SELECTION OF STABLE BANANA CLONES WHICH DO NOT PRODUCE DWARF SOMACLONAL VARIANTS DURING IN-VITRO CULTURE

The use of shoot-tip culture for banana micropropagation, conservation and exchange of germplasm may be reduced by the occurrence of undesir ed somaclonal variants at high percentages. Dwarf variants which accou nt for approximately 80% of the off-types among 'Cavendish' bananas ar e difficult to detect at the in vitro and nursery stages. A significan t economic loss is caused when they are detected only at the productio n stage in the field. In the present study the hypothesis that the rat e of occurrence of dwarf variants among 'Cavendish' vitroplants is gov erned by clonal-inherent genetic factors was studied. For this purpose , a selection in favour of highly stable clone(s) was performed. Based on results obtained in an earlier study, 11 families of cv. 'Williams ' were selected for further large-scale multiplication and evaluation. Six of the families that did not produce dwarf variants in the initia l study are referred to as 'stable' families. Five of the families whi ch did produce dwarf variants are referred to as 'non-stable' families . At the first stage, a few hundred plants of each of the families wer e evaluated. All of the 'non-stable' families and four of the six 'sta ble' families produced dwarf variants. One of the 'stable' families pr oduced a very low rate of dwarfs and one did not produce any dwarfs. T he two stable families were tested further, especially the one which e xhibited the greatest stability. In that clone only five dwarf variant s were recorded among more than 21 000 vitroplants over a span of 8 ye ars. During that period plants for evaluation were regenerated from ne wly initiated cultures with primary explants of the clone. Some of the cultures were kept before evaluation for many cycles and for a longer period in vitro than the regular ones. The fact that plants of the cl one remained stable with regard to restoring a negligible rate of dwar f variants with time, both in situ and in vitro, is attributed to a ge netic trait that was revealed by the selection procedure employed. The origin and causes of dwarf somaclonal variation in bananas is discuss ed. It is suggested that Cavendish banana shoot tips are of chimeric c onstitution. The dissociation of these chimeras may result in dwarf so maclonal variants.