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a Dep. of Plant and Microbial Biology, Univ. of California, Berkeley, CA 94720 USA
mjcho{at}nature.berkeley.edu
Plants from in vitro culture can exhibit somaclonal variation, two characteristics of which are structural rearrangements and variation in chromosome number. These characteristics were studied in barley (Hordeum vulgare L. cv. Golden Promise) callus and plants derived from nontransgenic and transgenic callus of approximately the same age; chromosomes were studied in cells from callus and root tips from plants. Analysis of these data revealed greater variation in ploidy in transgenic compared with nontransgenic plants. Of 59 independent transgenic lines, only 32 (54%) had normal diploid complements of 2n = 2x = 14, while 27 (46%) were tetraploid (2n = 4x = 28) or aneuploid around the tetraploid level (i.e., 26, 27, 29 and 30 chromosomes); no aneuploidy around the diploid number was observed. Nontransgenic plants regenerated after in vitro culture alone had a much lower percentage of tetraploids (0–4.3%). Most diploid plants had normal gross morphology, while tetraploid plants had abnormal morphological features. Ploidy determinations were made on randomly selected cells from callus of immature embryos cultured for 0 to 14 d. The number of tetraploid cells in 1-d- to 7-d-old callus was around 2 to 4%; in callus comparable in age to that used to regenerate both the transgenic and the nontransgenic sets of plants, 23% of the cells were tetraploid. This percentage is lower than the percentage (46%) of tetraploid plants from the transgenic lines; however, it is considerably higher than the percentage (0–4.3%) of tetraploid plants from nontransgenic callus. Therefore, although chromosomal variation and abnormalities occur in callus and nontransgenic plants, the extent of ploidy changes in transgenic plants is exacerbated, perhaps due to the additional stresses that occur during transformation.
Abbreviations: BAP, 6-benzylaminopurine • 2,4-D, 2,4-dichlorophenoxyacetic acid • IE, immature embryo
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