242 – II.3. BRASSICA CROPS (BRASSICA SPP.)
The development of protoplast fusion technology has been highly successful in
circumventing the natural sexual barriers that separate the Brassicaceae species and
genera. The technology has the potential to access desirable genes present in distant
relatives (Glimelius, 1999; Christey, 2004; Navrátilová, 2004; Liu, Xu and Deng, 2005).
Prakash et al. (2009) have compiled a list of intertribal somatic hybrids in the Brassiceae
and the desirable traits to be transferred (Table 3.17). Additional intergenomic hybrids
have been produced but failed to establish in soil e.g. Camelina sativa + B. carinata
(Narasimhulu et al., 1994), C. sativa + B. oleracea (Hansen, 1998) and Barbarea
vulgaris + B. napus (Fahleson, Eriksson and Glimelius, 1994).
With some exceptions, the somatic hybrids so far obtained have exhibited a high
degree of sterility and/or morphological abnormalities that have limited their use.
However, the importance of somatic hybridisation is not so much the direct use of the
resulting amphidiploids, containing both parental genomes, but rather to utilise the
somatic hybrids as a bridge to transfer desirable traits to target species (Glimelius, 1999).
Cell fusion not only brings together the nuclear contents of both parents but also
combines the cytoplasm and organellar content of fused cells. Frequently, to improve the
outcome, the nucleus of one parent is eliminated by X-ray, centrifugation or chemical
treatment before fusion but the fused cell contains the cytoplasm of both parents.
The resulting plant is termed a “cybrid”. This technique allows cytoplasmic substitution
which frequently results in cytoplasmic male sterility (CMS). Cell fusion among the
Brassicaceae, where the cytoplasm of both parents are combined, can also generate novel
cytoplasmic variability, bringing about organellar reassortment and DNA rearrangement,
which is not possible using sexual hybridisation.
Chloroplast segregation is independent of mitochondrial segregation and while
mitochondrial recombination has been frequently observed in the Brassiceae (Glimelius,
1999), recombination is rarely found in the chloroplasts. It is also rare to have a mixture
of the two chloroplasts occurring in the same hybrid. In general, the chloroplasts are
usually contributed by crop species. This may occur because many of the fusions are with
the allopolyploid crop species that contribute large numbers of chloroplasts per cell
(Butterfass, 1989).
