Physcomitrella patens, recently renamed Aphanoregma patens, has been transformed with the plasmid, pBI426.
On selective medium approx. 30% of regenerants expressed the transformed phenotype transiently (transients).
The remaining 70% (transformants) retained their transformed phenotype (GUS-positive and resistant to G418)
indefinitely when subcultured repeatedly on selective medium. However, most lost this phenotype after one or two
passages through nonselective medium (unstable transformants). Approximately 0.2% of transformants retained
their transformed phenotype after numerous passages through nonselective medium (stable transformants). Using
PCR methodology, it has been shown that loss of the transformed phenotype by unstable transformants is
invariably accompanied by disappearance of the transgenic DNA. Southern blot analysis data argue strongly that
unstable transformants cultured under selective conditions contain unintegrated pBI426 as circular concatenates
consisting of 3–40 copies of the plasmid. Under selective conditions, it appears that replication and/or partitioning
of these extrachromosomal concatemers might be growth rate-limiting. This is the first report of a transgenic,
autonomously replicating extrachromosomal element in a photosynthetic plant. A single copy of pBI426 has been
inserted into the moss genome in each of three stable transformants analysed.