The Bifrost Eucalyptus project
Research, context and planning
In the scientific tradition, it is common to use mythological or fictional names to illustrate an object or a concept (for example celestial bodies in astronomy, genes in developmental biology). I have chosen symbolism from old Norse mythology, which nicely serves as a metaphor for the material and the aims of the project. Bifrost was the rainbow bridge between the mortal realm (Midgård) and the realm of the gods (Asgård) in Norse mythology. The rainbow bridge is a commonly used metaphor and symbol for a transition from the material and mundane to the ideal and fantastic. The rainbow also symbolizes diversity, hope, peace and LGBTQ(…) rights. The rainbow has often been used as a symbolic bridge spanning not only space but also time, for example Michael Jones McKean’s The Rainbow (www.therainbow.org). My artistic take on the rainbow starts from the rainbow gum tree Eucalyptus deglupta, an imposing (>60m) tree with striking rainbow-colored bark. The rainbow gum is unusual in that it is one of the few species of Eucalyptus growing naturally outside of Australia and in the northern hemisphere (close to the equator). Attempts at growing it outside its natural habitat are hampered by its frost sensitivity, and it can only grow in USDA hardiness zones 9 or higher , and the tree does not reach its full potential grandeur even in warmer climates like California due to periods of cold challenge. Until recently, the rainbow gum tree was classified as the subgenus Minutifructus, but molecular studies have placed it in the Symphyomyrtus subgenus  – the subgenus with the largest number of Eucalyptus species. The subgenus placement is relevant, since interspecific Eucalyptus hybrids have been successful within subgenera but not between subgenera . More significantly, what was once thought to be a rare case of intersubgeneric hybridization between E. deglupta and other Symphyomyrtus eucaluptus species has already been reported . Another member of the Symphyomyrtus subgenus is the cider gum Eucalyptus gunnii, which (by Eucalyptus standards) is a cold hardy plant tolerating temperatures down to between -10oC and -20oC, depending on individual variation within the species (Figure 1). A French breeding program is using E. gunnii, and especially the extra cold-hardy endangered subspecies divaricata (Miena or “Blue Ice” cider gum) for generation of resilient and high-productivity E. gunnii X E. dalrympleana (Gundal) hybrids . The snow eucalyptus Eucalyptus pauciflora has a greater cold hardiness, but belongs to another subgenus (former: Monocalyptus, now: Eucalyptus), which means that hybridization would be more difficult , and possibly involve advanced technologies like embryo rescue in vitro culture.
The Bifrost Eucalyptus project has three aims (Figure 2) : (a) to introgress the cold hardiness from cider gum (E. gunnii) into the rainbow gum (E. deglupta), and (b) to explore the phenotypic variation in multi-generation intercrosses where equal contribution from both species is maintained – in order to combine and refine the best traits of both species.A third aim (c) is to introgress the rainbow bark trait from E. deglupta into the much smaller and hardier E. gunnii.All these aims are initiated using extremely low-tech equipment and technologies (grafting  and cut style manual pollination [47-48]) in a residential setting in Merelbeke, Belgium with a hope for a future distributed development model at multiple locations.
Copyright and participatory development
Biological material can in many ways be compared to digital creations – especially for plants and other organisms that can be clonally reproduced , since this produces a genetically identical copy. As a consequence of this, I aim to follow an open source development model where I encourage others to either independently replicate what I am planning to do or to take material from me and breed further according to their own ideas and needs (in open source software terms : “forking” the project). This open source participatory “bazaar” model has proven to be very successful in software development . The copyright situation for biological material is not quite the same as for other artistic works, since it often is covered by the Plant Breeders Rights. In order to stay compatible with this system and still provide a free license which allows for further independent commercial development, I am opting for a very permissive distribution of biological material under terms similar to Creative Commons or the GNU general public license [51-52]. The Open Source Seed Initiative (OSSI) share these ideals and already have a copyleft license in place which I will make use of (http://osseeds.org/) .Seeds and clones will be freely distributed (except distribution costs) from each generation in order to encourage the project to branch out and take unexpected paths by other people. This free distribution will also ensure that seeds and clones are challenged in a wide range of environments by enthusiasts. At later stages in the project, much larger areas will be needed for screening and selection of interesting individual plants, which can be enabled by a distributed development model.
Planned Execution phase 1 : cheating Heimdall
To take the Bifrost metaphor one step further, the Norse god Heimdall (the name is believed by some to be a composite word of heim “world” and dallr “flowering trees” ) is an appropriate symbol for the early challenges in this project – having an unusual birth (by 9 women) and being gatekeeper on the rainbow bridge Bifrost (which could symbolize pre-zygotic reproductive barriers). In contrast to crossing inbred cultivars, the phenotypic effect of crossing outbred Eucalyptus is not always predictable due to large intra-species genetic variation, which means that many F1 hybrid individuals have to be phenotypically evaluated. Crossing two inbred cultivars usually has major advantages (hybrid vigor, heterosis), since it has been known for a long time that detrimental traits typically are recessive . The inbred Crossing two species with vastly different environmental adaptions can however lead to so-called outbreeding depression, which could either be that the intermediate phenotype of the hybrid is mal-adapted to the intended environment or that there are genetic incompatibilities leading to inviability of the offspring (a post-zygotic hybridization barrier). Experiences with interspecific Eucalyptus hybrids for cold resistance has shown that the F1 hybrids typically show an intermediate cold resistance with a slight bias towards the most sensitive parent . Because of this, it is unclear if the F1 hybrids will be able to survive a Belgian winter, which means that some clones will be saved in pots indoors to preserve successfully generated F1 hybrids.
Planned Execution phase 2a : Cold-hardy rainbow gum – a pot of gold at the end of the rainbow?
If some F1 hybrids can survive a Belgian winter, cold-hardy F1 Bifrost clones (first generation E. gunnii X E. deglupta) will be back-crossed to a non-parental E. deglupta in order to avoid inbreeding depression, and the offspring will be screened again for cold hardiness. This process will be repeated iteratively until most characteristic traits of E. deglupta are present. At that stage, independent cold-hardy backcross-lines will be intercrossed in order to attempt to further enhance the cold resistance. Intermediate generations of this breeding trajectory will be fed into the “Execution phase 2b” intercross population. If this breeding is successful, it could become an economically interesting plant for forestry applications, which means that it theoretically could be possible to get rainbow tree forests in cold climates. Imagine a cold winter morning in a forest of imposing huge rainbow-colored trees, a clear blue sky visible through the leaf ceiling, the ground covered with frost or snow. This experience alone would be an art installation.
Planned Execution phase 2b : Advanced intercross breeding and selection – finding offspring superior to the parents through transgressive segregation?
This breeding strategy aims to maintain approximately equal genetic contribution from both parental species and select F2 and later offspring based on combinations of the best or most attractive features from both species. If F1 offspring is too cold sensitive to survive a Belgian winter, intercrosses from backup clones kept indoors will be used to generate a much more diverse F2 population. While cold hardiness from E. gunnii and the rainbow bark from E. deglupta are the two major features to select for, there are other properties like the aromatic leaves and tasty sap from E. gunnii and the fast growth rate and impressive size of E. deglupta that also could be selected for. A segregating population from a cross of a very cold-hardy (E. gunnii) and a very cold-sensitive species (E. deglupta) could also be interesting for scientific studies of the genetics of cold hardiness.
Planned Execution phase 2c : Rainbow-colored cider gum – an appropriately sized and attractive garden ornament?
The huge size of E. deglupta makes it a problematic plant to grow in a regular residential garden. Because of this, a smaller tree similar to E. gunnii but with the attractive rainbow-colored bark could be a very interesting plant for ornamental purposes. If the rainbow bark is a (co-)dominant trait, cold hardy Bifrost F1 hybrids will thus be back-crossed to E. gunnii (cultivar: “azura”) to make a small and hardy rainbow-colored tree. After sufficient back-crosses, intercrosses of independent pedigrees of trees will be done to ensure the genetic diversity while maximizing the effect from the rainbow bark. If the F1 hybrids do not show rainbow-colored bark, a selection of breeding material will have do be done from the F2 intercross population in “Execution phase 2b” for back-crossing and subsequent inter-crossing. Material from this backcross breeding can also be fed back into the intercross populations.
This project does not have a well-defined start or end and will most likely span decades – if not centuries. Because of this, this public declaration of intent and description of its theoretical background is one possible starting point, which invites other interested parties to also pursue similar aims in collaboration or independently. This art project does not care about the “who”, only about the end results (the plants) which hopefully will outlive all people involved. At this moment, the E. gunnii parental plants are planted outside and E. deglupta seeds have been ordered on line and seeded. The E. deglupta seeds have been germinated during the winter months (January) to ensure plantlets big enough for grafting on E. gunnii rootstocks in the spring or early summer (April-May, when the lowest temperature is above 10oC). For technical details, see the supplemental information. If E. deglupta grafts are too cold-sensitive to survive the winter until flowering, I will attempt an alternative strategy: to graft buds from E. deglupta seedlings on adult E. gunnii host plants that are old enough to flower. That way, the florigen signal  from the E. gunnii host plant could induce flowering also in the E. deglupta grafts in the same season (June-August). If the time frame from grafting to flowering is too short, I will attempt multiple strategies to insulate the scions to allow them to grow further the next season. A clear advantage with this strategy is that we theoretically can go from seed to seed in a single year, significantly speeding up the breeding progress while reducing the need for large areas to grow full-sized trees for breeding. The cold sensitivity of the grafts could also be a simple high-throughput selection system for F1 and F2 generation plants, which would also solve the difficulties of having areas big enough for phenotypic evaluations in the offspring. Some phenotypic evaluations (for example, the rainbow bark and the size of the trees) will however require that trees are planted to grow to adulthood.
Future perspectives and physical expositions
This is my second “art-inspired science” project. In contrast to the first one, where I applied minimalist philosophy to the design of a small circular piece of DNA , this project has the potential to appeal to a much wider audience. In a way, this project is not limited in time nor space and every instance of trees grown from this project could be seen as part of a larger exposition presented at different scales – from single potted plants or trees in a garden to large forests. At a shorter-term, one aim will be to to try to plant a couple of the early generations (F1) Bifrost trees somewhere in the Citadelpark, Ghent, Belgium close to the modern art museum SMAK (https://smak.be/en). This site would have several symbolic values – the park is next to an actual botanical garden giving the connection to the plant sciences, the proximity to SMAK as a connection to the artistic nature of the project, and Citadelpark has a reputation to be a meeting place for the gay community , so the rainbow bark could also be a tribute to them.