When I first started breeding with converted tets, spontaneous doubling of chrom. was about 1/1000 unfertilised gametes. The likelihood of both a ovum and a pollen grain doubling and mating is 1 in a million. Your tet. likely came from an OP from an other tet rose. Perhaps you guys working with Max Graf should pollenate heavily with a very fertile tetraploid to catch these 1/1000 doubling. More reliable than bees doing it?
Just to recap here, given the information made available to me…
[u]David Z’s 3K20 rose (a confirmed tetraploid) very likely has the following parentage:
[/u]
Max Graf (a confirmed diploid) X John Davis (David Z’s specimen is apparently a confirmed triploid).
…and to be honest, none of this surprises me regarding the chromosomal behavior of Max Graf with other roses in hybridization from what I have read about it.