**1. Introduction**

26 The Dynamical Processes of Biodiversity – Case Studies of Evolution and Spatial Distribution

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Species diversity is unequally distributed across the globe, with more species found in the tropics than any other ecosystem in the world. This latitudinal gradient of species richness illustrates the complex evolutionary history of global biodiversity, and many studies have placed it in the context of geological history and rates of speciation and extinction (Mittelbach et al., 2007). Historical biogeographic studies, using molecular phylogenies calibrated with a relative dimension of time, indicate that the accumulation of this diversity is both ancient ("museum" model) and recent ("cradle" model) within groups (Bermingham & Dick, 2001; McKenna & Farrell, 2006). An additional layer of complexity that makes it difficult to untangle the evolutionary processes driving tropical speciation are biotic interactions, such as plant competition and parasite interactions (Berenbaum & Zangerl, 2006). Much of our understanding of the processes underlying speciation comes from mathematical models or studies of model organisms. However, some of the classical questions of evolutionary biology, such as what factors are driving speciation in species rich biomes, can only be understood by detailed evolutionary and ecological studies of specious groups.

*Begonia* is a genus of about 1550 described species, placing it in the top ten most speciose angiosperm genera (Frodin, 2004; Hughes, 2008). This makes it an ideal model for studying the processes and patterns underlying the generation of diversity (Forrest et al.,

The Origin of Diversity in *Begonia*:

Forrest & Hollingsworth, 2003; Forrest et al., 2005).

indigenous to continental Southeast Asia (de Wilde, 2011).

subsequent radiation (Plana, 2003; Plana et al., 2004).

Genome Dynamism, Population Processes and Phylogenetic Patterns 29

nuclear, mitochondrial and plastid markers strongly support a close relationship of

Only two genera are currently recognized in the Begoniaceae: the monotypic genus *Hillebrandia*, and the species-rich and morphologically diverse genus *Begonia* (Doorenbos et al., 1998; Forrest & Hollingsworth, 2003). A third genus, *Symbegonia*, was previously included in Begoniaceae, and separated from *Begonia* by floral characters (syntepalous perianth and a monadelphous androecium). Based on molecular data, the genus *Symbegonia* has been shown to be nested within *Begonia* section *Petermannia* (Forrest & Hollingsworth, 2003). *Hillebrandia sandwicensis*, which is endemic to Hawaii, can be differentiated from *Begonia* by a suite of morphological characters. These include more differentiated segments of the perianth, semi-inferior ovaries (inferior in *Begonia*), and fruit dehiscence between the styles in contrast to the usually loculicidal dehiscence in *Begonia* (Clement et al., 2004;

A reliable infrageneric classification and subdivision of large genera such as *Begonia* is crucial in order to inform taxonomic monographs, biogeographic and evolutionary studies. A revision of circumscriptions of *Begonia* sections by Doorenbos et al. (1998) provides a foundation for the subdivision of the genus. In this revision 63 sections were recognized, and another three sections have been subsequently proposed (de Wilde & Plana, 2003; Forrest & Hollingsworth, 2003; Shui et al., 2002). The distributions of all but one of the currently accepted *Begonia* sections are limited to single continental regions, i.e. Africa, Asia, or America. Only section *Tetraphila* can be found in multiple continents and a single, recently discovered, and still to be named species in this predominantly African section is

DNA sequence data from non-coding regions plays an important role in plant classification and barcoding (CBOL Plant working group, 2009), and has widely been used to resolve relationships at the species and sectional level in *Begonia*. A framework phylogeny of *Begonia* based on analyses of c. 13000 bases of plastid and mitochondrial DNA of 30 *Begonia* species (Goodall-Copestake et al., 2010; Fig. 1) indicates that African taxa form the earliest divergent clades in the *Begonia* phylogeny and that both Asian and American *Begonia* lineages are derived from African ancestors. The phylogenetic relationships within the relatively small group of African *Begonia*, which comprises around 160 species subdivided into 17 sections (de Wilde & Plana, 2003; Doorenbos et al., 1998), are relatively well understood. African *Begonia* species are not retrieved as monophyletic, but South African species placed in section *Augustia* were shown to be closely related to a clade of American taxa, and Socotran *Begonia* species (section *Peltaugustia*) were shown to form a monophyletic clade with Asian taxa (Forrest et al., 2005; Goodall-Copestake et al., 2010; Plana et al., 2004; Thomas et al., 2011). Revisions exist for the majority of the African sections (see references in Plana, 2003), and the intersectional relationships of African *Begonia* species have been studied using molecular systematic approaches and have been discussed in some detail in Plana (2003) and Plana et al. (2004). Most African sections are well circumscribed and seem to represent monophyletic taxa, but section *Mezierea* is polyphyletic (Forrest et al., 2005; Plana, 2003; Plana et al., 2004); and the lack of resolution or support in phylogenies makes the assessment of the monophyly of some sections problematic. Apart from the grade of continental African taxa, a major Madagascan radiation can be detected. Only one of the c. 50 Madagascan (incl. Comores and Mascarenes) *Begonia* species, *Begonia oxyloba*, is also widespread on the African continent (Keraudren-Aymonin, 1983). The other Madagascan species seem to be the result of a single dispersal event from continental Africa and a

Begoniaceae with Datiscaceae and Tetramelaceae (Schaefer & Renner, 2011).

2005; Neale et al., 2006). The distribution of *Begonia* diversity is uneven throughout tropical regions, with the greatest diversity in America and Asia (>600 species each), whilst being relatively species poor in Africa (160 species) and absent in Australia (Goodall-Copestake et al., 2010). The genus is thought to have originated in Africa, while South American and South East Asian species are the results of parallel radiations over the last 20 - 50 million years (Goodall Copestake et al., 2010; Plana et al., 2004; Thomas et al., 2011). Long distance dispersal is rare, for example *Begonia* species have failed to cross the Torres strait from Papua New Guinea to Australia.

Begoniaceae are easily recognizable by diagnostic characters such as asymmetrical leaves, unisexual monoecious flowers, twisted-, papillose stigmas, and dry-, three-winged capsules (Doorenbos, 1998). However, there are numerous deviations from these typical character states. Within the genus *Begonia* there is a large range of morphological diversity, particularly in vegetative form, and this is linked to adaptation to a variety of ecological conditions. Vegetative adaptations such as the evolution of perennating rhizomes, leaf micromorphology optimised for low, scattered light; or stomatal clustering may underlie their ability to thrive in diverse niches. Phenotypic polymorphism within populations occurs, most frequently in the anthocyanin patterns on the leaves, which although striking have not been shown to have measurable effects on light capture (Hughes et al., 2008).

The genetic and morphological diversity of the genus *Begonia* has been exploited through cultivation to produce over 10,000 cultivars. These are horticulturally divided into 5 classes: a) the tuberous begonias (*B*. x *tuberhybrida*), a complex group derived from crosses between species such as *B. boliviensis* or *B. pearcei*, b) Elatior begonias (*B*. x *hiemalis*), a cross between tuberous begonias and *B. socotrana*, c) Lorraine begonias (*B.* x *cheimanta*), a cross between tuberous hybrids and *B. dregei*, d) semperflorens begonias (*B. semperflorens*-cultorum), with *B. cucullata* and *B. schmidtiana* as important ancestors, and e) begonias grown for their ornamental foliage (*B*. *rex*-cultorum), Asiatic in origin (Haegeman, 1979; Hvoslef-Eide et al., 2007). Commercial interest in this group has promoted research into a variety of topics, including hybridisation and polyploidy.

Phylogenetic and cytological research in the last decade has significantly increased our knowledge of diversity within the genus *Begonia*. In this review, the current classification and the evolution of species diversity is discussed with reference to recent progress in: a) population genetic and phylogenetic techniques using genetic markers in association with morphological characters and b) cytological techniques such as mitotic or meiotic chromosome visualisation, linked to genome size studies. We also present new data on barriers to hybridisation between *Begonia* species. We focus on how genetic, cytological and local ecological effects may contribute to diversity in this genus, particularly the evolution of species diversity.
