With the advent of genetic
engineering, there has been a drastic change in the agricultural techniques. Gone
are those days when man used to depend solely on nature for rainfall, soil
fertility etc. Modern-man can toy with the genetic sequence and identify the
part of genome which is beneficial for him.
An attempt to improve the
crop varieties began almost simultaneously with the agricultural practices.
Around ten thousand years ago, man started to alter the genetic make up of
crops without his knowledge. The conventional plant breeding practices began
when early farmers selected the plant with desirable traits and saved its seeds
for the next generation. Later with the knowledge of genetics, plant breeders
were able to select the crops with desirable traits like faster growth, pest
and disease resistance, sweeter fruits, larger seeds etc. Two plants having two
different desirable characters were cross-pollinated artificially to obtain an
offspring containing a mixture of these characters (hybrid variety). As a net
result of these techniques new varieties of crops have developed which are
significantly different from their wild relatives. Conventional breeding
techniques largely depend on selection of superior varieties of crops and
incorporation of desirable traits into future generations.
Genetic engineering has
brought quantum change from conventional breeding practices and has introduced
combination of genes which do not exist naturally. Sequencing of genomes of
various plants has now become feasible with the faster progression in genetic
engineering. Predictive breeding is an advanced technique which comes up with a
promise of identifying the beneficial genes out of the entire genome. Genetic
markers like SNPs (single nucleotide polymorphism) or minisatellites are used
to construct linkage maps. The linkage maps help to identify the traits which
are inherited together. The knowledge of genome sequence along with the linkage
maps helps to associate the beneficial traits of the plant with specific parts
of genome sequence.
Complicated genomes of
polyploid crops pose a serious problem to predictive breeding. A cell of a
polyploidy organism has more than two sets of homologous chromosomes and each
set contains all the genes responsible for the biological traits of that
organism. Professor Ian Bancroft took this as a challenge and identified the
sequence of rapeseed (Brassica napus). This crop has been found to
originate from two related species that is Brassica rapa and Brassica
oleracea. Rapeseed (Brassica
napus) was declared as a third
leading source of vegetable oil in the world in the year 2000. Canola is the
name given to Canadian oil that is extracted from certain varieties of Rapeseed.
This oil is a rich source of essential fatty acids (omega-6 and omega-3 fatty
acids in the ratio of 2:1). Apart from these it is also an important source of
Biodiesel which can power motor vehicles. This oil is generally combined with
fossil-fuel diesel in ratios varying from 2% to 20% of biodiesel. This does not
damage the engine. Isn’t it an eco-friendly plant?
Sequencing the DNA of this
plant directly was difficult because of its complicated genome. Hence they
adopted a different strategy. Transcriptomes were sequenced from young juvenile
leaves. Transcriptome is a set of all RNA molecules, including mRNA, rRNA,
tRNA, and other non-coding RNA. It represents a small percentage of the genome.
The sequence of a transcriptome mirrors the sequence of the DNA from which it
was transcribed. This study gives an idea about all the genes which get
expressed into proteins in that tissue. Sequence of transcriptome will act like
a raw material for scientists struggling to locate the human friendly genes
within the entire genome of the polyploid plants. Based on this improved varieties
of seeds can be produced.
A large number of crops like
potato, wheat, oat, sugarcane, cotton etc are polyploid and few crops amongst
them are staple crops in certain parts of the world. Transcriptome sequencing
will help the scientists to identify the desirable genes even in the absence of
complete genome sequence. This new technique will make predictive breeding of
crops with complicated genomes feasible.
Reference:
Bancroft I, Morgan C et al.
Dissecting the genome of the polyploid crop oilseed rape by transcriptome
sequencing. Nat Biotechnol. 2011 Jul
31.
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