Fertility of the Arilpums

According to theory, crossing tetraploid arils with I. pumila should produce fully fertile amphidiploids. In practice, the fertility of these plants has not been as complete as might be desired.

Peter Werckmeister's 'Miltonia' (from crossing I. pumila with an oncogelia seedling) was reported fertile only as a pollen parent. However, he reported eventually producing another plant of the same type, and crossing it successfully with 'Miltonia' as pod parent to produce a number of seedlings (1968). Werckmeister also raised advanced generation seedlings from crossing tetraploid regelias with I. pumila, and reported ample fertility in both directions for this group (1968).

All registered seedlings from arilpums involve the arilpum as pollen parent. This includes Lynda Miller's seedlings of Paul Cook's 'Hoogpum Purple' and Elm Jensen's seedlings from his stol-pum hybrid. However, Jensen reported that his stol-pum seedlings were fertile both ways (1981). It is also interesting that the historic cultivar 'Zwanenburg' (thought to be an AAPT unbalanced tetraploid form I. lutescens aurea and an unreduced gamete of I. susiana) is also pollen fertile, but has apparently not produced any seedlings as a pod parent.

The two registered arilpums presently in commerce, Mathes's 'Barbarella' and Thoolen's 'Aladdin's Gem' are both fertile as pollen parents only. Both raised a number of other seedlings of this type, with minimal pod fertility (1990). Mathes reported raising another arilpum seedling with enough fertility as a pod parent to produce an F2 generation. However, fertility was not improved among these progeny. Mathes eventually gave up working with the arilpums because of this fertility barrier. Thoolen ascribed her lack of further progress to not having sufficient space to raise the number of seedlings that would be needed.

I recently noted from several seed exchange lists that Lawrence Ransom has produced seedlings from Tadzhiki Bandit x I. pumila, with both the F1 and F2 producing seeds, so there must be reasonable fertility in his arilpum line. (Note: Mr. Ransom has recently communicated to me that he does not believe I. pumila is the actual parent of these seedlings.)

The fact that most arilpums are readily pollen fertile indicates that there is no important problem in chromosome pairing involved; the arilpums behave as amphidiploids, as theory predicts. The difficulty must be a physiological one in the production of fertile ovules. Werckmeister described this as a plastid infertility, but did not elaborate on this explanation.

While pondering on these difficulties, I recalled that the earliest AATT amphidiploid arilbreds, 'Ib-Mac' and 'Capitola', were also fertile only as pollen parents. It was not until the varieties of C. G. White appeared from his large-scale breeding program that the fertility barrier was finally broken down completely.

All this leads me to believe that breeding pod fertile arilpums will be difficult, but not impossible. Persistence and the raising of large numbers of seedlings will be needed. I'm also hopeful that some benefit can be obtained by using a wide range of both pumila and aril parents, particularly the regelias, since regelia-pum hybrids have not been reported as having fertility problems.

With 'Barbarella' and 'Aladdin's Gem' available, I think arilpum breeding is now at a stage comparable to that of arilbred breeding circa 1945, when 'Ib-Mac' and 'Capitola' were the only reliably fertile arilbreds, and the fertile family had not yet gotten off the ground. If enough work can be done with the arilpums, I am optimistic that the next couple decades will see the fertility barriers broken down. The more hybridizers who pursue this goal with determination, the quicker success can be achieved.

References

All reports cited above are from the ASI Yearbook of the indicated year.

Tom Waters

December 2010

updated March 2014

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