Advances in the floral structural characterization of the major subclades of Malpighiales, one of the largest orders of flowering plants

Abstract

Background and aims: Malpighiales are one of the largest angiosperm orders and have undergone radical systematic restructuring based on molecular phylogenetic studies. The clade has been recalcitrant to molecular phylogenetic reconstruction, but has become much more resolved at the suprafamilial level. It now contains so many newly identified clades that there is an urgent need for comparative studies to understand their structure, biology and evolution. This is especially true because the order contains a disproportionally large diversity of rain forest species and includes numerous agriculturally important plants. This study is a first broad systematic step in this endeavour. It focuses on a comparative structural overview of the flowers across all recently identified suprafamilial clades of Malpighiales, and points towards areas that desperately need attention.

Methods: The phylogenetic comparative analysis of floral structure for the order is based on our previously published studies on four suprafamilial clades of Malpighiales, including also four related rosid orders (Celastrales, Crossosomatales, Cucurbitales, Oxalidales). In addition, the results are compiled from a survey of over 3000 publications on macrosystematics, floral structure and embryology across all orders of the core eudicots.

Key results: Most new suprafamilial clades within Malpighiales are well supported by floral structural features. Inner morphological structures of the gynoecium (i.e. stigmatic lobes, inner shape of the locules, placentation, presence of obturators) and ovules (i.e. structure of the nucellus, thickness of the integuments, presence of vascular bundles in the integuments, presence of an endothelium in the inner integument) appear to be especially suitable for characterizing suprafamilial clades within Malpighiales.

Conclusions: Although the current phylogenetic reconstruction of Malpighiales is much improved compared with earlier versions, it is incomplete, and further focused phylogenetic and morphological studies are needed. Once all major subclades of Malpighiales are elucidated, more in-depth studies on promising structural features can be conducted. In addition, once the phylogenetic tree of Malpighiales, including closely related orders, is more fully resolved, character optimization studies will be possible to reconstruct evolution of structural and biological features within the order.

Fig. 1.

Phylogeny of Malpighiales reproduced from Xi et al. (2012) (Peraceae and Rafflesiaceae follow Wurdack and Davis, 2009). Suprafamilial clades are discussed in the main text (numbers 1–12 follow Xi et al., 2012). The following families are treated differently in APG III (2009): (1) Peraceae are circumscribed with Euphorbiaceae; (2) Passifloraceae, Turneraceae and Malesherbiaceae are circumscribed as Passifloraceae s.l.; (3) Ochnaceae, Medusagynaceae, and Quiinaceae are circumscribed as Ochnaceae s.l. The distribution of ten prominent features of gynoecium (incl. ovule) structure is plotted: 1. Flowers with mainly 3 carpels. 2. Carpels with 2 ovules, collateral, antitropous, with obturator. 3. Carpels with 1 ovule (median). 4. Carpels with more than 2 ovules. 5. Ovules crassinucellar. 6. Ovules with thin nucellus (incompletely tenuinucellar or weakly crassinucellar). 7. Inner integument thicker than outer at the mature embryo sac stage. 8. Outer integument thicker than inner at the mature embryo sac stage. 9. Outer integument only 2 cell layers thick at the mature embryo sac stage. 10. Inner integument 5 or more cell layers thick at the mature embryo sac stage