Tissue culture in Haworthia. III. Occurrence of callus variants during subcultures and its mechanism.

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Abstract
A single original callus, that had been induced from a flower bud of H. setata Haw. (2n = 14), was successively subdivided during the 1st 4 culture generations finally to 236 calluses, of which 224 were successfully subcultured for 6 further culture generations. Several selected generations of each callus were examined during this period (2 yr), as to the following 6 characters; color, shape, growth rate, redifferentiation potentiality, greening and karyotype. Each of the 1st 5 characters was graded into 3-5 classes. Change in each character was traced systematically in individual callus lines (= clones), and the data were analyzed statistically. As a whole, the variation expressed by both the total variance and standard deviation greatly increased during the early culture generations when subdivisions of the calluses were made, and it remained almost constant during the later generations. In addition, clear differences in the relative proportions of different variant callus types were often recognized among the 4 1ry clones, among 8 2ry clones in each 1ry clone, and so on. The main mechanism of the occurrence of variant types of the callus is the production of different types (= cloned calluses) of homogeneous cell populations from a heterogeneous population (= stock callus) by continuous subdivisions of the stem callus. In general, the change in callus characteristics was reversible, suggesting that some callus variants are epigenetic in nature. However, great differences in reversibility were observed in several cases. Certain karyotypes were somewhat associated with 2 characters, callus shape and greening. Genetic changes also play an important role on the occurrence of callus variants.