TISSUE-CULTURE IN HAWORTHIA .3. OCCURRENCE OF CALLUS VARIANTS DURING SUBCULTURES AND ITS MECHANISM

The present investigation was aimed to elucidate how variant types occur during culture of the callus. A single original callus, that had been induced from a flower bud of Haworthia setata Haw. (2n=14), was successively subdivided during the first four culture generations finally to 236 calluses, of which 224 were successfully subcultured for further six culture generations. Several selected generations of each callus were examined during this period (two years in total), as to the following six characters; color, shape, growth rate, redifferentiation potentiality, greening and karyotype. Each of the first five characters was graded into three to five 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 on the relative proportions of different variant callus types were often recognized among the four 1ry clones, among eight 2ry clones in each 1ry clone, and so on. These results indicated that 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 on the reversibility were observed in several cases. Furthermore certain karyotypes were somewhat associated, at least, with two characters, callus shape and greening. These facts indicate that genetic changes also play an important role on the occurrence of callus variants. © 1979, The Genetics Society of Japan. All rights reserved.