Breeding of Sugarcane - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 38

represents only a subset of genetic information from existing genome of sugarcane.
The presence of markers with higher allele doses, as well as combinations of
markers with different doses, makes it imperative that additional segregation
patterns be considered.
The application of molecular markers for either trait or genotype selection in the
breeding of sugarcane has lagged behind other crops despite a substantial research
effort in the past decade on sugarcane molecular genetics. Most important traits in
sugarcane are explained by multiple quantitative trait loci, each only contributing a
small proportion of the overall phenotypic effect. The percentages of phenotypic
variation explained by QTLs were in general low, 4-26 %. The most studied traits
can be yield components (POL, tons of cane per hectare, fiber content) and disease
resistance (brown rust, leaf scald, Fiji leaf gall, Pachymetra root rot) [ 32 , 33 ]
There have been no reports of effective use of Marker-Assisted Selection (MAS)
in sugarcane; however candidate markers are described for durable rust resistance
gene Bru-1. Bru1 PCR diagnostic markers should be useful to identify cultivars
with potentially alternative sources of resistance to diversify the basis of brown rust
resistance in breeding programs [ 34 ]. However, the efficiency of this marker has not
been demonstrated on different germplasms.
In recent years, association mapping strategy has been used in sugarcane. This
methodology consists of evaluating marker-trait associations attributed to the
strength of linkage disequilibrium between markers and functional polymorphisms
across a set of diverse germplasm. The development of modern sugarcane cultivars
was based on a strong genetic bottleneck, followed by a small number of cycles of
intercrossing (small number of meiotic divisions) and vegetative propagation,
suggesting that linkage disequilibrium should be extensive. However, because
only low-density markers are available and statistical methods have not been
refined, association studies are at the initial stages [ 17 , 35 ].
Transgenes can be used to introduce genes from other species and have the
potential to incorporate new characteristics to elite genotypes. In the case of
sugarcane, there have been transgenes for tolerance for herbicides tolerance, pest
resistance ( cry genes), diseases resistance (mosaic and leaf scalding), resistance to
abiotic stresses (higher accumulation of proline and trehalose), higher accumulation
of saccharose, suppression of flowering, and other characteristics [ 36 ].
The main difficulty found in the research with transgenic cane is the gene
silencing, probably caused by the high complexity of the genome (polyploidy and
aneuploidy). To achieve transgenic events with a stable expression of the transgene,
some groups have been studying the influence of diverse promoters. Besides gene
silencing, another difficulty is the impossibility of backcrossing to a species. This
means that each genotype of transgenic cane for the same gene must be transformed
separately, which depends on the regeneration capacity and also makes the process
more expensive.
Institutions of several countries (Argentina, Australia, Brazil, Colombia, the
United States, South Africa, India, China, and Indonesia) already obtained trans-
genic cane in laboratory. Field tests with transgenic events also have been made in
several countries, but no commercial release was made. Because of the growing