Chapter 8 Discussion and Conclusions

Cover Page The handle http://hdl.handle.net/1887/78385 holds various files of this Leiden University dissertation. Author: Yu, R. Title: A monograph of the plant genus Trigonostemon Blume Issue Date: 2019-09-18 Chapter 8 Discussion and Conclusions A monograph of the tropical Asian plant genus Trigonostemon Blume is presented. Taxonomic revisions are made according to the ‘morphological species concept’ based on herbarium and living material (Chapter 2–4). The species circumscriptions and recognition have been optimised. The more than 140 species described in the historical literature have now been reduced to 59 accepted species (including 3 uncertain species). The species complexes (i.e., T. longifolius Wall. ex Baill., T. villosus Hook.f. and T. viridissimus (Kurz) Airy Shaw) in the genus are now clearly outlined and reduced to single, variable species that can be recognised by morphological characters. Species delimitation is a course of reasoning. By investigating a limited number of specimens, one has to decide what is part of the normal variability within a species and which are the specific discontinuities. Trigonostemon displays limited variation in the floral structures, but has highly variable vegetative characters. This was the problem in the species complexes, where (many) entities were discriminated based on variable characters. As soon as more material became available, it appeared that the variation was continuous and no species could be discriminated (e.g., obvious characters like the leaf shape and length of petiole, sometimes used as delimiting characters, can be highly variable within a species). Generally, the more inconspicuous characters (e.g., the number of stamens is a species indicative character) are often very useful to circumscribe the species complexes. The more variable species might be more common than usually expected, as they are often the ‘result’ of higher collecting densities, because ‘the general (logical) experience is that increase of specimens brings along increase of variability, hence of the necessity of wider specific delimitation’ (after van Steenis 1957). This is a drawback in making revisions. This means that the revision presented in this dissertation will not be the last and final conclusion. More molecular studies may reveal cryptic species within the present species delimitations. An exploration of the pollen morphology of Trigonostemon and its close relatives is made using light and electron microscopy (Chapter 5). Two major types of pollen are found and correlate well with the macromorphology. Species with the Trigonostemon reidioides type pollen (‘croton pattern’ ornamentation) have deeply divided stigmas and a protruding appendage on the connective; species with the Trigonostemon verrucosus type pollen (verrucate exine) have slightly (or not) bifid stigmas and a connective without an appendage. Dimorphic pollen within a genus is possible, but it often occurs in different sections (Erdtman 1952). Trigonostemon pollen adds a good example. The correlations between pollen and macromorphology support the infrageneric classification of the genus (see below, also Chapter 6). Molecular phylogenetic studies are made of Trigonostemon, Dimorphocalyx Thwaites and other closely related or morphologically similar taxa (Chapter 6). Five markers are used: the nuclear ITS and chloroplast trnK intron, trnT-L, trnL-F and rbcL. The generic circumscription of Trigonostemon has been solved. Trigonostemon and Dimorphocalyx are found to be two separate monophyletic groups within the inaperturate crotonoids. Because they are not sister groups, they are treated as separate genera. An infrageneric classification of Trigonostemon is proposed. Three monophyletic sections are supported based on the molecular phylogeny; one section, not included in the phylogeny, is treated based on its exceptional morphology and limited distribution. The main characters used to define the sections (the selection of these characters is discussed below) are the division of stigmas, the presence of a protruding appendage on the connective, the disc shape and the exine sculpture of the pollen grains. In addition, Dimorphocalyx and Tritaxis are phylogenetically and morphologically congeneric, and are merged under the latter genus name. 240 A monograph of the plant genus Trigonostemon Blume ― Chapter 8 In phylogenetic systematics, ‘a monophyletic group can only be confirmed by demonstrating their common possession of derivative characters (synapomorphies)’ (after Hennig 1965). When the pollen characters are mapped on the molecular phylogeny, it has become clear that the verrucate pollen is such a derived character shared in clade T3 (Figure 42), and thus supports sect. Tylosepalum (Kurz) Benth; while the ‘croton pattern’ ornamentation is a shared primitive character state (symplesiomorphy) among all major clades in Trigonostemon. This is a phylogenetic interpretation of the pollen morphology. Furthermore, the molecular phylogeny proves to be a powerful tool in the (infra)generic (and higher ranks) classification. Morphological characters of Trigonostemon show a high degree of variability (discussed above, also in Chapter 2) or homoplasy (e.g., although the number of stamens is a good indicator for species, but it seems to be ‘randomly distributed’ among the species when mapped on the phylogeny and shows a high amount of homoplasy, see Figure 42). The combination of a molecular phylogeny and the morphological characters mapped on it works well to find the synapomorphies by which the sections are recognised. Therefore, molecular and morphological research should be a permanent union in classical taxonomy. The historical biogeography of Trigonostemon and Dimorhocalyx is analysed. A Bayesian dating analysis indicates that both genera originated in the Late Eocene to Oligocene. The S-DIVA and DEC analyses reveal the probable geographical origin and migration routes of the genera. Trigonostemon originated on the SE Asian mainland, but sect. Trigonostemon reached the Malay Peninsula through radiation in the Early to Middle Miocene. Diversification occurred subsequently in both SE Asian mainland and W Malesia. In contrast, Dimorphocalyx originated and diversified mostly on Borneo. In addition, the frequent change in sea levels during the Pleistocene accelerated the diversification of sect. Trigonostemon in Borneo and the Malay Peninsula. The dot map of the georeferenced specimens (Figure 2) more or less reflects the species diversity and individual density of Trigonostemon among its distribution areas (as defined by Figure 43), although bias caused by collecting preferences exists (for distribution maps of Dimorphocalyx, see van Welzen & van Oostrum 2015). The connections in the floras of these areas depend largely on floristic exchanges (e.g., dispersals). Areas in West Malesia have a strong connection, partly because of the frequent dispersal events (for the detailed routes, see Chapter 7); in East Malesia, there appears a weak connection along the Outer Melanesian Arc, which was a chain of islands including part of the Philippines, N and E Sulawesi, N Moluccas, north coast of New Guinea and the Pacific Islands (nicely exemplified by de Boer 1995; the distributions of Trigonostemon and Dimorphocalyx more or less fits in with this island chain, except both genera are absent in Sulawesi); the Philippines acts as a bridge, connecting West and East Malesia. An indication of the study (although it is only a single case) is that the floristic compositions of Malesia are a dynamic network and cannot be simply demarcated by one single line (e.g., Wallace’s line and other lines, also referred to as the area Wallacea, van Welzen et al. 2011). Future studies in biogeographic zones should not only focus on the static species occurrence, but also see from a dynamic angle and take the floristic exchanges into account..

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