Acanthaceae

Introduction

The large and largely tropical plant family Acanthaceae includes at least 4000 species. The group is part of the order Lamiales which includes plants such as the familiar snap dragon (Plantaginaceae), mint (Lamiaceae) and African violet (Gesneriaceae). These plants have sympetalous corollas (i.e., with petals fused into a tube) and four stamens (sometimes reduced further to two).

Acanthaceae includes the 221 genera treated by Scotland & Vollesen (2000) plus the black mangrove genus, Avicennia. Notably, as a result of the largely tropical distribution of Acanthaceae, species-level diversity remains poorly understood, and there is little doubt that many new species remain to be discovered. In particular, the Neotropics house the richest and most poorly documented angiosperm flora on Earth. It is thus not surprising that Neotropical acanths remain incompletely known, and we expect many more species to be described from this region. As a quantitative indication of the pace of new discovery in Acanthaceae, Index Kewensis reports 81 new species described in the genus Justicia alone between 1986 and 1995.  We expect the species count eventually to exceed 4000.

Characteristics

Plants belonging to the Core Acanthaceae (i.e., Acantheae through Ruellieae as indicated in the phylogeny presented at the top of this page; Acanthoideae sensu Scotland & Vollesen 2001) may be recognized by their fruit: a few-seeded, explosively dehiscent capsule within which seeds are borne on hook-like structures called retinacula (the lignified derivatives of the funiculus). These retinacula are a unique and unreversed synapomorphy for Acanthoideae: among these plants–all 4000 of them–fruits vary in size and in seed number but are otherwise remarkably homogeneous. The hooks presumably serve to propel the seeds away from the parent plant when the fruit dehisces but studies are lacking.

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Retinacula of the fruit of Asystasia gangetica.  © 2006 Gerald Carr

Within Core Acanthaceae, Acantheae are well marked as monophyletic by morphological and molecular synapomorphies. Its sister group which we have here called Ruellioideae is also marked by a number of morphological synapomorphies including cystoliths (crystals) in most plant structures.

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Cystoliths are the small linear structures that 'dot' the lower surface of this leaf of a South American Ruellia species. © C. Ezcurra

In addition to this core group, four other small lineages are very closely related and are now treated as part of a broad Acanthaceae: the pantropical and subtropical black mangroves, Avicennia; Old World Thunbergia (including the commonly cultivated black-eyed Susan, Thunbergia alata) and related genera, Old and New World Mendoncia; and Nelsonioideae (Nelsonia, Elytraria, Staurogyne, a handful of other small genera). Each of these four groups has its own distinctive fruit type. Unfortunately, we have yet to identify a synapomorphy for all Acanthaceae that is as notable and easy to recognize as fruit type is for the core group.  Compared to most other Lamiales, Acanthaceae have relatively few ovules.

Beyond fruits of Acanthaceae, diversity in other traits is spectacular. Plants are rosette and caulescent herbs, shrubs and trees (rarely vines except Thunbergia and Mendoncia which are almost entirely climbing plants). Most acanths have solid green leaves, but a few have remarkably colored leaves, and some are cultivated for their foliage (e.g., polka dot plant, Hypoestes phyllostashya; Persian shield, Strobilanthes dyerianus). Inflorescences of many acanths have large, colorful bracts that often eclipse the corollas in showiness (e.g., Pachystachys, Megaskepasma; see images below). However, bracts of plants of many other species are diminutive, green and clearly not involved in attracting pollinators. Many species have extremely showy flowers and a number are cultivated for their beauty (e.g.,  Justicia brandegeana and many other species belonging to this large genus, Aphelandra squarrosa; you will see examples below); however, many others have diminutive green bracts and very small flowers. Across the family, flowers range in size from a few mm to more than 10 cm, with a parallel range of variation in shape and color. Variation in these corolla traits is no doubt associated with a wide range of animal pollinators, although studies are remarkably few.

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Pachystachys lutea Nees (left, © 2006 Lucinda A. McDade) and Megaskepasma erythrochlamys Lindau (middle, © 2003 Shigenobu Aoki Botanical Garden) have large, colorful bracts. Justicia betonica L. (right, © Gerald D. Carr) is cultivated because of its showy flowers.

Pollen and Chromosomes

Structural diversity continues below the visible level in Acanthaceae. In terms of number and type of apertures and, especially in terms of ultrasculpturing, pollen grains of Acanthaceae are without doubt as diverse as any other family-level lineage regardless of size.  

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Pollen grains of Acanthaceae. From left to right: Crossandra sulphurea, © 1990 Carol Furness. Acanthus mollis, Acanthus spinosus, Neriacanthus grandiflorus, Aphelandra flammea, and Crossandra pungens, © T. F. Daniel.

Chromosome numbers are similarly diverse, with documented haploid numbers ranging from 7 to 68, thus indicating many evolutionary transitions via both dysploidy and polyploidy.

Discussion of Phylogenetic Relationships

There is at present little disagreement among students of Acanthaceae regarding membership in the family and delineation of the main clades as presented in the phylogeny above.  There is, however, quite a bit of work to be done to resolve relationships at a number of levels.  For example, the results of Schwarzbach & McDade (2001) placed Avicennia sister to Thunbergioideae but without strong support from molecular data for this relationship and with no identifiable morphological synapomorphies supporting the relationship.  Similarly, the placement of Neuracanthus among the lineages of core Acanthaceae (i. e., the lineages with retinacula) have yet to be fully resolved.

There also remains a great deal of work to be done on relationships within lineages.  At present (August 2006), we present intra-lineage phylogenetic information for Acantheae, for Isoglossinae and the Tetramerium Lineage within Justicieae, and for Ruellieae.  Our continuing work and that of our colleagues will gradually add knowledge of relationships within the other lineages.     

References

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