The allopolyploid origin of several bryophyte species was reported approximately 20 years ago. Among them the best characterized is Pellia borealis, a hybrid of two cryptic sibling species: Pellia epiphylla-species N and P. epiphylla-species S. Genomes of the allopolyploid liverwort P. borealis and its progenitors were investigated using conventional as well as molecular cytogenetic techniques. The nuclear DNA content and cell cycle phase in the thallus nuclei was established using flow cytometry. Fluorescent differential staining, C-banding and fluorescent in situ hybridization (FISH) with 26S and 5S ribosomal DNA (rDNA) probes revealed new features of the chromosomes in the P. epiphylla-P. borealis complex. Some characteristics found in both the polyploid and haploid karyotypes support earlier suggestions about the allopolyploid origin of P. borealis. The banding pattern observed on P. borealis chromosomes suggests the occurrence of structural changes in the allopolyploid genome. The number and localization of rDNA sequences were established and simultaneous FISH with 26S and 5S rDNA probes showed colocalization of both types of ribosomal RNA (rRNA) genes in Pellia chromosomes. A nuclear DNA estimate showed that the P. borealis nuclear DNA content is about twice that of the largest known nuclear genome previously known in bryophytes.
Shoot organogenesis was induced from 2-and 6-week-old callus derived from the leaves of Arabidopsis thaliana ecotype Columbia (2n = 10). Regenerated plants were evaluated for chromosomal variations by means of flow cytometry and fluorescent in situ hybridization (FISH). Flow cytometric measurements revealed the occurrence of diploid, tetraploid, and octoploid plants among the regenerants of 2-week-old calli, whereas only diploid and tetraploid plants were regenerated from the 6-week-old calli. Chromosome counting showed that plants developed from the 2-week-old calli exhibited mixoploidy and a high frequency of aneuploid cells. These plants were infertile and displayed altered morphology. FISH with 5S and 25S rDNA probes allowed to detect some structural chromosomal rearrangements in regenerated plants. Along with cells which exhibited correct localisation of rDNA loci, also cells bearing chromosomal translocations, deletions or duplications were found. The type of structural aberrations varied between diploid and tetraploid regenerants.
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