The distinctness of different species is maintained in nature by a complex package of physical and biochemical factors, collectively called the ‘reproductive barriers’ which prevent natural hybridisation among them. Natural hybridisation is not very common even among related taxa but artificial hybridisation may be possible between related taxa. The reproductive barriers are so strong in some species that experimental hybridisation even between varieties of a species may almost be impossible. On the other hand, in some taxa, such reproductive isolation is poor and even intergeneric hybrids can be produced by artificial pollination, as in the case of orchids where up to five genera have been brought together. This can also be achieved using isolated cells from two unrelated species.

4 SOMATIC HYBRIDISATION

In the face of frequent failure to produce the desired sexual hybrids aimed at crop improvement, hybridisation by somatic cell fusion (somatic hybridisation) has opened up new avenues.

During fusion, the DNA content of two cells from different sources (parents) is made to mix, creating a hybrid containing all the genes of the fusion partners. Somatic cell fusion is not easy to achieve and viruses or chemicals are usually used to assist the process. The somatic hybrids themselves are not necessarily stable or viable, particularly if cells from two different species are involved. But it is possible to rescue these hybrids by tissue culture methods.

4 HYBRIDOMA AND MONOCLONAL ANTIBODIES

Cell fusion is possible with many types of cells, both prokaryotic and eukaryotic. The best known example of eukaryotic fusion is the so-called ‘hybridoma’ cell. This is a product of fusion between a myeloma (which is a lymphoid cell line capable of growing indefinitly in culture) and a normal lymphocyte. Lymphocytes are a type of white blood cells which are part of the body’s the immune system that produces the ‘antibodies’. Antibodies are large complex protein molecules which are capable of specifically recognizing other molecules, the ‘antigens’, which elicit the production of antibodies. Antibodies are very precise in distinguishing between small differences in the structure of the antigens they encounter and bind with, if related.

When an organism becomes accidentally infected or is deliberately immunised with a purified antigen, a wide range of antibodies are made, some reacting with each part or ‘determinant’ of the antigen which is recognised as ‘foreign’ to the organism. This immune response is the basis of vaccination, where killed or attenuated cells of a pathogen are injected into individuals at risk from a disease, such as diptheria or measles, to stimulate antibody production. Once immunised, the subject is unable to contract the disease if exposed to a virulent pathogen with the same antigens. After immunisation, a mixture of antibodies appears in the blood serum and can be isolated. This mixture of antibodies is produced by many different cells and so is called a ‘polyclonal’ anti-serum. Instead of isolating the individual types antibodies from the polyclonal pool, which is a very difficult process, antibody producing spleen cells are now isolated and induced to fuse with myeloma cells. Immortal antibody-producing hybrid cells can be made this way, and can be grown outside the body. By using myeloma cells with appropriate growth characteristics, successful hybrids can be selected and each such single cell cloned can be grown to form a separate cell line. Each such line will produce only one type of antibody. These are called ‘monoclonal’ antibodies (MCAs or MABs).

Chemically, monoclonal antibodies are just like any other antibodies, except that they are produced from cells grown in culture and that very pure preparations of one specific antibody can be made in large quantities. Some hybridomas are also grown inside mice using the peritonal (abdominal cavity) fluid as a nutrient medium. This process is expensive and laborious and is only really suitable for initial development and relatively small-scale production. For larger-scale work, in vitro culture techniques are needed.