Nuclear Cytology,Polyploid,Aneuploid,Endopolyploid Cells,Chromosome Fragmentation,Chromatid Separation,Culture System,Haplopappus Gracilis

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	Nuclear Cytology - Callus and suspension cultures show both numerical (polyploidy and aneuploidy) and structural (deletions, translocations, etc.) chromosome changes. The frequency of these changes tends to increase with time so that some cultures may become predominantly or even completely polyploid or aneuploid. Explants contain endopolyploid cells, which may give rise to a portion of polyploid cells in cultures. But most polyploid cells appear to originate through endoreduplication (additional rounds of DNA replication without intervening cell division) although selection for such cells can not be ruled out.
Aneuploid cells originate mainly due to anaphase irregularities like unequal chromatid separation, lagging chromatids or chromosomes, anaphase bridges giving rise to breakage-fusion-bridge cycle, chromosome fragmentation, etc. The cytogenetic status of cultured cells is influenced by several factors of the culture system, e.g., GR concentrations and combination, culture age, liquid or agar medium, subculture interval, sucrose concentration, etc. Suspension cultures of many diploid species show a selection for diploid cells so that they remain predominantly diploid for long periods, e.g., Vida hajastana and Haplopappus gracilis cultures for over 300 days.