of the gene. Once a transposon leaves the gene, the resulting gap might not be repaired correctly, leading to mutations in the gene. In mammalian cells transposons create mutations that are implicated in cancer.
Vitis vinifera has been widely cultivated around the world for thousands of years, and over this time period grapevines have been selected for traits that are important for its cultivation and use. Until recently, little was known about jumping genes in grapevines. However, after sequencing the first Vitis vinifera genome in 2007, it has been clearly shown that many transposons are present in the Pinot noir genome.
How widely are transposons distributed in Vitis vinifera? We are currently sequencing the genomes of 16 Chardonnay clones, each of which has a distinctive property. This project is being done in collaboration with the Australian Wine Research Institute. These 16 different clones of Chardonnay have distinct phenotypic properties such as early or late ripening; loose bunch morphology; small bunch sizes; flavour ripening at lower sugar concentration; seedlessness, to name a few. It would be interesting to see if jumping genes were involved in creating mutations in the Chardonnay genome that lead to these phenotypic differences.
The presence of these mobile genetic elements in grape vines, previously thought to have genetically stable genomes, has a big impact on so called “organic grape growing” claims. Jumping genes continuously integrate into any locus of the DNA of grape vines and destroy genes or create new genes in the vine.
Growers cannot control the movement of jumping genes, in fact, it is claimed that transposons have been involved in evolutionary development of Vitis vinifera
over the ages. The genomes of all grapevines planted around the world have probably been genetically altered through jumping genes, and this process will continue as long as vineyards exist.