Environmental Engineering Reference
In-Depth Information
105 days as compared to B. rapa varieties, which are earlier-maturing (approximately 88 days).
Therefore, B. napus varieties are better adapted to southern Manitoba and central Saskatchewan/
Alberta.
The primary and secondary gene pools of Brassica species are very diverse and offer allelic
variants for FA profiles as well as for tolerance to various biotic and abiotic stresses. This diversity
plays an important role in their adaptation to specialized regional requirements.
18.2.4 g EnEticS , g EnomicS , and B iochEmiStry
The genomic relationships among the cultivated Brassica species are well known (Downey and
Rimmer 2003). Three diploid species, B. rapa , B. oleracea , and B. nigra , are the primary species
with their genomes designated as A ( n = 10), B ( n = 8), and C ( n = 9), respectively. Early
cytogenetic studies (188) conducted on the interspecific hybrids of these diploid species established
that B. juncea ( n = 18; AB), B. napus ( n = 19; AC), and B. carinata ( n = 17; BC) are amphidiploids
resulting from natural crosses between pairs of corresponding species. Extensive studies (Parkash
and Hinta 1980; Coulthart and Denford 1982; Palmer 1988; Warwick and Black 1991; Truco and
Quiros 1994; Simonsen and Heneen 1995) using cytogenetics such as meiotic chromosome pairing,
isozyme patterns, restriction fragment length polymorphism (RFLP) and restriction patterns of
chloroplast/mitochondrial DNA have not only verified these genomic relationships among the
Brassica species but also have elucidated their genomic composition and evolution. These studies
reveal that B. juncea and B. carinata are the recipients of cytoplasm from B. rapa and B. nigra ,
respectively, whereas the origin of the B. napus cytoplasm is still not known. It is also apparent
from these studies that the evolution of three diploid species of Brassica involved duplication of
chromosomes followed by chromosomal aberrations such as deletion, translocation, and inversion.
Phylogenetic relationships as illuminated by molecular analysis of Brassica genomes also pointed
toward two evolutionary pathways—one leading to the origin of B. rapa and B. oleracea and the
other leading to the origin of B. nigra . Various gene mapping studies (Song et al. 1990, 1991; Truco
et al. 1996; Quiros 1999; Inaba and Nishio 2002; Ananga et al. 2008) using molecular markers have
thrown some light on the intragenomic and intergenomic homoeology of the chromosomes of the A,
B, and C genomes of Brassica species. It appears that all three genomes share homologous regions;
however this colinearity is confined to some chromosomal regions only and broken for most other
chromosomal segments resulting in complex relationships within and between chromosomes of the
three diploid species. Based on gene marker arrangements on the conserved chromosomal regions
it can be concluded that the C genome is the possible progenitor of the A genome and the genes have
undergone extensive reordering during the evolution of Brassica species (Quiros 1999).
Oil content of rapeseed is a complex quantitative trait involving several genes. These direct
a battery of physiological processes leading to the accumulation of oil in the seeds. This trait is
mainly controlled by genetic makeup of the species/variety but is also significantly affected by both
environmental conditions and interaction between the genotype and environment (McVetty and
Scarth 2002). Various studies based on classical genetic analyses have been conducted to partition
the variation in oil content into genetic and environmental components and determine the type of
gene action involved in the inheritance of traits. Some findings indicate that (Han 1990; Grami et
al. 1997) oil content had higher heritability (both broad and narrow sense) in summer rape than
in winter rape. Others (Chen and Beversdorf 1990; McVetty and Scarth 2002) indicated that both
additive and dominance effects govern the inheritance of oil content in Brassica . The unpredictable
or inconclusive gene action for oil content in Brassica has lead to nonconventional approaches for
selecting high oil genotypes in breeding programs. These approaches include the detection of major
quantitative trait loci (QTL), exploitation of hybrid vigor, and use of high oil mutants (Uzunova et al.
1995; Burns et al. 2003; Zhao et al. 2005; Delourme et al. 2006; Qiu et al. 2006; Gao et al. 2007).
The quality and utility of rapeseed oil is mostly determined by its fatty acid composition, and
considerable research efforts have been directed to understand the genetic control of major FA
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