218 – II.3. BRASSICA CROPS (BRASSICA SPP.)
perspective of intra- and interspecific outcrossing in the field, it has been noted that the
incidence of interspecific crossing in mixed species populations is likely to increase as the
number of plants in the self-incompatible species decreases, due to scarcity of pollen of
the same species and increasing pollen competition from other nearby species.
Although nearly all the mono-genomic Brassica species are self-incompatible, the
natural amphidiploids species – B. napus, B. juncea and B. carinata – are all
self-compatible (Takahata and Hinata, 1980). Okamoto et al. (2007) note that
interspecific crosses between B. rapa and B. oleracea are difficult to make and, when the
chromosome complement is doubled, produce self-incompatible amphidiploids plants
(Beschorner, Plümper and Odenbach, 1995; Nishi, 1968). They suggest that a single
mutation in a dominant S-haplotype could result in a self-compatible B. napus plant that
could reproduce itself through the production of self seed. Amphidiploid plants without
such a mutation would be forced to cross with one or the other diploid parent and rapidly
be assimilated into one or the other parent species. Fujimoto et al. (2006) provide
evidence for such mutations in B. rapa and B. oleracea.Interspecific hybridisation and introgression
Introduction
With the introduction of genetically modified (GM) B. napus, the potential for
inserted genes to transfer and introgress into related Brassicaceae species has been the
subject of much speculation and research. There are many conditions which have to be
met for such an event to occur. First, the cross of interest must occur. However, crossing
success depends on a series of preconditions that include physical proximity of the
parents, pollen movement and longevity, synchrony of flowering, breeding system of the
parents, flower characteristics, pollen-style compatibility and competitiveness of foreign
pollen. If all these pre-fertilisation conditions are met, the next series of hurdles include
sexual compatibility, embryo-endosperm imbalance as well as hybrid fertility and
viability in nature. In addition, the hybrid must have sufficient fitness to backcross with
the recipient parent producing fertile progeny through several generations. For example,
Wei and Darmency (2008) found crosses between male sterile B. napus and B. juncea,
B. nigra, H. incana and R. raphanistrum produced only small seed, resulting in poor
seedling establishment of the hybrids under field conditions. Even if all the conditions are
met, introgression will not occur unless there is pairing between a chromosome of the
recipient parent and a donor parent chromosome segment that carries the inserted gene.
Gene transfer cannot occur in nature if any one of these requirements is not met.
However, it has been speculated that strong selection pressure over many backcross
generations could result in the transgene existing in a stable strain carrying an extra
chromosome pair (Chèvre et al., 2001).
Modern researchers have overcome many of the natural barriers to interspecific and
intergeneric crosses within the tribe Brassiceae. Techniques such as ovule, ovary and
embryo culture, as well as protoplast fusion have produced hybrids that would otherwise
fail due to sexual barriers. Success has also been achieved by crossing induced polyploids
from one or both parents. Such techniques have been used to try to integrate important
agronomic or quality traits from a foreign species into a cultivated crop. However,
success using such techniques is no indication that the same result could occur through
sexual crossing in nature.