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.
September 2010