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COLCHICINE CONVERSION by Julian Coker

Recently the orchid world lost one of its great gentlemen and gentle men in the passing of Don Wimber. Don was involved in many of the most notable scientific innovations related to orchids. He was the father of colchicine conversion of diploids to tetraploids in a number of genera, especially Cymbidium, Phragmipedium and Odontoglossum, which led to great advances in hybridising. Being such a modest man and not one who sought the headlines, his contribution to clonal propagation was understated. While Morel is generally credited with the development of clonal propagation, there are those who consider Don's contribution was of equal if not greater importance.

Much of Don Wimber's early work was carried out in USA and involved Cymbidium. Later this work was extended to chromosome counting and colchicine conversion, especially involving Odontoglossum and Phragmipedium, for the Eric Young Orchid Foundation. He spent a considerable amount of his later time in Sydney with his wife who was working at Sydney University.

Wimber's earliest work with colchicine involved many different plants, but fortunately for us he did a considerable amount of work with diploid cymbidiums. This involved both large-flowered cymbidiums, leading in considerable part to the development of our quality cymbidiums of today, and especially small-flowered cymbidiums that in the diploid state were impossible or difficult to use as parents for further breeding. Most notable was the conversion of Cymbidium Peter Pan 'Greensleeves' from the diploid to the tetraploid state.

Colchicine, an alkaloid derived from the autumn crocus, has the ability to prevent the dividing cell wall from forming during cell division. During mitotic division the chromosomes are duplicated and drawn to opposite poles prior to the formation of the cell wall that will divide them. However, under the influence of colchicine this cell wall does not form and thus the chromosome number doubles. As these newly doubled cells continue to divide in the absence of colchicine, a plaque of cells forms; if the initial shoot develops from within this plaque then a plant with double the initial chromosome count results. Generally this represents a conversion from the diploid to the tetraploid state, but it can equally apply to triploids, converting them to hexaploids, or tetraploids to octaploids. These are generally of little use because of their undesirable growth habit.

In colchicine conversion, actively growing protocorms are exposed to a liquid or solid medium containing colchicine for a period of time, before being returned to a normal culture medium. It is a random exercise, with some protocorms having converted areas on their epithelium, and others none, while some are killed by the treatment. As the protocorms develop into plants, the trained eye can identify the converted clones by the nature of their growth and these can then be individually selected and grown on into mature plants. The ultimate test is chromosome counting, but stomatal guard cell size or visual observation will generally suffice to distinguish tetraploids from diploids.

Colchicine conversion is equally applicable to mericlones or seedlings. With our considerable bank of converted clones, together with chance tetraploids that sporadically occur, and their progeny, we now have an excellent base of vigorously growing, free-flowering, quality clones for future breeding. We owe Don Wimber a considerable debt of gratitude for his worthy contribution to improving the quality of flowers in converted clones, and in expanding the numbers of breeding parents, past, present and future.

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