Credit: R. Carayol
CIRAD and its partners had to use cunning to establish the first sugarcane reference sequence. The plant's genome is so complex that conventional sequencing techniques had proved useless. This meant that sugarcane was the last major cultivated plant to have its genome sequenced.
A novel sequencing method
The team took up the gauntlet using a nifty approach based on a discovery made at CIRAD some 20 years previously: the genome structures of sugarcane and sorghum are very similar. The term used for this is "colinearity," which means that there is a degree of parallelism, with numerous genes occurring in the same order. Olivier Garsmeur, a CIRAD researcher and lead author of the study, was thus able to use the sorghum genome as a template to assemble and select the sugarcane chromosome fragments to sequence. "Thanks to this novel method, the reference sequence obtained for a cultivar from Réunion, R570, is very good quality," says Angélique D'Hont, a CIRAD geneticist who coordinated the study.
A key stage in investigating the genome
That reference sequence is a vital step toward fully sequencing the sugarcane genome and analyzing the variations between the sugarcane varieties more effectively. Angélique D'Hont had the same experience with the banana genome in 2012. As she says, "having a reference sequence for a species radically changes all the genomic and more broadly genetic approaches for that species."
A new era for sugarcane breeding
As with all other cultivated plants before it, sugarcane breeding will now be able to enter the age of molecular biology. Until now, for want of a reference sequence, sugarcane cultivar breeding programs were restricted to hybridization, followed by conventional, very cumbersome field assessments. Molecular screening techniques can now be developed to supplement field trials. This is a major breakthrough, since almost 80 percent of the world's sugar comes from sugarcane. Moreover, the plant has also recently become a frontrunner in the race to produce biomass. This new genetic knowledge will serve to create new varieties for a wider range of uses.
Sugarcane, a complex genome
The sugarcane genome is complex for several reasons:
- High polyploidy (large number of copies of each chromosome category)
- Aneuploidy (variable number of copies depending on the chromosome category)
- Bispecific origin of the chromosomes
- Structural differences and interspecific chromosome recombinants.