Although molecular studies focussing exclusively on vireyas started during the last decade, it is interesting to note that the first genetic study on vireyas was carried out by Henslow (1891) using non-molecular methods, and thus gave an insight into the complex relationships among vireyas. One interesting outcome of this study was the observation that the hybrid produced resembled either parent in either way and in various degrees, contrary to the popular belief at that time, that a hybrid resembles the male parent in the flowers, and the female parent in foliage. Moreover, Henslow also observed that
characters in the grandparents or higher ancestry were shown to reappear in the hybrid, having been more or less absent in the parents.
The majority of the molecular studies on Rhododendron in the past included a few representatives of vireyas: Kron & Judd (1990); Scheiber et al. (2000); Kurashige et al. (2001); Tsai et al. (2003); Milne (2004); Goetsch et al. (2005); Chung et al. (2007); Craven et al. (2008). These studies were aimed at resolving the phylogeny of the genus Rhododendron in general, and have been discussed in detail in the Section 2.1.3. The limited number of species used in these studies restricted the results to general comments on the relationship between vireya and other sections. There were insufficient taxa to draw any firm conclusions about relationships within vireya.
In contrast, the analyses carried out by Brown et al. (2006a, 2006b; 2006c) are the most comprehensive molecular studies on vireyas to date and marked a milestone in vireya research. These studies gave a new insight into the relationships within the vireyas, and relationships between the vireyas and the rest of the genus Rhododendron. The first of these studies by Brown et al. (2006a) was a phylogenetic analysis using the sequence data for the ITS nrDNA region, using 39 accessions of vireyas (representing 32 taxa). This study showed that Subgenus Rhododendron Endlicher is monophyletic (Figure 16).
Figure 16 ITS nrDNA region 50% majority rule tree of vireyas from Bayesian analysis. Bayesian analysis iteration number 5; log-likelihood range −2470 to −2490. Nodes that are not resolved by the parsimony analysis (tree length 135, CI=0.72, RI=0.93) are marked with *. Bayesian posterior probability values are shown above the node and bootstrap values are shown below the nodes they support. All species of Section Vireya are indicated by a V and an abbreviation of the subsection: Albovireya, VA; Euvireya, VE; Malayovireya, VM; Phaeovireya, VPh; Pseudovireya, VPs; Siphonovireya, VSi; Solenovireya, VSo. The subgenus, or section of Subgenus Rhododendron taxa, and outgroup taxa are also indicated: Subgenus Rhododendron, Section Rhododendron, RR; Subgenus Rhododendron, Section Pogonanthum, RP; Subgenus
Azaleastrum, Az; Subgenus Mumeazalea, M; Subgenus Hymenanthes, H; Subgenus Pentanthera, P. Parentheses are used to identify the different sequence accession (see Table 1) or the GenBank numbers of sequences sourced from the GenBank database. General area distributions are also shown. Adapted from Brown et al. (2006a).
Brown et al. (2006a) also showed that Pseudovireya was paraphyletic, and formed two clusters (nodes 9 and 11 in Figure 16), a small cluster corresponding to mainland Asian species (node 9), and a larger cluster (node 11) with two subclusters corresponding to Taiwanese (node 12) and Malesian (node 13) species respectively. Pseudovireya was also shown to be sister to the rest of the vireyas, labelled as ‘Euvireya’ (node 15). This Euvireya clade consists of a mixture of all the subgroups within Vireya excluding Pseudovireya, and these subgroups were shown to be paraphyletic. Strikingly, the subclades formed within the Euvireya clade correspond to specific geographic regions (Figure 16). This research thus showed that the relationships among the species of Section Vireya do not correspond to the traditional classification based on morphology, instead correlate strongly with geographic areas, with a disjunction between an Australian–New Guinea clade and clades of west and middle Malesian taxa.
Figure 17 Combined cpDNA strict consensus tree for 59 vireya taxa. Strict consensus of 144 trees of length 191 from Parsimony analysis, using the psbA-trnH and trnT-trnL data sets combined, and including indel characters; CI = 0.62 and RI = 0.85. Nodes not resolved by the four long Bayesian analyses (log likelihood range –3970 to –4010) are indicated *. Nodes not resolved in the individual analyses, but resolved in all combined analyses, are underlined. Bayesian posterior probability values are shown above the node and bootstrap values are shown below the nodes that they support. Posterior probability values marked # were found in the short Bayesian analysis (log likelihood range -3690 to -3740) but not the long Bayesian analyses (log likelihood range –3970 to –4010). The three main clades of ‘Euvireya’ are indicated by arrows.
Hymenanthes = Subgenus Hymenanthes; Rhododendron = section Rhododendron. Adapted from Brown et al. (2006b).
The second study by Brown et al. (2006b) further examined the phylogeny of vireyas using two non-coding regions of cpDNA (psbA-trnH and trnT-trnL). The phylogenetic analyses of these two cpDNA regions, representing 75 vireya taxa, showed that the Section Vireya (Blume) H. F. Copeland was monophyletic (node 1 in Figure 17). Similar to the study of Brown et al. (2006a), Pseudovireya was shown to be paraphyletic, and formed two clades corresponding to mainland Asian species (node 2), and Malesian and Taiwanese species (node 4). The clade containing taxa belonging to the subsections Euvireya, Siphonovireya and Malayovireya were shown to be monophyletic, while the individual subsections were shown to be paraphyletic as in the ITS nrDNA study of Brown et al. (2006a). The groups supported by the cpDNA analyses strongly relate to geographic regions rather than taxonomic groupings, the most obvious of these being the general split between the regions eastern Malesia (node 28), and western and middle Malesia (node 14).
Brown et al. (2006c) is a cladistic biogeographic study of vireya rhododendrons, combining the results of Brown et al. (2006a) and Brown et al. (2006b). The molecular phylogenetic analysis of the vireya rhododendrons showed that a major clade divergence correlates with a distinct biogeographic pattern: one major clade restricted to the east of Wallace’s Line35 and another to the west. Based on geographic pattern, it was argued that the vireyas are an old Gondwanan group, the alternative hypothesis being that the group is young, assuming that low molecular distances between taxa within clades reflects a young age, which in turn requires long-distance dispersal to explain distribution patterns. It may be that deep divergences within the vireyas have an old history, but that diversification within clades is more recent (Brown et al. 2006c).
35 An imaginary line drawn in 1859 separating the ecozones of Asia and Wallacea (a group of Indonesian islands separated by deep water straits from the Asian and Australian continent shelves), a transitional zone between Asia and Australia (Whitmore 1981, 1982).
Figure 18 Maximum parsimony strict consensus tree for Rhododendron Subgenus
Rhododendron based upon rpb2i sequences. All bootstrap values > 50% are shown. Adapted from Craven et al. (2008).
The study by Craven et al. (2008) included the results of the studies by (Brown et al. 2006a, 2006b; Goetsch et al. 2005; Hall et al. 2006), and marked another milestone in the classification of the genus Rhododendron. Based on molecular data, this study did not agree completely with the morphological classifications of Sleumer (1966a), Chamberlain et al. (1996) and Argent (2006). The relative positions of three vireya groups
were made clear in this study, agreeing with Argent’s views that a practical method is required for dealing with the large number of vireya species (Argent 2006). This was achieved by treating the relevant subsections sensu Sleumer (1966a) as informal groups, thus facilitating identification but not compromising the principle that formal classification should be based on evolutionary relationships (Craven et al. 2008). Molecular data supported the taxonomic groups Malayovireya and Euvireya sensu Sleumer (1966a), however did not support the other subsections and series of Sleumer (Figure 18). The core vireya36 complex was shown to be comprised of a large group of actively evolving (including radiating and interbreeding) species, of which the speciose and morphologically ultradiverse New Guinea clade was notable (Craven et al. 2008).
Craven et al. (2008) clearly showed that the Subgenus Vireya (sensu Argent) is polyphyletic and embedded within Subgenus Rhododendron (sensu Craven), and therefore cannot be a sister taxon to it. Figure 18 illustrates the maximum parsimony strict consensus phylogenetic tree for Rhododendron Subgenus Rhododendron based upon rpb2 sequences. A major difference seen is the paraphyly of Vireya (sensu Argent and Sleumer). Section Pseudovireya is sister to the Section Vireya. Section Discovireya is not sister to the clade formed by the sections Vireya and Pseudovireya, but more closely related to the temperate sections Rhododendron and Pogonanthum. The group Euvireya remains intact in all these classification systems (Brown et al. 2006a, 2006b; Craven et al. 2011). Distinguishing between the Pseudovireya and Siphonovireya can be difficult, but Malayovireya, Albovireya and Phaeovireya all seem coherent and largely monophyletic (Stevens 1985). The long, narrowly tubular corolla of Solenovireya and Siphonovireya is distinctive, but it is unclear whether these two sections are monophyletic (Heads 2003), and have not been shown to be so in any of the molecular studies discussed above.
36 Taxa of Subgenus Vireya sensu Argent (2006) excluding those belonging to the sections Pseudovireya and Discovireya.
Figure 19 Inferred phylogeny of Rhododendron Section Schistanthe based upon rpb2i, rpb2d and rpc1 sequence data. The numbers indicate bootstrap support. Adapted from Craven et al. (2011).
Craven et al. (2011) further investigated the evolutionary relationships of the Vireya group of Rhododendron, utilising nuclear DNA sequence data, and demonstrated that this group of species is monophyletic, and a revised classification was presented (Figure 19). As the name Vireya was predated at sectional level by several other valid names, the correct name for the section is now Schistanthe (Craven et al. 2010). Within Schistanthe, four subsections are recognised: Pseudovireya, Discovireya, Malayovireya and Euvireya. The study proposed a revised classification of the vireyas (with identification keys):
Genus Rhododendron L. Subgenus Rhododendron
Section Rhododendron
Section Pogonanthum G. Don Section Schistanthe Schlechter Subsection Discovireya Sleumer
Subsection Pseudovireya (C. B. Clarke) Sleumer Subsection Malayovireya Sleumer
Figure 20 Maximum parsimony strict consensus tree based upon the combined data for rpb2i,
Goetsch et al. (2011) is the most comprehensive molecular study of the Vireya group published to date, consisting of a phylogeny derived from analysis of sequences from multiple nuclear genes, rpb2i, rpb2d and rpc1. An analysis based on the combined sequences of these three nuclear genes supported a phylogeny in which the reinstated rank Section Schistanthe (excluding R. santapaui) is monophyletic, with well-defined clades corresponding to the subsections Euvireya, Malayovireya, Pseudovireya, and Discovireya (Figure 20). Within the Subsection Euvireya, the subclades follow geography more closely than traditional taxonomic groupings based on morphology. One of the two most derived clades contained exclusively species from New Guinea, Australia, and the Solomon Islands. The results are consistent with a stepwise phylogeographic history of Section Schistanthe, originating in Asia, spreading eastward to New Guinea within the last 15 mya, when movement of the Australian tectonic plate brought New Guinea into the Malesian domain.
A study conducted on the Rhododendron of Taiwan by Tsai et al. (2012) showed that R. kawakamii (belonging to the Subsection Pseudovireya) is separated from the other Rhododendron species in the study. The results were in agreement with the habitat preference of Rhododendron in Taiwan, in which R. kawakamii is epiphytic (a trait shared by the majority of the vireyas) and thus ecologically separated from the other Rhododendron species, which are either shrubs or trees. Molecular data showed that this species is distinct from the other Rhododendron species in Taiwan.