Cytogenetic stability of maize tissue cultures: a cell line pedigree analysis

Abstract

Cytogenetic instability in maize plants regenerated from tissue culture is a commonly observed phenomenon. In an attempt to understand the origin of this instability meiotic analyses were performed on 370 regenerated plants that were initiated from cell lines of 22 immature embryos of 9 maize genotypes. Cell lineage pedigrees were maintained on these cultures to record the familial relationship between regenerated plants. Overall, 12.4% of the 370 régénérants contained cytological aberrations. The largest category of aberrations involved chromosome breakage (translocations, deletions, and an inversion) between a heterochromatic knob or knob site and the centromere. Late replicating knob heterochromatin might have been responsible for the chromosome breakage events. Cytologically identical aberrations were observed in plants regenerated at various time points in the pedigreed cultures, indicating an early occurrence of the aberration during culture maintenance. Heterogeneity of cell karyotypes also was observed within embryo cell lines. Multiple independent aberrations occurred within a single embryo cell line. The pedigree of the cultures provided evidence that the sectoring of regenerated plants was due to a multicellular origin of meristematic areas. The frequency of cytogenetically abnormal plants recovered increased with culture age. The data presented are consistent with the age effect not being due to an increased mutation rate but due to mutational events that occurred throughout culture development with a subsequent maintenance and accumulation of aberrant cells over time.Key words: Zea mays, cytogenetic stability, tissue culture, somaclonal variation.

Cytological elucidation of somaclonal variation in grasspea (Lathyrus sativus L.)

“…These facts suggest that the conditions which are favourable for polyploidization of the calli probably are unfavourable for the morphogenesis of higher levels of polyploids. It may be concluded that in cultured tissues, there is a progressive accumulation of irregularities of chromosomes which increases with the aging process during long term sub-culturing and that these tissues have a reduced potentiality for morphogenesis (Benzion and Phillips (1988).…”

Mechanisms of Somaclonal Variation

“…In culture, when cells arc induced into rapid cell division, the presence of heterochromatin may therefore lead to enhanced levels of breakage in those chromosomes which contain it. Correlations between the location of tissue culture-induced chromosome breakage and the distribution of heterochromatin have been demonstrated in many species (2,12,18). However, chromosome breakage still occurs in plants wit11 little heterochromatin (I) suggesting tl1at other aspects of genome composition must also be i.mport:mt.…”

Do We Understand Somaclonal Variation?

“…In oats, plants regenerated from 20-month-old cultures of the Tippecanoe and Lodi varieties carried a detectable structurally-altered chromosome 49% and 88% of the time, respectively (41). In maize, 1036 regenerated plants were analyzed meiotically (2,42,43,44). Of these, 217 (21%) possessed detectable structurally-altered chromosomes.…”