Hannah Banks

A new subfamily classification of the leguminosae based on a taxonomically comprehensive phylogeny

The classification of the legume family proposed here addresses the long-known non-monophyly of the traditionally recognised subfamily Caesalpinioideae, by recognising six robustly supported monophyletic subfamilies. This new classification uses as its framework the most comprehensive phylogenetic analyses of legumes to date, based on plastid matK gene sequences, and including near-complete sampling of genera (698 of the currently recognised 765 genera) and ca. 20% (3696) of known species. The matK gene region has been the most widely sequenced across the legumes, and in most legume lineages, this gene region is sufficiently variable to yield well-supported clades. This analysis resolves the same major clades as in other phylogenies of whole plastid and nuclear gene sets (with much sparser taxon sampling). Our analysis improves upon previous studies that have used large phylogenies of the Leguminosae for addressing evolutionary questions, because it maximises generic sampling and provides a phylogenetic tree that is based on a fully curated set of sequences that are vouchered and taxonomically validated. The phylogenetic trees obtained and the underlying data are available to browse and download, facilitating subsequent analyses that require evolutionary trees. Here we propose a new community-endorsed classification of the family that reflects the phylogenetic structure that is consistently resolved and recognises six subfamilies in Leguminosae: a recircumscribed Caesalpinioideae DC., Cercidoideae Legume Phylogeny Working Group (stat. nov.), Detarioideae Burmeist., Dialioideae Legume Phylogeny Working Group (stat. nov.), Duparquetioideae Legume Phylogeny Working Group (stat. nov.), and Papilionoideae DC. The traditionally recognised subfamily Mimosoideae is a distinct clade nested within the recircumscribed Caesalpinioideae and is referred to informally as the mimosoid clade pending a forthcoming formal tribal and/or cladebased classification of the new Caesalpinioideae. We provide a key for subfamily identification, descriptions with diagnostic charactertistics for the subfamilies, figures illustrating their floral and fruit diversity, and lists of genera by subfamily. This new classification of Leguminosae represents a consensus view of the international legume systematics community; it invokes both compromise and practicality of use.

The flower of Hibiscus trionum is both visibly and measurably iridescent

Living organisms can use minute structures to manipulate the reflection of light and display colours based on interference. There has been debate in recent literature over whether the diffractive optical effects produced by epoxy replicas of petals with folded cuticles persist and induce iridescence in the original flowers when the effects of petal pigment and illumination are taken into account. We explored the optical properties of the petal of Hibiscus trionum by macro-imaging, scanning and transmission electron microscopy, and visible and ultraviolet (UV) angle-resolved spectroscopy of the petal. The flower of Hibiscus trionum is visibly iridescent, and the iridescence can be captured photographically. The iridescence derives from a diffraction grating generated by folds of the cuticle. The iridescence of the petal can be quantitatively characterized by spectrometric measurements with several square-millimetres of sample area illuminated. The flower of Hibiscus trionum has the potential to interact with its pollinators (honeybees, other bees, butterflies and flies) through iridescent signals produced by its cuticular diffraction grating.

Tetrad pollen in the subfamily Caesalpinioideae (Leguminosae) and its significance

Pollen in permanent tetrads occurs in only eight species from three genera placed in widely separated tribes in the Caesalpiniodease. Tetrads are recorded in all four species of Diptychandra (including one yet to be described) in the tribe Caesalpinieae. They occur in only two species of about 300 in Bauhinia (B. phoenicea and B. pottsii and its varieties) in the tribe Cercideae. Of 13 species of Afzelia in the tribe Detarieae tetrad pollen has been found only in A. pachyloba and A. bella and its varieties. There are few common features in the tetrad pollen found in the subfamily. Diptychandra has acalymate tetrads with distinctive interconnecting channels between individual grains. Bauhinia phoenicea has calymate pollen while B. pottsii has acalymate pollen with cross wall cohesion and exine bridges. There is a wide range of variation in the shape of the pollen, colpal length and ornamentation among the four varieties of B. pottsii. The tetrads are acalymate in Afzelia, held together by a shared tectum. The pollen has a thin endexine, very thin lamellated foot layer, short columellae and very thick, coarsely reticulate tectum, very distinct from the perforate tectum found in the other genera. The limited occurrence of tetrad pollen within the subfamily and the significance of its great variation in Bauhinia, and to a lesser extent in the other genera, is discussed. The range of structure and plasticity of arrangement of tetrads and their wide taxonomic distribution, suggest that these pollen structures arose more than once in the Caesalpinioideae and this might be expected in the subfamily which is regarded as ancestral in the Leguminosae.

Aperture variation in the pollen of Nelumbo (Nelumbonaceae)

The evolutionary and developmental origin of tricolpate pollen is of great interest because pollen of this kind defines a major clade of angiosperms (eudicots), a clade that is also well supported by molecular data. We examined evidence that tricolpate and monosulcate pollen types are produced alongside each other in the anthers of Nelumbo flowers, as has previously been reported. Observations of pollen in situ within individual anthers revealed mainly tricolpate pollen produced in tetrahedral tetrads, but also a small percentage of clearly aberrant pollen grains that have a great variety of aperture configurations. Previously published evidence for tetragonal tetrads is not supported, and previously reported monosulcate grains are part of a continuum of variation among the aberrant grains in aperture number, position and form. Other eudicots show similar variability in their pollen apertures. The variation in the pollen of Nelumbo is not exceptional, and may not be more significant than variation seen in the other taxa with regard to the origin of the tricolpate and tricolpate‐derived pollen characteristic of eudicots. Nevertheless further studies of aberrant pollen in Nelumbo and other eudicots, together with comparisons of pollen development in “normal” eudicots and closely related species that show radical, and developmentally fixed, reorganization of apertures and pollen polarity, may be helpful in understanding the processes that controlled the transition from the monosulcate to the tricolpate condition.

Discovering Karima (Euphorbiaceae), a New Crotonoid Genus from West Tropical Africa Long Hidden within Croton.

Croton scarciesii (Euphorbiaceae-Crotonoideae), a rheophytic shrub from West Africa, is shown to have been misplaced in Croton for 120 years, having none of the diagnostic characters of that genus, but rather a set of characters present in no known genus of the family. Pollen analysis shows that the new genus Karima belongs to the inaperturate crotonoid group. Analysis of a concatenated molecular dataset combining trnL-F and rbcL sequences positioned Karima as sister to Neoholstia from south eastern tropical Africa in a well-supported clade comprised of genera of subtribes Grosserineae and Neoboutonieae of the inaperturate crotonoid genera. Several morphological characters support the relationship of Karima with Neoholstia, yet separation is merited by numerous characters usually associated with generic rank in Euphorbiaceae. Quantitative ecological data and a conservation assessment supplement illustrations and descriptions of the taxon.

Pollen structure and function in caesalpinioid legumes

PREMISE OF THE STUDY A diverse range of pollen morphologies occurs within the large, paraphyletic legume subfamily Caesalpinioideae, especially among early-branching lineages. Previous studies have hypothesized an association between surface ornamentation and pollination syndrome or other aspects of pollen function such as desiccation tolerance and adaptations to accommodate volume changes. METHODS We reviewed caesalpinioid pollen morphology using light microscopy, scanning and transmission electron microscopy, in combination with a literature survey of pollination vectors. KEY RESULTS Pollen structural diversity is greatest in the early-branching tribes Cercideae and Detarieae, whereas Cassieae and Caesalpinieae are relatively low in pollen diversity. Functional structures to counter desiccation include opercula (lids) covering apertures and reduced aperture size. Structures preventing wall rupture during dehydration and rehydration include different forms of colpi (syncolpi, parasyncolpi, pseudocolpi), striate supratectal ornamentation, and columellate or granular wall structures that resist tensile or compressive forces respectively. Specialized aperture structures (Zwischenkörper) may be advantageous for efficient germination of the pollen tube. CONCLUSIONS In Detarieae and Cercideae in particular, there is potential to utilize pollen characters to estimate pollination systems where these are unknown. Supratectal verrucae and gemmae have apparently evolved iteratively in Cercideae and Detarieae. At the species level, there is a potential correlation between striate/verrucate patterns and vertebrate pollination.

Pollen morphology and phylogenetic analysis of Eperua Aublet (Detarieae: Caesalpinioideae: Leguminosae)

The diverse pollen morphology of fourteen species of the genus Eperua is described and illustrated using light, scanning electron and transmission electron microscopy. Six pollen types are described and a key for their identification is provided. A cladistic analysis was carried out using macromorphological and palynological characters to form a hypothesis of relationships between taxa. The pollen morphology is discussed with regard to systematic relationships, function and phylogenetic significance of certain pollen morphological structures within the genus.

Disorder in convergent floral nanostructures enhances signalling to bees

Diverse forms of nanoscale architecture generate structural colour and perform signalling functions within and between species. Structural colour is the result of the interference of light from approximately regular periodic structures; some structural disorder is, however, inevitable in biological organisms. Is this disorder functional and subject to evolutionary selection, or is it simply an unavoidable outcome of biological developmental processes? Here we show that disordered nanostructures enable flowers to produce visual signals that are salient to bees. These disordered nanostructures (identified in most major lineages of angiosperms) have distinct anatomies but convergent optical properties; they all produce angle-dependent scattered light, predominantly at short wavelengths (ultraviolet and blue). We manufactured artificial flowers with nanoscale structures that possessed tailored levels of disorder in order to investigate how foraging bumblebees respond to this optical effect. We conclude that floral nanostructures have evolved, on multiple independent occasions, an effective degree of relative spatial disorder that generates a photonic signature that is highly salient to insect pollinators.

Pollen morphology and systematics of Burseraceae

The Burseraceae are a medium‐sized family in which 18 genera are currently recognised. They are the subject of a long‐term project to describe the pollen morphology from light, scanning electron and transmission electron microscopy. The pollen morphology of tribe Protieae has been published, as well as an account of the pollen of the African taxa in the family. Pollen data for the other two tribes, Bursereae and Canarieae, are more or less complete. The pollen of all the genera have been examined, with the exception of the recently described Pseudodacryodes Pierlot for which, currently, there is no pollen material available. This paper summarises the results. There is considerable variation in exine and aperture features between, and occasionally within, the genera and 14 major pollen types are defined, including two previously undescribed types: ‘Canarium oleiferum’ and ‘Canarium gracile’. The distribution of pollen characteristics throughout the family is compared with previously published tribal and subtribal groupings, as well as with current ideas of generic relationships from molecular analyses. Comparisons show notable congruence of pollen data with molecular data. To some extent pollen morphology is different for each of the subtribes. Nevertheless, there are some notable exceptions, for example, the pollen of Garuga and Boswellia are remarkably similar, although Garuga has been included, somewhat tenuously, in tribe Protieae, and Boswellia is included in tribe Bursereae, subtribe Boswelliinae. In a recent molecular tree Garuga and Boswellia appear to be closely related, and this supports the conclusion, based on several macromorphological characters as well as pollen, that Garuga should be transferred to tribe Bursereae.

Is floral iridescence a biologically relevant cue in plant–pollinator signalling? A response to van der Kooi et al. (2014b)

van der Kooi et al. (2014a) presented optical analyses of the petals of six species of angiosperms. They investigated the flowers of 50 plant species (by scanning electron microscopy) and then selected four species with different surface structures (plus a further two in the Supporting Information) for optical characterization. Subsequently, van der Kooi et al. (2014b, in this issue of New Phytologist, pp. 18–20) repeated measurements of one of the species (Hibiscus trionum) and presented photographs of two of the species from their earlier paper (van der Kooi et al., 2014a), plus a further two previously unreported species. They did not record as iridescent any of the eight flowers that they optically characterized and/or photographed. By contrast, we presented optical characterization and photographs ofH. trionum (taken in the laboratory, in daylight, with no flash), which we showed to be visibly and measurably iridescent, with an associated regular surface structure termed a diffraction grating in the central part of the flower (Vignolini et al., 2014). Although the distribution of floral iridescence within the 300 000 species of angiosperms remains to be explored (Vignolini et al., 2014), this phenomenon is likely to be relatively uncommon, hence the apparent lack of records before Whitney et al.’s (2009) paper. van derKooi et al. (2014a,b) did not record floral iridescence in a sample of eight species. We are currently exploring this distribution in a phylogenetic context; so far we have identified iridescent flowers infrequently distributed across all major angiosperm groups. InH. trionum, we found the same measurable iridescence effect in the three independent lines we investigated – our laboratory stock sourced from Chiltern seeds (www.chilternseeds.co.uk), the line held by the Cambridge University Botanic Garden (maintained as selfed seed from germplasm supplied by Cluj-Napoca Botanic Garden in 1996), and a native New Zealand line grown from seed of Auckland Museum Herbarium voucher AK253689 (kindly supplied by Brian Murray). Our usual laboratory growth conditions are described in Vignolini et al. (2014), and the flower also develops well outdoors in sheltered glasshouse bays of the Cambridge University Botanic Garden (a relatively dry temperate habitat). The difference between our results and those of van der Kooi et al. (2014a,b) on this species could be due to one (or more) of several possible factors, but we currently lack details of their seed source or growth conditions to better resolve these discrepancies. (1) As they propose, they could be working with a distinct genetic lineage that lacks iridescence; the flower photographed in van der Kooi et al. (2014b) does appear morphologically distinct from our lines. (2) van der Kooi et al. (2014a,b) may lack access to fully developed specimens, since the flower of H. trionum only opens fully in optimal growth conditions. (3) There could be a temporal effect, because the flower ofH. trionum lasts for only a few hours on the plant, and the iridescent effect deteriorates rapidly when the flower wilts after removal. Whatever the explanation for the lack of iridescence recorded by van der Kooi et al. (2014a,b), the photographs and optical measurements presented in Vignolini et al. (2014) demonstrate, unequivocally, that some flowers are iridescent.