Acantheae

Introduction

Acantheae are well supported as the basal lineage of core Acanthaceae (i.e., those plants with retinaculate fruits) and also as a clade.  The group has  about 500 species that are currently placed in about 20 genera (but this taxonomy does not entirely reflect our recent phylogenetic results:  McDade et al. 2005).  Plants belonging to this lineage occur in both the New and Old Worlds; in the Old World, the vast majority of species occur in Africa with just a few in southern Europe and in Asia.

Characteristics

Among Acanthaceae, Acantheae are marked by the synapomorphy of monothecate anthers.  A few other distantly related lineages of Acanthaceae have monothecate stamens (e.g., core Isoglossinae of Justicieae, McDade et al. 2000a, Kiel et al. 2006) but, in these lineages, the loss of one theca per stamen seems to have occurred after reduction to two stamens.  In contrast, Acantheae retain the plesiomorphic trait of four stamens, and this combination of stamen (four) and thecae (one) number is, to our knowledge, unique among Acanthaceae. 

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Note the four stamens, each with a single theca or anther sac.  This combination of traits is synapomorphic for Acantheae. © Lucinda McDade

Most New World Acantheae, members of the Stenandriopsis clade, Crossandra, Sclerochiton, and Streptosiphon have a five-lobed calyx with at least slightly unequal segments: the dorsal lobe is widest, the paired anterior lobes are narrower, and the paired lateral lobes are narrowest.   We have proposed that the unequal, 1,2,2 configuration of the calyx is a synapomorphy for Acantheae (McDade et al. 2005).  The trait has been further modified in the members of the one-lipped clade that have a four-lobed calyx (see Fig. 2g in McDade et al. 2005), and also in at least some members of the Stenandrium clade that have narrow, apparently equal segments.  At least the basal members of the other main lineages of Acanthaceae s.s. have a calyx of five equal lobes, although there are further modifications in some lineages, notably Barlerieae.   

As the phylogeny above suggests, the two main lineages of Acantheae can be characterized as having a plesiomorphic two-lipped corolla or a synapomorphic one-lipped corolla.  As is typical of Acanthaceae and Lamiales in general, two-lipped corollas have two lobes in the upper lip and three lobes in the lower lip.  Plants in the one-lipped clade have all five lobes of the corolla directed ventrally such that the plants lack an upper lip (although in some, the dorsal sepal seems to fulfill that role functionally).  

References

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