Rumex, Linnaeus, 1753

Hybrids in Rumex View in CoL and considerations on the taxonomic and nomenclatural status of the Shchavnat Hybrid Dock

It is commonly accepted (or at least supposed) that the processes of hybridization played a considerable role in the evolution and diversification of the genus Rumex . Hybrids, representing various interspecific hybrid combinations, are frequently reported in many floras and taxonomic treatments ( Danser 1924a, Rechinger 1984, Elven et al. 2000, Holyoak 2000, Stace 2010, Stace et al. 2015, Preston & Pearman 2015, Pliszko 2019, etc.). For example, 47 hybrid combinations were reported by Elven et al. (2000: 316–318) for the Flora Nordica area ( Finland, Denmark excluding Greenland, Iceland, Norway, Sweden). Stace (2010: 447–452) reported for the flora of the British Isles 43 interspecific hybrids of Rumex . The widespread and ecologically successful species seem to be able to form numerous hybrid combinations: for example, Rumex crispus Linnaeus (1753: 335) is parent to 14 of about 40 Rumex hybrids of the British flora ( Brown et al. 2023). Forty-nine hybrid combinations were listed in the treatment of Rumex for the flora of Eastern Europe ( Borodina 1996); however, at least some of them were mentioned only as possibly occurring, without actual confirmed records from the area.

In should be noted that most of hybrids or nothospecies reported in the genus from various parts of the world were described or identified based mainly or exclusively on morphological characters and co-occurrence of the supposed parental species. As Bhandari & Park (2022, art. 5423: 1) promptly noted, “Several hypothesized combinations of parental species of hybrids based on their intermediate morphology have been suggested in the genus, but few of them have been phylogenetically tested”. Thus, only several hybrid combinations in Rumex have been experimentally, phylogenetically and/or genetically confirmed beyond any reasonable doubt ( Ziburski et al. 1986, Ruhsam et al. 2015, Bhandari & Park 2022 and references therein).

Considering hybridization in the group of species discussed here, Rechinger described a supposed hybrid between Rumex chalepensis Miller (1768 : without pagination; RUM n. 11 https://www.biodiversitylibrary.org/page/ 394481#page/1009/mode/1up) and R.tianschanicus (cited as ‘ thjanschanicus ’) as R. × khorasanicus Rech. f. ( Rechinger 1950: 125). Also, hybrids with participation of R. paulsenianus have been described: Rumex × kaschmirianus Rech. f. (Rechinger in Qaiser 2001: 163) derived from R. nepalensis Sprengel (1825: 159) × R. paulsenianus , and Rumex × munshii Rech. f. (Rechinger in Qaiser 2001: 162) derived from R. punjabensis K.M. Vaid & H.B. Naithani (1979: 802) × R. paulsenianus .

It is usually admitted that interspecific hybrids in Rumex are often either sterile or at least less fertile than the parental species; in addition to their intermediate morphology, hybrids often form underdeveloped pollen and fruits/ seeds, or at least produce less fruits/seeds than parent or related species. Rechinger (1984: 78) noted that “in nature hybrids of Rumex may be recognized by their appearance. The primary panicles sheds [shed—SM] most of the sterile flowers but the plants continue to grow and tend to form secondary flowering panicles. Thus the hybrids are often taller than the parents and assume an untidy habit. Additionally most hybrids fail to set fruit because the flowers dry and fall off before full development of the valves (i.e. the three inner perianth segments) occurs. The few valves which do reach full size exhibit the characters of the parents in various ways. Often these hybrid nuts, even if reaching full size, are not viable and can be compressed between the fingers” [also reproduced verbatim in the Flora of Pakistan ( Qaiser 2001: http://www.efloras.org/florataxon.aspx?flora_id=5&taxon_id=128864)]. Stace (2010: 444) mentioned that “All the hybrids [in Rumex —SM] are to some degree sterile, most highly so, with undeveloped achenes; they are not rare, but mostly occur as single or few plants and almost always with 1 or both parents”. Bhandari & Park (2022, art. 5423: 1) also confirmed that “Although interspecific hybrids have been reported frequently in subgenus Rumex , hybridization has not been recorded between species in different subgenera. Hybrids are often morphologically intermediate between the putative parents and generally exhibit partial or almost complete sterility. Spontaneous hybrids between species of Rumex are usually less fertile and ecologically successful than the parental species”.

In contrast to that, the cultivars of the Hybrid Dock derived from Rumex patientia × R. tianschanicus are rather stable in their morphological characters. In our opinion, it indicates that (1) R. tianschanicus and R. patientia are closely related taxa, both belonging to Rumex subgen. Rumex sect. Rumex ; and (2) their hybrids representing now various cultivars of the Hybrid Dock are genetically and morphologically stabilized and fully fertile. Moreover, despite its varietal diversity (see Rakhmetov et al. 2003, Rakhmetov & Rakhmetova 2006), the Hybrid Dock seems to be genetically rather uniform ( Zhang et al. 2004). For these reasons, we prefer here to describe the Hybrid Dock not as a species-rank hybrid / nothospecies but as a stabilized hybridogenous species.

One option that we also considered was coining a name for a nothospecies, in the sense of Art. 3.2 and Art. H.1.1 of the ICN (International Code of Nomenclature for algae, fungi, and plants: Turland et al. 2018). However, the distinctions between a hybrid or nothospecies, on the one hand, and a hybridogenous species, on the other hand, are rather elusive, and they are not defined in the ICN. For comparison, consider the usage of concepts of hybrids (nothospecies) and hybridogenous species as applied to European taxa of Sorbus Linnaeus (1753: 477) and its related genera in the treatment by Sennikov & Kurtto (2017), who recognized in their taxonomic treatment both hybrids and hybridogenous species (which have not been treated as nothospecies). We considered these cases ( Mosyakin et al. 2022) and proposed a preliminary definition in the context of the ongoing discussion on the concept of nothogenus versus hybridogenous genus: “For the purposes of this Code, a hybridogenous genus is a genus that is derived from and has evolved from an intergeneric hybridization event or events and that contains one or more hybridogenous species, i.e. evolutionarily stabilized species that, although of hybrid origin, are regularly treated similarly to other species of non-hybrid origin [emphasis added—SM]; however, a hybridogenous genus may also contain hybrids (including intergeneric ones)” ( Mosyakin & McNeill 2023: 462; see also comments by Turland & Wiersema 2024: 394).