There are two commonly grown types of aril irises: the oncocyclus that range from the southern Caucasus through Syria, Jordan, Lebanon, and Israel, and the regelias from Turkestan. The oncocyclus group includes the most spectacular flowers in the genus Iris, and has inspired the devotion of enthusiasts for centuries. The regelias, although less dramatic, are nonetheless interesting irises, with their elongated petals and unusual coloring. Two of the regelia species, Ii. hoogiana and stolonifera, are natural tetraploids with 44 chromosomes. The other regelias are diploids, as are all oncocycli, with 20 chromosomes. Although the regelias have chromosomes in sets of 11 and the oncocycli in sets of 10, the chromosomes pair readily and diploid or tetraploid hybrids are usually perfectly fertile.
Tetraploid arils would be expected to produce fertile seedlings when crossed with the 48-chromosome bearded irises or with I. pumila, thus bypassing the sterility barriers that arise when the diploid oncocycli are used. There are a few tetraploid aril hybrids with some oncocyclus ancestry, of which Persian Pansy is perhaps the best example. In the 1970s and 1980s, John Holden and Sam Norris used colchicine to induce tetraploidy in oncocyclus and oncogelia hybrids.The process is a difficult one, as many seedlings either die or revert to the diploid state. Eventually, though, a number of tetraploid seedlings of mostly oncocyclus ancestry were produced, and a small handful were named and registered.
It is unlikely that anyone will take up the work with colchicine again soon, so expanding the genetic base of this family requires crossing diploids with the available tetraploids, and hoping for an unreduced gamete of a triploid seedling with enough fertility to continue the line. This is extremely important work, not only to facilitate arilbred breeding, but because the oncocycli have very narrow habitats in a region of the world threatened by war and development.
My own priority with this group is to cross them with as many different oncocyclus species and hybrids as I am able to grow, in the hope of obtaining an occasional tetraploid. Given the difficulty of the crosses involved and the expected low germination rates, this is work that will require a great deal of patience.
I've compiled a listing of tetraploid arils that may be of use in building a collection of these irises.