Isoplexis

Isoplexis is a section of 4 species within the genus "Digitalis". The species of section Isoplexis differ from other plants in the genus "Digitalis" in that their monosymmetric (sometimes called zygomorphic) flowers have a distinctive large upper lip rather than large lower lip and the species are endemic to the Canary Islands (the species "D. canariensis", "D. chalcantha", and "D. isabelliana") and Madeira ("D. sceptrum").

Two "Isoplexis" species were first described by Linnaeus in 1753 as part of the genus "Digitalis", these were "D. canariensis" and "D. sceptrum". Since then the section has undergone several changes, the addition of two more species and more importantly being moved to a separate genus, under the genus name of "Isoplexis" or sometimes "Callianassa", back and forth many times (Lindley 1821, Loudon 1829, Bentham 1835, Webb 1845, Wetstein 1891, Himmelbaeur and Zwillinger 1927, Werner 1960-1966, Heywood 1972). The position of "Isoplexis" as a section within "Digitalis" was finally proven by Carvalho in 1999 using molecular data and published by Brauchler et al. in 2004."Isoplexis" species grow in woody habitats: "D.canariensis" in humid woodland areas and rarely in dry woodland areas, "D.isabelliana" in "Pinus canariensis" woodland and open disturbed areas," D.sceptrum" in the cloud zone community "Clethro-Laurion" (Sjogren 1972) near streams on steep slopes, and "D.chalcantha" in Canary Island laurel forest.The flowers of "Isoplexis" species appear to be adapted for bird pollination. It was once thought that the original pollinators of "Isoplexis" and the other Canarian bird pollinated plants (such as members of the genera "Canarina" and "Lotus") were sunbirds which had become extinct on the Canary Islands; this might explain why "Isoplexis" species are rare and considered endangered species (Vogel 1954; Vogel et al. 1984; Valido et al. 2004). However more recent work has shown that these plants are adequately pollinated by non-specialist flower visiting birds, particularly the Canary Island chiffchaff ("Phylloscopus canariensis"), and the Canary Sardinian warbler ("Sylvia melanocephala leucogastre") (Olesen 1985, Ollerton et al. 2008), and in fact show some specific adaptations to infrequent pollination by these birds, such as extended flower lifespans (Ollerton et al. 2008), and a hexose-dominated sugar ratio in the composition of the nectar (Dupont et al. 2004).

Some of this text has been modified from Toomey, N (2004) Investigation of the phylogenetic utility of Cycloidea-like genes in various angiosperm lineages. PhD Thesis, The University of Reading, England.

References

*Brauchler, C, Meimberg, H, Heubl, G (2004) Molecular Phylogeny of the genera Digitalis. L and Isoplexis (Lindley) Loudon (Veronicaceae) based on ITS- and trnL-F sequences. Plant Systematics and Evolution 248(1-4) 111-128

*Bentham, G (1835) In: Edwards’s Botanical Register (Lindley, J.). Vol. 21 pp. [I] , [9, ind.] ad 1770 [3] . 1 Jun 1835, London

*Carvalho, JASS (1999) Systematic studies of the genera Digitalis L. and Isoplexis (Lindl.) Loud. (Scrophulariaceae: Digitaleae) and conservation of Isoplexis species. PhD thesis, The University of Reading, England

*Dupont, YL, Hansen, DM, Rasmussen, JT & Olesen, JM (2004) Evolutionary changes in nectar sugar composition associated with switches between bird and insect pollination: the Canarian bird-flower element revisited. Functional Ecology 18 670-676

*Heywood, VH (1972) Flora Europaea: notolae systematicae ad floram Europaen spectantes no. 13. Scrophulariaceae: Digitalis L. Sect. Macranthae Heywood, sect nov. Botanical Journal of the Linnean Society 65 (4) 357

*Himmelbaeur, W, Zwillinger, E (1927) Biologische-chemische Formenkreise in der Gattung Digitalis L. Biologia Generalis 3 595-684

*Lindley, J (1821) Digitalium Monographia. H. H. Bohte, Londini, pg 27

*Linnaeus, CV (1753) Species Plantarum. Vol II 561-1200. Impensis Laurentii Salvii, Holmaniae.

*Loudon, JC (1829) Encyclopaedia of Plants. 528 528-529

*Olesen, JM 1985. The Macaronesian bird-flower element and its relation to bird and bee opportunists. The Botanical Journal of the Linnean Society 91: 395-414

*Ollerton, J, Cranmer, L, Stelzer, R, Sullivan, S, Chittka, L (2008) Bird pollination of Canary Island endemic plants. Nature Precedings

*Sjögren, E (1972) Local climatic conditions and zonation of vegetation on Madeira. Agrronomia Lusitana 36(2) 95-139

*Valido A, Dupont YL, Olesen JM (2004) Bird-flower interactions in the Macaronesian islands. Journal of Biogeography 31: 1945-1953

*Vogel, S (1954) Blütenbiologische Typen als Elemente der Sippengliederung. Botanische Studien (Jena) 1: 1-338

*Vogel S, Westerkamp C, Thiel B, Gessner K (1984) Ornithophilie auf den Canarischen Inseln. Plant Systematics and Evolution 146: 225-248

*Webb, P. B (1845) Histoire Naturelle des Isles Canaries 3 (2-3) 144

*Werner, K (1960) Zur nomenklatur und taxonomie von Digitalis L. Botanische Jahrbucher fur Systematic, Pflanzengeschichte und Pflanzengeographie 79(2) 218-254

*Werner, K (1962) Die kultivieten Digitalis – Arten. Feddes Repertorium 70 167-182

*Werner, K (1964) Die verbreitung der Digitalis – Arten. Wiss. Z. Marin-Luther Univ. Halle-Wittenberg, Math. – Naturwiss. Reihe 13 453-486

*Werner, K (1965) Taxonomie und phylogenie der gattungen Isoplexis (Lindl.) Benth und Digitalis L. Feddes Repertorium 70 109-135

*Werner, K (1966) Die wuschformen der gattungen Isoplexis (Lindl.) Benth. und Digitalis L. Botanische Jahrbucher fur Systematic, Pflanzengeschichte und Pflanzengeographie 85(1) 88-149

*Wettstein, V (1891) Scrophulariaceae. Rhinantoideae-Digitaleae. In: Die Naturlichen Pflanzenfamilien IV 3b (Engler, A., Prantl, K. Eds.) pp.83-90 Wilhelm Engelmann, Leipzig