246 – II.3. BRASSICA CROPS (BRASSICA SPP.)
dihaploid plant that is fully fertile and totally homozygous. Thus, complete homozygosity
is reached in a single generation, and all seeds arising from self-fertilisation of that plant
will be genetically identical. It is this single step to homozygosity that reduces the number
of generations and time required to develop a new variety or hybrid parent. However, in a
breeding programme, large populations of DH lines must be generated and evaluated
since no prior selection has taken place. DH lines are usually derived from F 1 donors,
although the use of F 2 and F 3 donor plants allows for more recombination and some
preselection.
Molecular markers and their application
Marker-assisted selection and chromosome mapping came into general use in the
1980s with the development of restriction fragment length polymorphisms (RFLP)
techniques that resulted in the first linkage maps for B. oleracea (Slocum et al., 1990),
B. rapa (Song et al., 1991) and B. napus (Landry et al., 1991). This technique was
important in identifying genomes and their chromosomes, locating genes and qualitative
trait loci (QTLs), which are DNA regions containing a gene or genes that regulate traits
of agronomic or quality interest.
The discovery of the polymerase chain reaction (PCR) by Mullis and Faloona (1987)
resulted in new types of genetic markers such as amplified fragment length
polymorphisms (AFLPs) that are more sensitive than RFLPs and simultaneously detect
various polymorphisms in different genomic regions.
Additional marker systems have since been added to the toolbox including: random
amplified polymorphic DNAs (RAPDs); sequence tagged sites (STS); simple sequence
repeats (SSRs) or microsatellites and single nucleotide polymorphisms (SNPs). Breeders
use these molecular markers to produce densely marked chromosome maps that can then
be used to: 1) characterise germplasm and its genetic variability; 2) estimate the genetic
distance between gene pools, inbreds and populations; 3) detect and locate QTLs and
monogenic traits of interest; 4) select genotypes based on the presence or absence of
specific markers; 5) identify useful candidate genes for sequencing (for more detailed
information on genome mapping and molecular breeding in B. napus, see Snowdon, Lühs
and Friedt, 2007; Snowdon et al., 2007). The marker systems differ in their ease of use,
cost and other characteristics. It is expected that the SNPs system will become the marker
system of preference, dispite its initial high cost, due to its ease of use, low cost per
analysis and high level of reproducibility (Korzun, 2003).
Comparative genomic gene identification
The distantly related and intensively studied species Arabidopsis thaliana provides
information that is highly relevant for gene isolation and characterisation in Brassica
crops. However, the genomes of Brassica species are much more complex (Snowdon,
Lühs and Friedt, 2007).
A comprehensive comparative RFLP linkage map of A. thaliana and B. napus
genomes indicated the 5 Arabidopsis chromosomes could be allocated to a minimum of
22 conserved, duplicated and rearranged blocks throughout the B. napus genome (Parkin
et al., 2005).
Such information highlights the complexity of genome rearrangements between the
two species, but also the great potential the model genome offers for comparative genetic
analysis of the Brassica crops (Snowdon, Lühs and Friedt, 2007).
