All of the studies described above have involved documenting the nature of the genomic
rearrangements present in one or a few isolated T-DNA lines; the frequency with which
translocations occur in collections of T-DNA lines was not addressed in those studies. There
have, however, been two studies published that investigated the frequency of translocations
in collections of T-DNA lines. Castle et al. (1993) found evidence of chromosomal
translocations or inversions in seven of the 56 Arabidopsis lines (12.5%) carrying embryo
lethal mutations that they genetically mapped, while Budziszewski et al. (2001) found that
11 of the 32 lines (34%) carrying seedling lethal mutants that they analyzed had potential
chromosomal abnormalities such as translocations. Because both of these studies involved
estimating the frequency of translocations using T-DNA lines that were pre-selected for the
presence of lethal mutations, these samples may have been enriched for plants carrying
chromosomal abnormalities. We were therefore interested in measuring the frequency with
which chromosomal translocations occur in T-DNA lines using a less biased sample.
Accordingly, all of the Salk T-DNA lines (Alonso et al., 2003) used in our study were viable
when the T-DNA mutant allele was in the homozygous state. In total, we characterized 64
independent T-DNA insertional lines from the Salk collection and found that 12 of the 64
lines (19%) had chromosomal translocations.