Thunbergioideae

Introduction

The acanthaceous subfamily Thunbergioideae consist of around 170 species distributed widely throughout tropical and subtropical regions of the world. Of the five presently recognized genera (Scotland & Vollesen 2000), Thunbergia and Mendoncia contain almost all the species, ca. 100 and 60, respectively. Thunbergia is well represented in tropical and subtropical regions of Africa and Madagascar, Asia and Australia. The range of Mendoncia partially overlaps with that of Thunbergia, with a few species native to tropical Africa and Madagascar, but the greatest diversity is found in tropical America. The remaining genera are Pseudocalyx, with around 8 species in tropical Africa, and monotypic Anomacanthus (tropical Africa and Madagascar) and Meyenia (India and Sri Lanka).

Characteristics

Thunbergioideae are mainly climbing herbs, often with showy flowers presenting elaborated pollination mechanisms (Endress 1994). The flowers have a much reduced calyx, seen as a very small rim or crown around the floral base. Protection of the young flowers is instead provided by two enlarged bracteoles which initially enclose the bud and usually persist during flowering. The fruits of Thunbergia, Meyenia and Pseudocalyx are dry dehiscent capsules, but they differ from those of core Acanthaceae in that they lack retinacula (hook-like modified funiculi). Furthermore, the fruits of Mendoncia and Anomacanthus are unique among Acanthaceae in being fleshy indehiscent drupes. The anthers of Thunbergioideae are distinctive in that they lack endothecium (a layer of cells involved in anther dehiscence), and the thecae open by pores or short slits, which in some Thunbergia have evolved into long slits (Borg et al. 2006).

Discussion of Phylogenetic Relationships

The monophyly of Thunbergioideae is strongly supported by both molecular and morphological evidence. The monophyly of the three major genera Thunbergia, Mendoncia and Pseudocalyx is also supported by morphology (Schönenberger and Endress 1998) as well as DNA data (Borg et al. 2008; Borg unpublished data). The remaining two genera, Anomacanthus and Meyenia, are monotypic, although the generic status of Meyenia is disputed and some authors have considered it a species belonging to Thunbergia.

Presently, no phylogenetic analyses have included all five genera. Molecular analyses of Thunbergia, Pseudocalyx and Mendoncia (Borg et al. 2008) suggest the two former are sister groups, and together they are sister to Mendoncia. Also morphology demonstrates the close relationship between Thunbergia and Pseudocalyx. Both Thunbergia and Pseudocalyx have dry capsules, like all other Acanthaceae, whereas Mendoncia is exceptional among Acanthaceae because of the possession of fleshy drupes; a character only shared with Anomacanthus. Furthermore, the fruits of Thunbergia and Pseudocalyx have two fertile locules, while in Mendoncia only one of two initiated locules develops fully. These dissimilarities have led some authors to place Mendoncia in a separate subfamily Mendoncioideae (Lindau 1895) or even a separate family Mendonciaceae (Bremekamp 1953). However, there are also characters linking Mendoncia to Thunbergia and, in particular, to Pseudocalyx. These include e.g. stigma shape and anther dehiscence, and several authors have rejected the raise of Mendoncia to family rank (e.g. Brummitt 1989). More recently, Schönenberger & Endress (1998) showed that the drupaceous fruits in Mendoncia and the dry capsules in Thunbergia and Pseudocalyx actually have an identical bauplan and that differences arise late during floral development.

References

Benoist, R. 1944. Contribution a la conaissance des Acanthacées africaines et malgaches. Notul. Syst. 11: 137-151.

Borg, A. J., McDade, L. A. and J. Schönenberger. 2005. Molecular systematics and biogeographic patterns in Thunbergioideae s.l. (Acanthaceae) based on phylogenetic analyses of nuclear and chloroplast sequence data. XVII International Botanical Congress, Vienna, Austria.

Borg, A. J., McDade, L. A. and J. Schönenberger. 2008. Molecular phylogenetics and morphological evolution of Thunbergioideae (Acanthaceae). Taxon 57: 811-822.

Bremekamp, C. E. B. 1953. The delimitation of the Acanthaceae. Proc. Kon. Ned. Akad. Wetensch. C 56: 533-546.

Bremekamp, C. E. B. 1965. Delimitation and subdivision of the Acanthaceae. Bull. Bot. Surv. India 7: 21-30.

Brummitt, R. K. 1989. Against separating Mendonciaceae from Acanthaceae. Acanthus 5: 1-3.

Endress, P. K. 1994. Diversity and evolutionary biology of tropical flowers. Cambridge University Press, Cambridge, U.K.

Lindau, G. 1895. Acanthaceae. Pp. 274-354 in: Engler, A. & Prantl, K. (eds.), Die Natürlichen Pflanzenfamilien, vol. 3b. Engelmann, Leipzig.

McDade, L. A., Masta, S. E., Moody, M. L. & E. Waters. 2000. Phylogenetic relationships Acanthaceae: Evidence from two genomes. Syst. Bot. 25: 106-121.

Schönenberger, J., & P. K. Endress. 1998. Structure and development of the flowers in Mendoncia, Pseudocalyx, and Thunbergia (Acanthaceae) and their systematic implications. Int. J. Pl. Sci. 159: 446-465.

Schönenberger, J. 1999. Structure and development of flowers and systematics in Thunbergioideae s.l. (Acanthaceae). Ph.D. dissertation, University of Zürich, Switzerland.

Scotland, R. W., & K. Vollesen. 2000. Classification of Acanthaceae. Kew Bull. 55: 513-589.

van Tieghem, M. P. 1908. Structure du pistil et de l'ovule, du fruit et de la graine des Acanthacées. Ann. Sci. Nat., Bot. 7: 1-24.