Resolving basal lamiid phylogeny and the circumscription of Icacinaceae with a plastome-scale data set

PREMISE OF THE STUDY

Major relationships within Lamiidae, an asterid clade with ∼40000 species, have largely eluded resolution despite two decades of intensive study. The phylogenetic positions of Icacinaceae and other early-diverging lamiid clades (Garryales, Metteniusaceae, and Oncothecaceae) have been particularly problematic, hindering classification and impeding our understanding of early lamiid (and euasterid) character evolution.

METHODS

To resolve basal lamiid phylogeny, we sequenced 50 plastid genomes using the Illumina sequencing platform and combined these with available asterid plastome sequence data for more comprehensive phylogenetic analyses.

KEY RESULTS

Our analyses resolved basal lamiid relationships with strong support, including the circumscription and phylogenetic position of the enigmatic Icacinaceae. This greatly improved basal lamiid phylogeny offers insight into character evolution and facilitates an updated classification for this clade, which we present here, including phylogenetic definitions for 10 new or converted clade names. We also offer recommendations for applying this classification to the Angiosperm Phylogeny Group (APG) system, including the recognition of a reduced Icacinaceae, an expanded Metteniusaceae, and two orders new to APG: Icacinales (Icacinaceae + Oncothecaceae) and Metteniusales (Metteniusaceae).

CONCLUSIONS

The lamiids possibly radiated from an ancestry of tropical trees with inconspicuous flowers and large, drupaceous fruits, given that these morphological characters are distributed across a grade of lineages (Icacinaceae, Oncothecaceae, Metteniusaceae) subtending the core lamiid clade (Boraginales, Gentianales, Lamiales, Solanales, Vahlia). Furthermore, the presence of similar morphological features among members of Aquifoliales suggests these characters might be ancestral for the Gentianidae (euasterids) as a whole.

Floral structure of Emmotum (Icacinaceae sensu stricto or Emmotaceae), a phylogenetically isolated genus of lamiids with a unique pseudotrimerous gynoecium, bitegmic ovules and monosporangiate thecae

BACKGROUND AND AIMS

Icacinaceae sensu stricto consist of a group of early branching lineages of lamiids whose relationships are not yet resolved and whose detailed floral morphology is poorly known. The most bizarre flowers occur in Emmotum: the gynoecium has three locules on one side and none on the other. It has been interpreted as consisting of three fertile and two sterile carpels or of one fertile carpel with two longitudinal septa and two sterile carpels. This study focused primarily on the outer and inner morphology of the gynoecium to resolve its disputed structure, and ovule structure was also studied. In addition, the perianth and androecium were investigated.

METHODS

Flowers and floral buds of two Emmotum species, E. harleyi and E. nitens, were collected and fixed in the field, and then studied by scanning electron microscopy. Microtome section series were used to reconstruct their morphology.

KEY RESULTS

The gynoecium in Emmotum was confirmed as pentamerous, consisting of three fertile and two sterile carpels. Each of the three locules behaves as the single locule in other Icacinaceae, with the placenta of the two ovules being identical, which shows that three fertile carpels are present. In addition, it was found that the ovules are bitegmic, which is almost unique in lamiids, and that the stamens have monosporangiate thecae, which also occurs in the closely related family Oncothecaceae, but is not known from any other Icacinaceae sensu lato so far.

CONCLUSIONS

The flowers of Emmotum have unique characters at different evolutionary levels: the pseudotrimerous gynoecium at angiosperm level, the bitegmic ovules at lamiid level and the monosporangiate thecae at family or family group level. However, in general, the floral morphology of Emmotum fits well in Icacinaceae. More comparative research on flower structure is necessary in Icacinaceae and other early branching lineages of lamiids to better understand the initial evolution of this large lineage of asterids.

Phylogeny of Lamiidae

PREMISE OF THE STUDY

The Lamiidae, a clade composed of approximately 15% of all flowering plants, consists of five orders: Boraginales, Gentianales, Garryales, Lamiales, and Solanales; and four families unplaced in an order: Icacinaceae, Metteniusiaceae, Oncothecaceae, and Vahliaceae. Our understanding of the phylogenetic relationships of Lamiidae has improved significantly in recent years, however, relationships among the orders and unplaced families of the clade remain partly unresolved. Here, we present a phylogenetic analysis of the Lamiidae based on an expanded sampling, including all families together, for the first time, in a single phylogenetic analyses.

METHODS

Phylogenetic analyses were conducted using maximum parsimony, maximum likelihood, and Bayesian approaches. Analyses included nine plastid regions (atpB, matK, ndhF, psbBTNH, rbcL, rps4, rps16, trnL-F, and trnV-atpE) and the mitochondrial rps3 region, and 129 samples representing all orders and unplaced families of Lamiidae.

KEY RESULTS

Maximum Likelihood (ML) and Bayesian trees provide good support for Boraginales sister to Lamiales, with successive outgroups (Solanales + Vahlia) and Gentianales, together comprising the core Lamiidae. Early branching patterns are less well supported, with Garryales only poorly supported as sister to the above 'core' and a weakly supported clade composed of Icacinaceae, Metteniusaceae, and Oncothecaceae sister to all other Lamiidae.

CONCLUSIONS

Our phylogeny of Lamiidae reveals increased resolution and support for internal relationships that have remained elusive. Within Lamiales, greater resolution also is obtained, but some family interrelationships remain a challenge.