The pioneering experiments by MULLER (1927, 1928) have shown, that it is possible to knock down the stability of genes. This new possibility was looked upon with great optimism. MULLER himself thought even about breeding better human beings, but the plant breeder STADLER was sceptical about the usefulness of genetic alterations induced e.g. by x-rays. The scepticism resulted from experiments with cereals (STADLER 1930, 1932), where he was not satisfied with an impressive quantity of heritable variation, but rather looked for the kind of changes, that might lead towards crop improvement. He did not observe any dominant mutations and suspected, that a recessive one was „simply the destruction of a gene". He advised students to better make use of „natural" genetic variation for profitable crop improvement.

When after World War II funds became available for „peaceful uses of atomic energy", many young researchers in developed as well as developing countries embarked on the fashionable technology of using radiation and radioactive isotopes for inducing mutations, often without paying attention to the rather modest results obtained between 1930 and 1945 by using radiations as well as some chemical mutagens. It was confirmed, that mutagen treatments harm the delicate molecular structure of genes, and hardly produce the minor changes in nucleotides, which could be expected to lead to meaningful codes for different (or even new) enzymes (STADLER and ROMAN 1948). Some chemical mutagens were found to act less destructive and to cause a much higher proportion of minor changes. However, even these lead to missense and nonsense mutations and did not contribute much to crop improvement.

However, with international coordination and some financial assistance by IAEA and FAO from 1964 onwards it could be convincingly demonstrated, that ionizing radiations and also certain chemicals, when handled properly, could induce many useful alterations in the genomes of crop plants. Records maintained by the Joint FAO/IAEA Division in Vienna show, that ca. 2000 crop cultivars with one or more useful traits from induced mutations (mainly from x- and gamma-rays) were released worldwide over a period of 35 years. Included in these records are some outstanding examples of cultivars (e.g. „Diamant" and „Trumpf" in barley; „IRAT 13", „Yuanfengzao" and „Calrose 76" in rice; „Lumian 1" and „NIAB 78" in cotton; „Pervenets" in sunflower; „Star Ruby" in grapefruit), which had a remarkable economic impact (Anonymous 1991; MICKE et al. 1980, 1985; FAO/IAEA Mutation Breeding Newsletters 1972 - 1997). Inspite of the number of induced mutants recognized as valuable crop cultivars and used successfully in cross breeding, of course „new genes" in the strict sense could not be produced (MICKE 1991). Consequently, one would expect much interest in the question, what kind of molecular alteration in the chromosomal or cytoplasmic DNA, or what kind of structural/numerical alteration in the genome happened in those successful mutant cultivars. However, so far there were not many efforts to bring into accord the destruction caused by mutagens and the good performance of so many induced mutants. The answers to this question should be very relevant for any future investment into mutation breeding and gene engineering, to give a reliable forecast of what can be expected.

Induced mutations occur more or less randomly in the genome, even their target cannot be directed. Only one of the (two or more) alleles of a locus is affected, inheritance is almost ever recessive, therefore homozygosity is normally required for proper expression. Accordingly, results were more often useful in self-pollinating plant species. On the other hand, mutant heterosis has been repeatedly reported (MICKE 1968, 1969, 1976; RÃ"MER and MICKE 1974; MALUSZYNSKI et al. 1987, 1989) and specific mutations e.g. concerning male sterility (DASKALOV and MICHAILOV 1988) or grain quality traits (RÃ"BBELEN 1990) proved useful in cross-pollinating species. In vegetatively propagated crops, which usually are heterozygous and therefore could be improved also by deletions uncovering existing alleles, success has been tremendous, but mainly in ornamental plants (BROERTJES and VAN HARTEN 1988).

Today, mutation breeding is not anymore based only upon classical physical mutagens like x- or gamma rays or classical chemical mutagens like EMS or NMH, but also upon variation that occurs during in-vitro culture and has been termed „somaclonal variation" (SKIRVIN, R.M. 1978; OONO et al. 1984; NOVAK et al. 1988; NOVAK 1991; SUKEKIYO and KIMURA 1991). It was pointed out already, that the term „mutation" is rather woolly defined, but the term „somaclonal variation" is even worse! Initially, it described the unexpected, sudden occurrence of more or less heritable variants in the somatic or generative offspring of in-vitro cultured plant material (HEINZ and MEE 1969). In the meantime, it became known, that a wide array of alterations in nuclear and cytoplasmic genetic elements contribute to the observed phenotypic variation, and that many of them are of epigenetic nature (D'AMATO 1986; PHILLIPS et al. 1994). Initially directors of tissue culture laboratories provoked the interest of plant breeders and sponsors by promising new and particularly useful genetic variation (OONO 1978; LARKIN and SCOWCROFT 1981; CHALEFF 1983; EVANS and SHARP 1983), but soon one learned, that uncontrollable occurrence of „somaclonal variation" could ruin valuable genetic stocks (GÃ"BEL et al. 1986). An interesting feature of somaclonal variation was the repeatedly reported occurrence of 'homozygous mutants' (e.g. OONO et al. 1984). At least some of them were later identified as a kind of „Dauermodifikation", resulting from gene inhibitions, that could pass through meioses and were stable for several generations (OONO 1985). It seems, that also here more research is needed, to clarify the underlying causes of new variation before its use in crop breeding.

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