"For example, I was unable to get a dip cross from Dixie Land Band its first summer, so I tried some tet crosses with it and got tons of seedlings. Nowhere is it listed as a tet, so it must have been quietly converted and may now be the predominant form that is being sold. It is not the only "dip" cultivar that I've found to be a "closet tet," and since no one keeps track of tet conversions, from now on I am going to routinely cross all unsuccessful "dips" with tets."
There are other possible explanations.
Dixie Land Band may have been produced by a cross of two diploid daylilies. It may have low fertility with diploids but better fertility with tetraploids. That could mean that it is a triploid that occurred naturally. There are no tetraploids registered with Dixie Land Band as a parent; that suggests that it was not converted.
Some of the older cultivars are not many generations removed from having triploids such as Europa as their parents. If one uses triploids as one of the parents in a cross one can produce triploid seedlings. Since there were no tetraploids to test seedlings if a seedling from such a cross was fertile with diploids it would have been registered as a diploid. Then crosses of diploids with the fertile triploid could have produced triploid seedlings that were not fertile with diploids but would have been automatically assumed to be infertile diploids.
There may be quite a few older cultivars, especially those that have low fertility, that are triploids but are registered as diploids. They may be infertile (for all practical purposes) with diploids but have noticeable fertility with modern tetraploids.
In the past, that speculation has been published in the Daylily Journal for older spidery diploid cultivars. ["So where are the seeds already?" by Rosemary Whitacre in the Daylily Journal 46 1991 pages 281-289. Names 'Pinky Green' registered as a diploid as not accepting diploid pollen but accepting tetraploid pollen successfully and both Madrid and 'Beau Soleil' as chimeras. A quote from that article, "The thing about these old spiders that many people do not know is that quite a few are irregular diploids. This means they are not quite the standard 22 chromosome jobs. Wheeler's, Russell's and Carl Milliken's daylilies all have among them irregular polyploids - usually aneuploids - but some are outright triploids or even somewhat irregular spontaneous tets. This is the heritage of Hemerocallis fulva 'Europa'." ]
{Aneuploid means not having an even number of chromosome sets - for daylilies that would mean not having 22, 33 or 44 chromosomes; an aneuploid daylily might have 23 or 21, chromosomes, or it might have 22 chromosomes but made up of nine pairs (18) plus one single (1) and one triple (3) instead of the correct two sets of 11 or 11 pairs.}
A cultivar that is infertile with diploids but fertile with tetraploids may be a tetraploid but it may also be a triploid.
