Breakage-fusion-bridge cycles and de novo telomere formation on broken chromosomes in maize callus cultures

Molecular Karyotyping on Populus simonii × P. nigra and the Derived Doubled Haploid

The molecular karyotype could represent the basic genetic make-up in a cell nucleus of an organism or species. A doubled haploid (DH) is a genotype formed from the chromosome doubling of haploid cells. In the present study, molecular karyotype analysis of the poplar hybrid Populus simonii × P. nigra (P. xiaohei) and the derived doubled haploids was carried out with labeled telomeres, rDNA, and two newly repetitive sequences as probes by fluorescence in situ hybridization (FISH). The tandem repeats, pPC349_XHY and pPD284_XHY, with high-sequence homology were used, and the results showed that they presented the colocalized distribution signal in chromosomes. For P. xiaohei, pPD284_XHY produced hybridizations in chromosomes 1, 5, 8, and 9 in the hybrid. The combination of pPD284_XHY, 45S rDNA, and 5S rDNA distinctly distinguished six pairs of chromosomes, and the three pairs of chromosomes showed a significant difference in the hybridization between homologous chromosomes. The repeat probes used produced similar FISH hybridizations in the DH; nevertheless, pPD284_XHY generated an additional hybridization site in the telomere region of chromosome 14. Moreover, two pairs of chromosomes showed differential hybridization distributions between homologous chromosomes. Comparisons of the distinguished chromosomes between hybrid and DH poplar showed that three pairs of chromosomes in the DH presented hybridization patterns that varied from those of the hybrid. The No. 8 chromosome in DH and one of the homologous chromosomes in P. xiaohei shared highly similar FISH patterns, which suggested the possibility of intact or mostly partial transfer of the chromosome between the hybrid and DH. Our study will contribute to understanding the genetic mechanism of chromosomal variation in P. xiaohei and derived DH plants.

The r-X1 deletion induces terminal deficiencies in the maize B chromosome

In addition to causing the nondisjunction of maize B and normal A chromosomes at the second megaspore division during embryo sac development, the r-X1 deletion results in terminal deficiencies (TDs) in various A chromosomal arms, but whether the r-X1 deletion also induces TDs of the maize B chromosome remains unknown. To answer this question, the chromosomal composition in the r-X1-containing progeny of r-X1/R-r female parents carrying two standard B chromosomes was determined. Nine of 104 (8.7%) examined kernels contained a smaller telocentric B chromosome, and one of these (designated Bdef-1) was further identified as a TD with a breakpoint in the third distal heterochromatic region of the B chromosome. Thus, the results indicated that the r-X1 deletion could also induce TDs of the maize B chromosome during megaspore divisions. The Bdef-1 chromosome lacked nondisjunctional behavior, and this behavior was restored by the presence of the B chromosome in the cell. A transmission analysis of the Bdef-1 chromosome revealed that loss of the distal portion of the B chromosome reduced female but not male transmission of the B chromosome. Furthermore, the Bdef-1 chromosome was used to more finely map B-derived miRNA genes on the B chromosome. Our results indicate that the r-X1 deletion results in TDs of the B chromosome in maize, and the r-X1 deletion system can thus be used to generate a series of terminally truncated B chromosomes that may be used to map features of the B chromosome, including genes and properties related to B chromosome functions.

Cytogenetic events in the endosperm of amphiploid Avena magna × A. longiglumis

This study analysed cytogenetic events occurring in the syncytial endosperm of the Avena magna H. C. Murphy & Terrell × Avena longiglumis Durieu amphiploid, which is a product of two wild species having different genomes. Selection through the elimination of chromosomes and their fragments, including those translocated, decreased the level of ploidy in the endosperm below the expected 3n, leading to the modal number close to 2n. During intergenomic translocations, fragments of the heterochromatin-rich C-genome were transferred to the D and Al genomes. Terminal and non-reciprocal exchanges dominated, whereas other types of translocations, including microexchanges, were less common. Using two probes and by counterstaining with DAPI, the A. longiglumis and the rare exchanges between the D and Al genomes were detected by GISH. The large discontinuity in the probe labelling in the C chromosomes demonstrated inequality in the distribution of repetitive sequences along the chromosome and probable intragenomic rearrangements. In the nucleus, the spatial arrangement of genomes was non-random and showed a sectorial-concentric pattern, which can vary during the cell cycle, especially in the less stable tissue like the hybrid endosperm.