Environmental Engineering Reference
In-Depth Information
(erucic acid, oleic acid, linolenic acid, palmitic acid, etc.) and the inheritance of genes involved in
their synthesis and accumulation. The major biosynthetic pathways (Murphy 1999) and the genes/
enzymes involved in the FA synthesis (Broun et al. 1998), which have been studied or manipulated
for FA accumulation in rapeseed oil include plastid specific genes/proteins responsible for increasing
the carbon-chain length using acetyl-coA as base; a group of genes/enzymes (desaturases) located
in endoplasmic reticulum responsible for the production of double bonds; thioesterases enzymes in
the cytosol for carbon chain termination and genes controlling the esterification of glycerol and FA
molecules.
Erucic acid (C22:1) is a long-chain FA and requires C18:1-CoA as a substrate for its synthesis,
which is catalyzed by the beta ketoacyl-CoA synthase (KCS) enzyme. It was reported (Barret et al.
1998; Han et al. 1998) that alleles at two genetic loci encode for the KCS, and two KCS genes also
cosegregate with two major loci controlling erucic acid levels found on independent linkage groups
of
B. napus
(Gupta et al. 2004). These results substantiate the genetic analyses (Chen and Beversdorf
1990; Luhs et al. 1999) of erucic acid content in three amphidiploid
Brassica
oilseed species, which
indicated that C22:1 level is controlled by two genes with additive effects. The mapping studies
searching for QTL for erucic acid also confirmed the presence of two major loci controlling the
erucic acid levels in
Brassica
species (Mahmood et al. 2003; Gupta et al. 2004). However, two
additional genetic loci were identified, which influence the erucic acid level in
B. carinata
(del Rio
et al. 2003) and offer a novel source (other than the KCS gene) for manipulating the erucic acid
content in
Brassica
species (Scarth and Tang 2006).
Because of their importance in determining the oil quality of
Brassica
seed oil, the genetic
control of monounsaturated fatty acid (MUFA, 18:1) and polyunsaturated fatty acids (PUFA, 18:2
and 18:3) has also been a subject of interest. Gene mapping and gene cloning studies in
B. napus,
B, rapa
and
B. juncea
(Sharma et al. 2002; Tanhuanpaa and Schulman 2002; Laga et al. 2004)
have shown that allelic variation at two loci controlling the expression of the FA desaturase (fad2)
gene are associated with varying levels (up to 32 %) of C18:1. Two genetic loci with additive effects
control the level of C18:3 in
B. napus
(Scarth 1995), and were mapped close to two FA desaturase
(fad3) genes (Tanhuanpaa and Schulman 2002). Other studies have shown the presence of more
than one genetic loci (Mahmood et al. 2003) or minor genes (Pleines and Friedt 1989) influencing
the level of C18:3 in
B. rapa
and
B. juncea
.
The exploitation of DNA polymorphism to develop linkage maps for detecting or tagging the
genetic loci influencing economically important traits is routinely done in agricultural crops.
Brassica
species offer a high degree of DNA polymorphism, making them well suited to this
modern approach for the development of molecular linkage maps. Several linkage maps have been
constructed in
Brassica
oilseed species based on a variety of molecular markers (RFLP, AFLP,
RAPD, SRAP, SNP, SSR, STS, SINE, ACGMs, microsatellites) of mapping populations such
as doubled haploid, F
2
, substitution lines, recombinant inbred lines, and backcross inbred lines
(for review see Quiros 2003; Quiros and Paterson 2004; Snowdon and Friedt 2004; Mikolajczyk
2007). At least 15 linkage maps of
Brassica
species (Lakshmikumaran and Srivastava 2003) with
a conservative estimate of 1000 loci (Quiros and Paterson 2004) have been described and most of
them were constructed using
B. napus
populations.
Researchers have also attempted to use markers common to several
Brassica
species (Sun
et al. 2007) and markers targeting homologs of defined genes of
A. thaliana
(Qiu et al. 2006) to
align oilseed rape maps and maps between
Brassica napus
and
A. thaliana.
Originally, a common
nomenclature to represent the linkage groups across species included
B. napus
linkage groups
N1-N10 (A genome) corresponding to
B. rapa
linkage groups R1-R10, and linkage groups N-11-N19
(C genome) corresponded to
B. oleracea
O1-O9 (Kim et al. 2006). However, a new nomenclature
was recently adapted based on the recommendation of The Steering Committee of the Multinational
Brassica
Genome Project (Suwabe et al. 2006). Based on this, N1-N10 and R1-R10 were replaced by
A1-A10, and C1-C9 designation was used in place for O1-O9 and N11-N19 whereas B1-B8 was used
to represent
B. nigra
(B genome). In spite of the extensive straight or comparative genetic mapping
