Liverwort Genomes Display Extensive Structural Variations

PIKE, L. M., HU, A., RENZAGLIA, K. S. and MUSICH, P. R., 1992. Liverwort genomes display extensive structural variations. Analyses of the total genomic DNA of eight species of liverworts and two species of green algae by thermal denaturation and CsCl buoyant density gradient centrifugation reveal a high degree of structural complexity and interspecific heterogeneity. The hepatic taxa exhibit two or more DNA components of varying base composition. Average G4‐C contents of total cellular DNA calculated from melting profiles are similarly variable, ranging from 38% to 53% G + C. The green alga Chara, a member of the ancestral line to land plants, shows similarities with liverworts in possessing multiple DNA components of comparable complexity, whereas Hydrodiciyon DNA displays a single component. Detailed hybridization analyses of individual density gradient fractions using α‐tubulin, rRNA and ribulose 1,5‐bisphosphate carboxylase/oxygenase large subunit rbcL gene probes were performed to locate the low‐copy number and moderately repetitive nuclear genes, and the chloroplast chromosome, respectively. The location of each gene within the density gradient is highly variable among the organisms examined; a‐tubulin occurs in fractions ranging from 44–64% G + C, rDNA in 50–64% G + C fractions, and the RbcL gene is located in fractions from 30–59% G + C. For a given species, the two nuclear genes normally overlap in their distributions within the gradient. In most instances, neither gene occurs in the major DNA components, indicating that these components may contain repetitive DNAs. The observed variation in the density of the rbcL gene implies substantial reorganization of the chloroplast genome. The overall differences in the genomic components within and between taxa provide insight into the dynamics of DNA structure that have occurred during the extended evolutionary history of these organisms