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Tree TSAR — Taxonomy, Systematics, and Review
Tree TSAR information

Hybrids, Chimeras, and Synthetic Plants

Understanding ×, +, and ∇ in Tree TSAR

Plants can combine biological material in fundamentally different ways. Two species may reproduce sexually and produce hybrid offspring. Tissues from two plants may coexist within one shoot after grafting. Somatic cells can be fused experimentally, and modern biotechnology can edit or assemble genomes in ways that ordinary reproduction would not produce.

Tree TSAR uses three symbols to keep those histories distinct:

Symbol Tree TSAR Meaning Nomenclatural Status
× Hybrid or nothotaxon arising from hybrid parentage Established botanical hybrid notation governed by the ICN
+ Graft chimera composed of genetically distinct tissues Established cultivated-plant convention
∇ Qualifying highly synthetic, engineered, or artificially assembled entity Tree TSAR editorial convention

The symbols are not decoration. They communicate something about how the organism came into being.

The Multiplication Sign: ×

The Madrid Code devotes Chapter H to hybrid nomenclature. Hybrid character can be indicated through the multiplication sign × and through the prefix notho- in the appropriate nomenclatural context (Turland et al. 2025).

At species level, the multiplication sign appears before the specific epithet, as in Prunus × yedoensis Matsum. At generic level, it precedes a nothogenus. A hybrid formula can instead display the parents directly with × between them.

A real intergeneric example is × Amelasorbus jackii Rehder, which Plants of the World Online treats as a naturally occurring hybrid of Amelanchier alnifolia and Sorbus scopulina.

The multiplication sign communicates hybrid nature. It does not, by itself, tell the reader whether the event occurred naturally or in cultivation.

A Hybrid Formula and a Hybrid Name Are Different

A hybrid formula identifies parentage:

Parent A × Parent B

A named nothotaxon gives the resulting entity its own nomenclatural name. Tree TSAR can display both when useful: the accepted name identifies the taxon, while the parentage element explains the biological relationship responsible for its origin.

Parentage is biological information. The accepted scientific name is nomenclatural information.

Hybridization Does Not Make the Parent Species Invalid

Plant species can remain morphologically, ecologically, and evolutionarily distinct while exchanging genes occasionally. Introgression can transfer parts of one genome into another lineage without causing the parental taxa to collapse into one population. Hybridization and allopolyploidy can also contribute to the origin of new species (Rieseberg & Willis 2007; Soltis & Soltis 2009).

Tree TSAR therefore does not treat the existence of hybrids as proof that the parents must be merged. The relevant questions are how much gene flow occurs, whether parental identities persist, whether the hybrid lineage is stable, and what nomenclature correctly describes the entities involved.

Hybrid Origin Can Become Evolutionary History

Some hybrid populations are transient; others recur wherever their parents meet; still others become stable lineages with their own ecology, fertility, morphology, or reproductive behavior.

A stable hybrid-origin lineage can function as an independently evolving biological unit. Its hybrid origin nevertheless remains part of its history, and biological stability does not automatically erase the nomenclatural status of a formally established nothotaxon.

Conversely, an ordinary accepted taxon can have ancient reticulate ancestry without requiring a multiplication sign merely because hybridization occurred somewhere deep in its evolutionary history.

Tree TSAR therefore keeps evolutionary origin and nomenclatural status distinct.

The Addition Sign: +

A graft chimera is not a sexual hybrid. When different plants are grafted, their tissues normally remain genetically distinct. In rare cases, shoots can develop in which persistent cell lineages from both partners coexist within one plant body.

Plant chimeras have been important developmental systems precisely because distinct genetic cell layers can remain together without forming one sexually recombined genome (Frank & Chitwood 2016).

The International Code of Nomenclature for Cultivated Plants uses the addition sign + in the nomenclature of graft chimeras. A classic example is + Laburnocytisus ‘Adamii’, involving tissues associated with Laburnum anagyroides and Cytisus purpureus (Brickell et al. 2016).

Tree TSAR uses + for this fundamentally different biological origin.

Why a Graft Chimera Is Not a Sexual Hybrid

In a conventional sexual hybrid, hereditary material from the parents is combined through reproduction. In a graft chimera, genetically distinct cell populations coexist within the same organism.

The plant may express features associated with both source taxa, but the underlying developmental structure is different. That is why Tree TSAR does not use × and + interchangeably.

× indicates hybrid parentage; + indicates chimerism.

Somatic Hybrids Are Different Again

Somatic hybridization can fuse protoplasts so that genetic material from different cells enters one hybrid cellular lineage. This can circumvent reproductive barriers that prevent ordinary sexual crossing.

Patel et al. (2011), for example, produced stable somatic hybrids involving Nicotiana × sanderae and N. debneyi through protoplast electrofusion. The resulting plants are biologically unlike graft chimeras because the source genomes are combined within hybrid cells rather than maintained in separate tissue lineages.

They are also not ordinary sexual hybrids. Artificial biological combinations therefore require attention to the actual mechanism rather than a generic label of “hybrid.”

The Nabla Symbol: ∇

Tree TSAR uses ∇, the nabla, for a narrowly defined class of synthetic, genetically engineered, or artificially assembled plants whose construction makes ordinary hybrid, cultivar, or taxonomic notation inadequate.

The ∇ marker is a Tree TSAR editorial convention. It is not part of the Madrid Code and does not create a new botanical rank.

Its purpose is explanatory. If artificial construction itself has become a major part of an entity’s biological identity, displaying that organism exactly like an ordinary evolutionary taxon can conceal important information about its origin.

Most Genetically Modified Plants Do Not Receive ∇

Genetic modification alone is not enough. Tree TSAR does not apply ∇ automatically to ordinary gene-edited plants, cisgenic plants, single-trait modifications, mutation-bred cultivars, conventional sexual hybrids, or graft chimeras.

The marker is reserved for cases with unusually extensive synthetic novelty: for example, functionally significant traits assembled from deeply divergent source organisms or constructs whose biological identity cannot honestly be represented as an ordinary product of lineage descent.

The threshold is intentionally high. If every herbicide-resistant cultivar or small gene edit received ∇, the symbol would communicate little beyond the fact that biotechnology was used.

Why × Does Not Cover Every Artificial Combination

The multiplication sign already has a specific botanical meaning. Using it indiscriminately for graft chimeras, somatic fusions, transgenic plants, and highly synthetic constructs would make very different biological processes appear equivalent.

Tree TSAR instead asks what kind of combination occurred. A conventionally named hybrid uses ×. A graft chimera uses +. A qualifying highly synthetic entity can receive ∇. Other modified plants retain their ordinary botanical or cultivated-plant names without special Tree TSAR notation.

∇ Does Not Create a New Botanical Taxon

Adding ∇ to a name does not validly publish a genus, species, cultivar, or any other formal rank. Where a qualifying organism already has an appropriate botanical or cultivated-plant name, Tree TSAR can retain that name while using ∇ as an explanatory qualifier.

The same boundary applies to other Tree TSAR editorial notation: explanation can communicate an interpretation, but it cannot substitute for formal publication under the applicable nomenclatural Code.

Why the Nabla?

Tree TSAR uses the nabla symbol ∇, not the Greek capital delta. Nabla is visually distinct from × and + and is reserved for the synthetic-plant category described here.

The three symbols therefore remain easy to distinguish both conceptually and typographically.

Symbols Describe Origin, Not Biological Worth

None of these symbols means that a plant is less real. A naturally occurring hybrid is a biological organism; a stable hybrid lineage can become an important ecological entity; a graft chimera is a genuine developmental organism; and an engineered plant can be stable and reproducible.

The symbols answer historical and biological questions about origin. Tree TSAR uses them to preserve those differences rather than rank the organisms by value.

Authoritative Tree TSAR display retains the correct Unicode symbol. Search is more forgiving. A reader who types an ordinary x instead of × should still be able to find a recognized hybrid, while the accepted page continues to display the proper multiplication sign.

This follows a general rule: search accommodates the reader; the page preserves the nomenclature.

A Classification That Can Represent Networks

Phylogenies are often drawn as trees because descent commonly creates branching patterns. Plant evolution also contains networks produced by hybridization, introgression, allopolyploidy, organellar capture, and artificial combination.

Tree TSAR does not solve that complexity by pretending those events did not occur. Hybrid taxa are marked when nomenclature calls for it, reticulate histories are described where they matter, and fundamentally different artificial combinations receive different notation.

The goal is not to make plant evolution look tidier than it is. It is to make the complexity legible.

Explore Further

References and Further Reading

Brickell CD, Alexander C, David JC, Hetterscheid WLA, Leslie AC, Malécot V, Jin X, Cubey JJ (eds.) (2016) International Code of Nomenclature for Cultivated Plants, 9th ed. Scripta Horticulturae 18. International Society for Horticultural Science. https://www.ishs.org/scripta-horticulturae/international-code-nomenclature-cultivated-plants-ninth-edition

Frank MH, Chitwood DH (2016) Plant chimeras: The good, the bad, and the ‘Bizzaria’. Developmental Biology 419(1): 41-53. https://doi.org/10.1016/j.ydbio.2016.07.003

Patel D, Power JB, Anthony P, Badakshi F, Heslop-Harrison JS, Davey MR (2011) Somatic hybrid plants of Nicotiana × sanderae (+) N. debneyi with fungal resistance to Peronospora tabacina. Annals of Botany 108(5): 809-819. https://doi.org/10.1093/aob/mcr197

Rieseberg LH, Willis JH (2007) Plant speciation. Science 317(5840): 910-914. https://doi.org/10.1126/science.1137729

Soltis PS, Soltis DE (2009) The role of hybridization in plant speciation. Annual Review of Plant Biology 60: 561-588. https://doi.org/10.1146/annurev.arplant.043008.092039

Turland NJ, Wiersema JH, Barrie FR, Gandhi KG, Gravendyck J, Greuter W, Hawksworth DL, Herendeen PS, Klopper RR, Knapp S, et al. (eds.) (2025) International Code of Nomenclature for algae, fungi, and plants (Madrid Code). Regnum Vegetabile 162. University of Chicago Press. https://www.iaptglobal.org/madrid-code-online