Oilseed Brassicas - Bioenergy Crop Plants - page 466

in Brassica species and Arabidopsis (for resources see http:// Brassica .bbsrc.ac.uk; http://www.
tigr.org/tdb/e2k1/bog1/ ) at the genome and microstructure levels, including the bacterial artificial
chromosome (BAC) based physical mapping, a comprehensive integration of maps across species
and genera is still in its infancy. There is a need to expand the marker analysis at the microstructure
level (Suwabe et al. 2008).
The power of the high density genetic maps lies in their use to detect, tag or even clone the genes or
quantitative trait loci (QTL) for economically important traits by their association with easily scored
markers. There are several examples in oilseed Brassica (Snowdon and Friedt 2004) where genetic
maps/markers have been used to discover genes/QTL and to clone genes for disease resistance,
oil content/quality, abiotic stresses, male sterility and morphological traits. QTL associated with
oil content in oilseed rape have been identified using different populations and different mapping
methods (Burns et al. 2003; Zhao et al. 2005; Delourme et al. 2006; Mahmood et al. 2006). The
number of QTL involved in oil content has ranged from 1 to 18 in these reports. Moreover, these
studies revealed that a single QTL could explain from 1.2% to 15.7% of the phenotypic variance,
and collectively the detected QTL could explain up to 51% of the total phenotypic variance for oil
content (Ecke et al. 1995), or explain up to 80% of oil content variation based on additive effects of
QTL and additive x additive epistasis (Zhao et al. 2005).
QTL for FA composition have also been identified on genetic maps of the B. napus genome
and reported in several articles (Ecke et al. 1995; Burns et al. 2003; Zhao et al. 2005; Zhang
et al. 2007). Although several loci (2-8 QTL) for almost all important FA in Brassica oil (C16:0,
C18:0, C18:1, C18:2, C18:3, C20:1, C22:1) were reported in these studies, the results vary widely,
probably due to the different genetic materials and marker systems used. This is reflected in
the broad range of phenotypic variability for FA which could be ascribed to their linkage with
molecular markers. For example, 8% of linolenic acid (C18:3) was significantly associated with
random amplified polymorphic DNA (RAPD) markers in B. napus as compared to 73.5% of the
linolenic acid in B. rapa (Tanhuanpaa and Schulman 2002). As with other crop species, B. napus
has benefited from advances in plant genomics, plant biotechnology and genetic engineering in
the past 20 years.
The technologies in these disciplines have been efficiently used in Brassica due to their relative
ease of tissue culture and transformation as compared to other dicots. Microspore-derived doubled
haploids are routinely produced in B. napus (Zhao et al. 1996; Murphy and Scarth 1998) to produce
homozygous lines for quick transfer of traits across the genotypes or species. Somatic hybridization
or protoplast fusion has been successfully used to conduct wide crosses for transfer of disease
resistance (Hu et al. 2002), to produce male sterile lines (Liu et al. 1999) and to produce asymmetric
hybrids (Yamigashi et al. 2002). Creation of useful genetic variability for breeding in Brassica
has been achieved by induced mutation (Kott et al. 1996) and somaclonal variation (Hoffmann
et al. 1982). However, most progress for trait improvement/transfer in Brassica has been achieved
by transformation technologies. B. napus is predominantly used for genetic transformation and
herbicide resistance (HR) is the most prominent trait for improvement. Herbicide (glufosinate,
glyphosphate, etc.) resistance genes have been successfully transferred into commercial canola
varieties through transformation.
Transformation of Brassica for modification of FA composition is another area where interesting
progress has been made. The targeted genes mostly encode for enzymes (ACCase, KAS, acyl-ACP
thioesterases, destaurases, elongases, acyltransferases) involved in FA biosynthesis pathways, and
are introduced into Brassica not only from related and unrelated plant species but also from yeast,
bacteria and mammals (Scarth and Tang 2006). Some notable achievements include the elevation
of oleic acid (C18:1) levels up to 89% in B. napus and up to 73% in B. juncea for plants transformed
with desaturase sense or antisense genes (Sivaraman et al. 2004); enhancement in the ratio of C18:2/
C18:3 and C18:1/C22:1 in B. juncea transgenic lines engineered with a novel thioesterase from
Diploknema butyracea (Sinha et al. 2007); production of high (<40%) gamma-linolenic acid (GLA)
canola by the introduction of delta-12-destaurase genes from the fungus Mortierella alpine (Liu et al.