Size and number of tandem repeat arrays can determine somatic homologous pairing of transgene loci mediated by epigenetic modifications in Arabidopsis thaliana nuclei

The chromosomal arrangement of different transgenic repeat arrays inserted at various chromosomal positions was tested by FISH in Arabidopsis 2C leaf and root nuclei. Large lacO (similar to 10 kb) but not tetO (4.8 kb) or small lacO (similar to 2 kb) arrays were, in general, more often spatially associated with heterochromatic chromocenters (CC) than flanking regions (that either overlap the array insert position or are between 5 and 163 kb apart from the insert site). Allelic and ectopic pairing frequencies of lacO arrays were significantly increased only in nuclei of lines with two large lacO arrays inserted at different positions on the same chromosome arm. Within the same lines, root nuclei showed a significantly lower increase of pairing frequencies at the insert position compared to leaf nuclei but still a higher frequency than in the wild-type situation. Thus, the frequencies of homologous pairing and association with heterochromatin of transgenic repeats may differ with the construct, the chromosomal insertion position, the cell type and with the number and repetitiveness of inserts. Strong CpG methylation is correlated with a high frequency of homologous pairing at large repeat array loci in somatic cells but has no impact on their association with CCs. These results show that single low-copy arrays apparently do not alter interphase chromatin architecture and are more suitable for chromatin tagging than multiple high copy arrays.