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The handle http://hdl.handle.net/1887/78385 holds various files of this Leiden University dissertation.

Author: Yu, R. Title: A monograph of the Blume Issue Date: 2019-09-18

Summary 251

Summary

Trigonostemon is a plant genus in the family comprising 59 species. These are small trees or shrubs growing in the lowland rainforests in and the adjacent areas. The vegetative characters of these plants are often variable, but the small unisexual flowers with colourful petals (in both ♂ and ♀ flowers) and the 3 or 5 united stamens (in the ♂ flowers) are typical of the genus. The present thesis is a monograph ofTrigonostemon , studying four aspects of the genus: the , pollen morphology, molecular phylogeny and historical biogeography.

Trigonostemon was scientifically described by Carl Ludwig Blume in 1825. More than140 species have been treated within the genus ever since. However, some of these species are identical to each other (synonyms) and some actually do not belong to the genus. The taxonomic studies revise the species based on observations of comparative morphology of herbarium and living material. The historical literature is compiled, and the characters that are useful in species delimitation are discussed. A total of 59 species (including 3 uncertain species) are accepted and scientifically described. Dichotomous identification keys, type information, geographical distributions and taxonomic notes are also provided (Chapters 2, 3, 4).

Pollen grains of plants differ by a vast array of shapes and sizes, as well as complex surface patterns and apertures (furrows and holes). The variation of the pollen grains often provides microscopic evidence for plant systematics. An exploration of the pollen morphology of Trigonostemon and its close relatives is made with light and electron microscopy. The pollen of a group of species in the Euphorbiaceae (often known as inaperturate crotonoids, Trigonostemon also belongs to this group) is characterised by a ‘ pattern’ ornamentation (that consists of rings of five or six raised elements arranged around a circular area) on the surface. However, the ‘croton pattern’ ornamentation in the pollen of Trigonostemon is found to have reduced to spines or become completely absent in a number of species. The potential implication of the pollen morphology for the (infra)generic delimitation of Trigonostemon is discussed (Chapter 5).

Molecular phylogenetic studies use DNA sequences to evaluate the relationship between the species by modelling their evolutionary history. The DNA sequences of five regions of the plant genomes are used to reconstruct the phylogeny of Trigonostemon, and other closely related species. Some previous studies argued that Trigonostemon and Dimorphocalyx should be merged into one single genus, but the molecular phylogeny shows that they are descendants of two different ancestors, and are thus two separate genera. Based on the molecular data and morphological characters, Trigonostemon is divided into four sections. The main characters used to define the sections are the division of stigmas, the apex of anthers, the shape of the nectar disc and the surface ornamentation of the pollen grains (Chapter 6).

Southeast Asia is the main distribution area of Trigonostemon and Dimorphocalyx. This area harbours a great . The migration routes of plants and animals in this area have always been of great interest in biogeography. The historical biogeographic analyses have revealed that Trigonostemon and Dimorphocalyx originated at similar times (between the Oligocene and Early Miocene) but the two genera had different migration histories: Trigonostemon originated on the Southeast Asian mainland, expanded its distribution area to first the Malay Peninsular and then Borneo, and diversified in all these areas; in contrast, Dimorphocalyx originated and diversified mostly on Borneo. The Philippines is inferred to have played an important role in helping the plants to disperse to the east part of the Malay Archipelago. In addition, the frequent 252 A monograph of the plant genus Trigonostemon Blume changes of sea levels during the Ice Ages are considered to have propelled the diversification of Trigonostemon in Borneo and the Malay Peninsula (Chapter 7).