Stability of monocentric and dicentric ring minichromosomes in Arabidopsis

A dicentric ring minichromosome (miniδ) was identified in transgenic Arabidopsis thaliana and added to a wild type as a supernumerary chromosome. This line is relatively stable and has been maintained for generations, notwithstanding its ring and dicentric structure. To determine the mechanism for stable transmission of miniδ, the structure and behavior of two new types of ring minichromosomes (miniδ1 and miniδ1-1) derived from miniδ were investigated. Fluorescence in situ hybridization analysis revealed that miniδ1 is dicentric just like miniδ, whereas miniδ1-1 is monocentric. The estimated sizes of miniδ1 and miniδ1-1 were 3.8~5.0 and 1.7 Mb, respectively. The sizes of the two centromeres on miniδ1 were identical (ca. 270 kb) and similar to that of miniδ1-1 (ca. 250 kb). Miniδ1 was relatively stable during mitosis and meiosis, as is miniδ, whereas miniδ1-1 was unstable during mitosis, and the number of minichromosomes per cell varied. This possibly resulted from misdivision caused by a short centromere on monocentric miniδ1-1. Transmission through the female was quite limited for all three ring minichromosomes (0-3.2%), whereas that through the male was relatively high (15.4-27.3%) compared with that of other supernumerary chromosomes in Arabidopsis. Ring structure without telomeres itself seems not to limit the female transmission.