In 1955 (in translation, ASI Yearbook 1966), Marc Simonet reported a chromosome count of 42, and suggested that the stated parentage was in error, and that the pollen parent was 'Ib-Mac', which Denis grew and used extensively. This would make it an unbalanced tetraploid of the APTT type. Simonet also cited the general resemblance of 'Zwanenburg' to other arilbreds from I. lutescens x 'Ib-Mac'. I would have to concur with the observation. On an established clump, 'Zwanenburg' produces stalks with two terminal buds plus a full branch (sometimes reduced to a spur). This seems unlikely if the single-flowered I. susiana were the parent, as lutescens-type dwarfs have just one or two terminal buds.
In the 1978 ASI Yearbook, however, Randolph and Mitra reported counting 'Zwanenburg' as 2n=40. Their interpretation was that the published parentage is correct, and that I. susiana contributed an unreduced gamete, making 'Zwanenburg' AAPT. Interestingly, Sidney Mitchell stated that he believed the correct parentage is I. susiana X 'Statellae' (also a dwarf of the I. lutescens type). This adds plausibility to the unreduced gamete interpretation. Note that the remark "counted as an amphidiploid", which has appeared in several earlier ASI checklists, is not correct in either case, because the two sets of bearded chromosomes from I. lutescens are different from each other (PT).
Who is correct? I initially tended to follow Randolph and Mitra. With their being aware of Simonet's count, we can assume they would take extra care to ensure that their contradictory result was correct. Their interpretation also has the advantage of being consistent with the published parentage (and Mitchell's suggested correction). On the other hand, Simonet's suggestion cannot be ruled out. 'Zwanenburg' produces rather tall stems, often bearing a branch. Although this is not impossible for a hybrid coming solely from I. lutescens and I. susiana, it seems very unlikely. The stem is much more like what one would expect from Simonet's suggested parentage.
The two chromosome configurations should result in different breeding behaviors. While either would be expected to produce AT gametes about half the time, if Simonet is correct the other half should be PT, whereas if Randolph and Mitra are correct, the other half should be AP. If Simonet is correct, crossing 'Zwanenburg' with SDBs should produce some fertile SDB seedlings. If Randolph and Mitra are correct, crossing 'Zwanenburg' with arilpums should produce some fertile arilpum seedlings. Alas, the latter hypothesis cannot be readily tested, since the only arilpums presently available, 'Barbarella' and 'Aladdin's Gem' are, like 'Zwanenburg', only fertile as pollen parent. (Werckmeister did report once getting a pod with two viable seeds from 'Zwanenburg' during an especially warm and dry season.)
Of the registered offspring of 'Zwanenburg', five were produced by using its pollen on SDBs, and two from TBs. One of the SDB x 'Zwanenburg' seedlings, 'Brownie', was registered as an MDB, is fertile both ways, and was subsequently used to produce other MDBs, SDBs and IBs. Although not conclusive, this suggests that Simonet may have been correct. For confirmation, one would need to produce a substantial number of SDB x 'Zwanenburg' seedlings and test them for fertility.
updated November 2017
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