Review of the Buddleja x weyeriana hybrids and future prospects for the introgression of yellow flower colour into Buddleja davidii
Updated March 2014.
Buddleja globosa and Buddleja davidii
Flower Colour in Buddleja
Van de Weyer's Original Plants
The original report of van de Weyer (1920) makes fascinating reading. His B. globosa x B. davidii F1 generation were virtually all greyish white tinted with lilac but for one yellowish-white near-sterile hybrid, and had 'ball-shaped' flowers. He selected one grey-white plant crossed presumably to a sibling or self-pollinated, the latter assuming the self-incompatibility of B. davidii wasn't inherited (Chen et al, 2011), to collect seeds from to form his F2. The F2 population was a mixed bag: grey, violet flowers and yellowish flowers in ball-like clusters borne on spikes, as well as a larger number of plants with violet panicles much like B. davidii var. Magnifica. 'Golden Glow'(Fig. 6) was his chosen best plant to exhibit from the former group. He also sowed a F3 generation but there are no details about these, but it is intriguing to speculate that 'Moonlight' (Fig. 7), first exhibited in 1923 (Anon., 1923), may in fact be from this F3 population although no details are given. Other than these two named cultivars his plants are now lost.
Fig. 7: Moonlight, another one of Weyer's original plants.
It could be assumed that B.x weyeriana would be triploid as the progeny of diploid and tetraploid parent species, however it has been known for some time that it is tetraploid like B. davidii with 76 chromosomes (Moore 1960). It also has a somewhat larger genome that either parent as determined by the DNA content of the cell, being some 26% greater than B.davidii (Van Laere et al, 2009). Genomic in situ hybridisation (GISH) analysis of the chromosomes from B.x weyeriana Sungold revealed that of the 76 chromosomes 36 were from B. davidii, 28 from B. globosa and 12 recombinant chromosomes (a mix of B. globosa and B. davidii) (van Laere et al, 2010). This nearly equal contribution from each parent suggests that van de Weyer's original plants were the result of so-called unreduced gametes in his B.globosa flowers and the F1 was also tetraploid. Unreduced gametes (B. globosa ovules in this case) have not undergone meiotic reduction and still have the parental complement of chromosomes (2n = 38). These 2n gametes are then more inclined to successful fertilisation by the normal 1n pollen of B.davidii with an equal number of 38 chromosomes, as a mismatch can be a barrier to successful seed formation and germination (van Laere 2008).
Unreduced gametes are well documented in the diploid Asian Buddleja B. lindleyana. These have been demonstrated as pivotal in the breeding of a B. lindleyana x B. davidii hybrid (Elliot et al, 2004) and reciprocal B. davidii x B. lindleyana hybrids (van Laere et al 2008), the latter giving rise to the Buddleja 'Argus' cultivars. However this is not the case with all Buddleja hybrids between species of differing ploidys. A cross of B. davidii and B. asiatica (a tender Asian diploid species) resulted in a triploid, fully sterile hybrid released commercially as Buddleja 'Asian Moon' (Renfro et al 2007).
In order to cross B. globosa and B. davidii successfully it seems likely that one must rely on unreduced gametes being produced in B. globosa, which isn't necessarily a common occurrence. So two separate projects have come to the same conclusion: it would be a good first step to artificially produce a tetraploid B. globosa. In the work of Rose et al (2000) a cultivar CSS18 was chosen for manipulation as it reliably set seed i.e.: it was female. Colchicine (a mitotic inhibitor) was used to induce polyploidy in nodal section culture and plants micro-propagated. When mature the tetraploid plants obtained had smaller leaves, smaller flowers and shorter internodes than the diploid. Although several crosses were made and later generations were also grown at the East Malling Research Station, Kent (K. Tobutt, personal communication) no further reports have been published so we don't know how successful the progeny were.
In the work of van Leare (2011) several methods were employed to induce tetraploidy, both in seedlings and shoots, using oryzalin and trifluralin (more potent mitotic inhibitors). Many tetraploid plants were successfully propagated and one was chosen (GLO34) to pollinate B.davidii 'Nanho Alba'. GL034 must have been male as the reciprocal-cross failed and in any case the induction of tetraploidy is unlikely to also induce hermaphrodism. It should be noted here that neither report mentions that B. globosa is a dioecious species. The F1 progeny of the cross (Fig. 8) were rather different to van de Weyer's F1 plants, possibly as a result of the different B. davidii cultivar used in the cross. Of seven plants five were yellow and two purple but with evidence of yellow pigmentation. The numbers are too few to work out the genetics but certainly support the assertion that Nanho Alba is dominant heterozygous at one locus for anthocyanin suppression (Tobutt 1993). The flowers are less globose that B.x weyeriana 'Sungold', and in interrupted panicles intermediate between the species (Fig. 8). Breeding is ongoing (van Laere, personal communication) but the results have yet to be reported.
These are not the only reports of artificial chromosome doubling in Buddleja. B. madgascarensis is an African species (section: Nicodemia) with yellow panicles, although is not at all cold hardy (Leeuwenberg 1979). It also has potential to be used for the introgression of yellow colour into B. davidii, selecting resulting hybrids with good cold hardiness. A cross of B. madgascarensis x B. crispa, both diploid species, has been made and gave rise to a diploid, sterile hybrid. Sterility precludes further breeding and diploidy limits its application in crosses with B. davidii. Oryzalin was used to induce tetraploidy in the hybrid and fertility was reinstated; these tetraploid plants were successfully crossed with B.davidii 'Nanho Alba' (Dunn and Lindstrom 2007). None of the surviving seedlings of this cross are yellow but they fade to orange/yellow as the flower ages. They are not sterile and the plants are also large which limits their appeal. They are extremely drought tolerant and winter-hardiness has not been an issue (Dunn and Lindstrom, personal communication).
Van de Weyer (1920) reported that Golden Glow was not very fertile but all the B.x weyeriana cultivars do in fact produce seed sporadically. Pollen sterility is suspected for many of the cultivars. They are readily pollinated by B. davidii. A number of B.x weyeriana x B. davidii hybrids are becoming popular garden varieties: Pink Pagoda and Blue Boy from Peter Moore (Longstock Nursery); Bicolor from Mike Dirr (University of Georgia); Flutterby Grande Peach Cobbler and Vanilla from Peter Podaras (Cornell University). Each one gains its colour from the pollen (B. davidii) parent, but with a difference that seems due to a small residual persistence of yellow pigment in the corolla. Bicolor's purple colour changes to orange-bronze as the flower fades, due to the partial breakdown of anthocyanins allowing the yellow-orange pigment to show through. Flutterby Vanilla is off-white retaining almost nothing of the yellow outside of the coralla tube.
Van Laere (2008) carried out reciprocal crosses of B. davidii 'Nanho Alba' with B.x weyeriana 'Sungold', managing to extract a small amount of poorly viable Sungold pollen. The seedlings were a mix of pale purple and white, as would be expected from what is known about the genetics of white flower colour (Tobutt, 1993). Sadly, no residual yellow pigment seems to be present in the corolla in the white seedlings from this cross. I have grown a number of B.x weyeriana x B. davidii seedlings out of Honeycomb open-pollinated in a densely Buddleja-planted garden - they are mixed colours as a result and very variable in many ways. A couple of plants with pale blue flowers show a phenomenon similar to Bicolor in that the blue in the flowers would fade leaving a primrose yellow background (Fig.9), a phenomenon noted by van de Weyer in his F1 plants (Weyer, 1920). The pigments responsibles for the yellow colour of the B.x weyeriana corolla are unknown but it is probably necessary to have the gene(s) that segregates the yellow into the corolla in a B. davidii type plant such as in the seedling below (Fig. 9). The potential is there to retain yellow in the corolla and not just the corolla tube, whilst breeding out the lilac/purple anthocyanins (or perhaps more correctly breeding in their suppression) leading to yellow flowers with the inflorescence morphology of B. davidii.
Sophisticated and traditional plant breeding efforts are underway to produce the first yellow flowered ostensible B. davidii although such a plant hasn't been released to commerce yet. The tendency to sterility in hybrids may prove a real hindrance in many cases. The full range of orange flowered New World Buddlejas has yet to be utilised in hybridisations with Asiatic plants, as most of them are not common or even known to be in cultivation. Some of these may prove excellent partners for hybridisation with B. davidii: the series Stachyoides are hermaphrodite and there are tetraploid species in other series. Several are high altitude species that may imbue cold hardiness (Norman 2000).
The alternative path to success may be traditional plant breeding with the B.x weyeriana x B. davidii hybrids but the fertility status of many of these plants is a disadvantage. The potential does appear to be there to breed at the very least a primrose yellow B. davidii-type plant. Whichever method is ultimately successful we eagerly await such a cultivar.
My thanks for Katrijn van Laere (Instituut voor Landbouw- en Visserijonderzoek / Institute for Agricultural and Fisheries Research, Belgium) for checking the text for accuracy and to Ann Croft for the photo of the Sungold reversion.
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