**4.1 Restricted gene flow**

Whilst the genus *Begonia* has a very broad distribution, differentiation occurs over very local scales (Hughes, 2008). Three studies have used molecular markers to study local patterns of differentiation. Matolweni et al. (2000) investigated allozyme variation in 12 populations of *B. dregei* and 7 populations of the closely related *B. homonyma* from isolated forest patches in South Africa. This data showed little, if any, gene flow among populations of either *B. dregei* or *B. homonyma*, even between populations that are located only a few kilometers apart within the same forest. Allelic variation and heterozygosity were low, alleles were frequently unique to individual populations, and population differentiation (Fst values) for each locus were very high. This suggests strong and longterm isolation between populations generating extensive genetic divergence and high potential for speciation.

A comparable study was performed by Hughes & Hollingsworth (2008). Seven populations of the South African *B. sutherlandii* were sampled throughout the mist belt forests of

The Origin of Diversity in *Begonia*:

**5.1 Chromosome number** 

2010; Thomas et al., in press).

*socotrana* chromosomes in some Elatior hybrids.

Genome Dynamism, Population Processes and Phylogenetic Patterns 35

The occurrence of particular chromosome numbers in a given group is important for predicting reproductive barriers between species and the potential fertility of the hybrids, and can be indicative of a close evolutionary relationship. Among *Begonia* species, chromosome numbers range from 2n = 16 for *B. rex* to 2n = 156 for *B. acutifolia*. Between these extremes, a wide range of chromosome numbers have been described (Doorenbos et al., 1998; Legro and Doorenbos, 1969; Legro and Doorenbos, 1971; Legro and Doorenbos, 1973). Many species or cultivars exhibit chromosome numbers of 2n = 26 or 28 (x = 13 or 14) or a multiple of this number. Within the horticultural tuberous begonia group, derived from interspecific crosses between American *Begonia*, chromosome numbers of 2n=27,28 (diploid), 41,42 (triploid) and between 52 and 56 (tetraploid) are most common (Legro and Haegeman, 1971; Haegeman, 1979), but variation outside this sequence exists. In Asian *Begonia*, 2n = 22 (x = 11) is the most frequently observed chromosome number. A phylogeny of non-coding cpDNA also indicates a base chromosome number of x = 15 may be ancestral within Asian *Begonia*, with chromosome counts of 30 or 44 as diploid and triploid derivatives (Thomas,

The search for a basic chromosome number is complex as there is no common number observed in the group, even taking into account the prevalence of polyploidy in the horticutural varieties assayed. Some authors (Matsuura & Okuno, 1936; Matsuura & Okuno, 1943; Okuna & Nagai, 1953; Okuna & Nagai, 1954) have suggested x = 6, x = 7 and x = 13 as the basic chromosome number, where x=13 may be of secondary origin. By using genomic *in situ* hybridisation (GISH), Marasek-Ciolakowska (2010) concluded that x = 7 may be the basic chromosome number of *B. socotrana*. They based this conclusion on the presence of 7 *B. socotrana* chromosomes and 56 chromosomes derived from tuberous *Begonia* in Elatior hybrids. An alternative explanation of the genomic composition of these Elatior hybrids is selective chromosome elimination of *B. socotrana* chromosomes after hybridisation. Selective chromosome elimination is a genome stabilisation process, and cytological investigation by Arends (1970) supports a role for it in the breeding of Elatior *Begonia*, observing 9 or 12 *B.* 

The inferred African origin for *Begonia* may suggest a basic chromosome number will be found in these taxa, especially early branching lineages. However it is uncertain how the genomic composition, particularly in terms of chromosome number, has changed in extant African species relative to their ancestors. Most of the described chromosome numbers in African taxa vary between 36 and 38, but counts of 22, 26 and 28 have also been made. Chromosome numbers of 22, 26 and 28 appear to be prevalent in the East-African seasonally adapted *Begonia* from the sections *Rostrobegonia* and *Sexalaria,* which diverged very early during *Begonia* evolution, and from the sections *Augustia* and *Peltaugustia.* These sections show a closer relationship to American and Asian sections than to other African sections. The closely related *Hillebrandia sandwichensis* has a chromosome number of 2n = 48 (Kapoor, 1966), probably the result of a polyploidisation of a 'diploid' with 2n = 24. Within the most related family Datiscaceae, 2n = 22 is reported for *Datisca cannabina* (Gupta et al., 2009). The chromosome numbers within the family Cucurbitaceae are very diverse, but many species posses chromosome numbers between 2n = 20 and 26 or multiples of these numbers, and 2n = 14, 16 and 18 are also widely reported. In the Coriariaceae, multiples of 20 were observed (2n = 20, 40 and 60) while in Corynocarpus, 2n = 44 or 46 is reported. An exact list of chromosome numbers and references in the abovementioned families is available at the TROPICOS® database (www.tropicos.org). These numbers indicate that basic chromosome

Kwazulu-Natal, and population structure assessed using microsatellite markers. A similar population structure was observed to *B. dregei* and *B. homonyma*. Levels of population differentiation were high and there was significant differentiation between populations, even within subpopulations at a small spatial scale. The genetically isolated nature of *B. sutherlandii* populations suggest effective interpopulation dispersal is rare. There was no significant link between genetic and geographic distance suggesting that this differentiation is caused by genetic drift rather than through long-term isolation.

A third study of population structure in *Begonia socotrana* and *B. samhaensis* at the Socotra archipelago also shows the same pattern of strongly isolated populations (Hughes et al., 2003).

The low intraspecific gene flow could be due to both poor seed dispersal in the sheltered conditions of the forest floor and to limited pollen flow. *Begonia* flowers do not attract specialist pollinators but practice deceit pollination by generalist pollinators such as small bees and flies. Analysis of this pollination mechanism in the wild has confirmed that it does results in a low seed set (Ågren & Schemske, 1993; de Lange & Bouman, 1999).

Isolation-by distance may contribute to speciation in the genus. As described by Hughes & Hollingsworth (2008), the population-level data are congruent with the macro-evolutionary patterns observed in the genus. Molecular phylogenies confirm that *Begonia* is characterized by geographically constrained monophyly, species with narrow geographical ranges, very few widespread species, and high levels of morphological differentiation between populations of the few widespread species (Hughes & Hollingsworth, 2008).
