216 – II.3. BRASSICA CROPS (BRASSICA SPP.)
polymorphism (SNP) molecular markers are now being used routinely to expedite the
introduction of novel genetic variation in Brassica breeding programmes.
Maternal and/or paternal inheritance of organelle genomes
Analysis of the chloroplast DNA of the cultivated diploid Brassica species, and their
close relatives, divided the subtribe Brassicinae into two ancient evolutionary lineages
(Warwick and Black, 1997), the “Nigra” lineage, which contained the diploid B. nigra
and the related wild mustard Sinapis arvensis, and the “Rapa/Oleracea” lineage, which
contained the diploid progenitors of B. napus (Figure 3.31). There has been little work
studying the origins of the cultivated amphidiploids B. carinata or B. juncea. However,
studies of organellar and nuclear DNA of B. napus and related species suggested that a
species closely related to B. montana gave rise to the cytoplasm of both B. rapa and
B. oleracea (Song and Osborne, 1992). The same study and an earlier study on
chloroplast evolution in amphidiploid Brassica species (Palmer et al., 1983) suggested
that oilseed rape (B. napus) evolved from multiple hybridisations between B. oleracea
and the closely related n=9 species, B. montana and B. rapa. Some of these lineages may
have been subject to introgression from post-hybridisation with their diploid progenitor.
Self- incompatibility, “S” alleles
Self-incompatibility (SI) occurs in many flowering plants and is one of the most
important systems to prevent inbreeding (Takayama and Isogai, 2005). SI is defined as
the inability of plants to produce functional gametes to effect fertilisation upon
self-pollination or when crossed with certain relatives (De Nettancourt, 1971). Although
the amphidiploid Brassica species, B. napus, B. juncea and B. carinata are largely
self-pollinating (autogamous), the diploid species, with some exceptions, are
self-incompatible and are obligatory out crossers. Among Brassica species and their close
relatives, 50 out of 57 species are self-incompatible (Hinata, Isogai and Isuzugawa,
1994). The self-/non-self recognition in most species is controlled by a single locus,
termed the “S locus” that inhibits the self pollen from penetrating the style when the same
S-allele specificity is expressed by both the pollen and pistil. In the Brassica
incompatibility system, over 30 B. rapa alleles and 50 B. oleracea alleles have been
identified: S 1 , S 2 , S 3 ....S50+ (Nou et al., 1993; Ockendon, 2000). Self-compatible (Sf)
alleles are also known.
Among angiosperms there are two major types of physiological SI systems:
gametophytic (GSI) and sporophytic (SSI) (Briggs and Knowles, 1967). In a GSI system,
the pollen reaction is controlled by the genotype of the individual pollen grain, i.e. a plant
heterozygous at the S-locus would produce two possible types of pollen with each
microspore receiving one of the two possible S-alleles. However, in the SSI system that is
present in the Brassicinae, all pollen released by a plant has the same phenotype with
respect to the compatibility reaction, regardless of the genotype of the individual pollen
grain. The S-locus consists of at least three tightly linked transcriptional units arranged in
pairs, with one functioning as the female determinant and the other the male. This
multi-gene complex at the S-locus is inherited as one segregating unit so the gene
complexes are called “S-haplotypes”. Self-/non-self recognition operates at the level of
protein-protein interaction of the two determinants (Takayama and Isogai, 2005). When
the SI system is activated in a Brassicinae species, a recognition reaction occurs between
the papilla cells of the stigma and the pollen (Hinata and Nishio, 1980).
