Thunbergia is a genus from the Old World tropics (mainly Africa and Asia). As currently circumscribed it counts ca. 90-100 species. At least nine species are cultivated as ornamentals in Europe (Brown & Hyam 2000) but all are frost-sensitive and unlikely to survive out-of-doors in western Europe. One species, Thunbergia alata (black-eyed Susan), is a very popular ornamental in Belgium as well and has been recorded as an escape.
The monophyly of Thunbergia is supported by molecular data (Borg & McDade 2008) as well as by morphology (Schönenberger & Endress 1998).
Borg A.J. & McDade L.A. (2008) Molecular phylogenetics and morphological evolution of Thunbergioideae (Acanthaceae). Taxon 57(3): 811-822.
Brown N. & Hyam R.D. (2000) Thunbergia. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 356-358.
Retief E. & Reyneke W.F. (1984) The genus Thunbergia in southern Africa. Bothalia 15(1 & 2): 107-116.
Schönenberger J. & Endress P.K. (1998) Structure and development of the flowers in Mendoncia, Pseudocalyx, and Thunbergia (Acanthaceae) and their systematic implications. Int. J. Pl. Sci. 159: 446-465.