off-types
The use of shoot-tip culture for banana micropropagation, conservation and exchange of germplasm may be reduced by the occurrence of undesired somaclonal variants at high percentages. Dwarf variants which account for approximately 80% of the off-types among ‘Cavendish’ bananas are difficult to detect at the in vitro and nursery stages. A significant economic loss is caused when they are detected only at the production stage in the field.
In the present study the hypothesis that the rate of occurrence of dwarf variants among ‘Cavendish’ vitroplants is governed 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 initial study are referred to as ‘stable’ families. Five of the families which did produce dwarf variants are referred to as ‘non-stable’ families.
At the first stage, a few hundred plants of each of the families were evaluated. All of the ‘non-stable’ families and four of the six ‘stable’ families produced dwarf variants. One of the ‘stable’ families produced a very low rate of dwarfs and one did not produce any dwarfs.
The two stable families were tested further, especially the one which exhibited the greatest stability. In that clone only five dwarf variants were recorded among more than 21 000 vitroplants over a span of 8 years. During that period plants for evaluation were regenerated from newly 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 clone remained stable with regard to restoring a negligible rate of dwarf variants with time, both in situ and in vitro, is attributed to a genetic trait that was revealed by the selection procedure employed.
The origin and causes of dwarf somaclonal variation in bananas is discussed. It is suggested that Cavendish banana shoot tips are of chimeric constitution. The dissociation of these chimeras may result in dwarf somaclonal variants.
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