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Islands have held the fascination for biologists interested in natural history. Since Darwin's observations on the Galapagos, oceanic island archipelagos have provided clues about evolutionary patterns and processes and may rightly be considered as among the best places on earth to seek an understanding of the origin and elaboration of biological diversity. This idea has stimulated considerable modern work on the systematics, genetics, and ecology of island groups. Island floras are of great interest to evolutionary biologists (Stuessy and Ono, 1998), typically because they often contain many unique endemic species. The floras of some islands have an extremely high ratio of endemic species; for example, in Hawaii approximately 90% of the angiosperms are endemic (Groombridge, 1992). The endemics are morphologically divergent from continental relatives and are considered to have evolved in environments that differ from those of the mainland. Often, gene flow from the mainland to an island is highly limited due to spatial isolation. This isolation can result in rapid fixation of mutations and subsequent speciation (Barton, 1998). In addition, a lack of competition with other species and the possibility for colonization of new habitats may promote speciation on islands (Crawford et al, 1987).
Most work on island plants in the last century has been systematic or biogeographic in focus addressing issues concerned with endemism, adaptive radiation, and the phylogenetic history of island taxa (Carlquist 1974; Bramwell 1976). Plants in islands have often evolved through adaptive radiation, providing the classical model of evolution of closely related species each with strikingly different morphological and ecological features and with low levels of genetic divergence. There are certain limitations with using morphology to study both the origin and causes of species diversity both within an archipelago and on individual islands. Many island plants have diverged so dramatically from putative ancestral groups that it is difficult to ascertain their relationships and thus, to trace the pattern of their evolution. The traits are subject to high level of convergence or linked genetically and developmentally to the characters under going radiation. On islands radiation in which much of the phenotypic variation among species can be concentrated in the relatively few characters that underline each radiation(Grant 1986; Baldwin and Robichaux 1995; Givnish 1995). Convergence evolution can seriously skew the phylogeny, because the characters converge independently as a result of selection imposed on several traits simultaneously by a shared environment. However, numerous molecular phylogenetic studies have subsequently shed further light on the historical patterns of organismal evolution and their underlying mechanisms in islands (see examples in Baldwin et al., 1998 more references).The ability to reconstruct the phylogenetic histories of taxa has been dramatically improved by the now fairly routine task of acquiring DNA sequence data from taxa. There have also been significant developments for phylogenetic analysis of this data. Within a phylogenetic framework one can answer fundamental questions such as whether ecologically and/or morphologically similar species on different islands are the result of island colonization or convergent evolution. Testing hypotheses about the ages of individual species groups or entire community assemblages is also possible within a phylogenetic framework. For these reasons recent years have seen an increasing number of molecular phylogenetic analyses of flora, primarily in the Canary Islands, Hawaiian Islands, Galapagos Islands, and the Caribbean Islands. In particular, the Macaronesian islands, especially the Canaries, have become a focus for study of the colonization and the diversification of different organisms (see review Juan et al. 2000).
The phytogeographic region of Macaronesia is comprised of five Atlantic archipelagos (Azores, Madeira, Selvagens, Canaries, and Caper Verde) off the western coasts of Europe and Africa, situated between latitudes 15 ° and 40° N. The region has 24 major islands that exhibit abroad range of variation both in their ecology and geology. Geological ages vary from 21 million years (Myr) for Fuerteventura to 0.8 Myr for El Hierro (Rothe 1982; Mitchell-Thomé 1985;Galopim de Carvalho and Brandão 1991; Boekschoten and Manuputty 1993; Carracedo 1994).The combination of latitudinal gradients and northeastern trade winds has produced a number of distinct ecological zones (Bramwell 1972). The great habitat diversity and insular isolation are the main factors responsible for the rich flora of Macaronesia; at least 831 species and 40 genera are endemic to the region (Humphries 1979; Hansen and Sunding 1993; La Roche and Rodrígues-Piñero 1994).
The woody Sonchus alliance is one of the largest diverse endemics (6 genera and approximately 31 species) and represents a premiere example of adaptive radiation in Macaronesia(Aldridge 1975, 1979). The alliance is composed of 19 species of primarily woody members of Sonchus (subg. Dendrosonchus), seven species of Taeckholmia, one species of subg. Sonchus (S. tuberifer), and four monotypic genera Babcockia, Lactucosonchus, Sventenia, and Chrysoprenanthes (Kim et al. 1996a, b; Lee et al. 2005). These taxa are all endemic to the Canary Islands (except three species of Dendrosonchus in Madeira and S. daltonii in the Cape Verde archipelago; (Figure 1) and display extensive morphological, ecological, and anatomical diversity (Aldridge 1977, 1978). Despite this diversity, all taxa have a uniform chromosome number (n=9, 2n=18; Ardévol Gonzalez et al. 1993), and the high fertility of frequent interspecific and intergeneric hybrids suggests genetic cohesiveness within the alliance(Aldridge 1976; Hansen and Sunding 1993).
The Canary archipelago, where most of the alliance occurs, is located in the Atlantic
Ocean and consists of seven islands (Figure 1). These islands are of volcanic origin and form an approximately linear chain (McDougal and Schmincke 1976-77; Banda et al. 1981). In contrast to several remote archipelagos in the Pacific, such as the Hawaiian, Galapagos, and Juan Fernandez Islands, the Canaries have two unique biogeographical features. The proximity of the islands to the African continent (i.e., the eastern most island, Fuertuventura, is only about 100km distance from the west cost of Morocco) suggests that colonizers could reach the islands easily, and thus multiple colonization events may have occurred for some closely related taxonomic groups, like the woody Sonchus alliance. In addition, the broad range of geological ages of the archipelago, from 0.8 to 20 Myr, raises the possibility that some elements
of the Canarian flora are much older in origin than others (Fernandez-Palacios and Anderson 1993; Fuster et al. 1993; Carracedo 1994). There also has been a long controversy over whether some of the woody Macaronesian endemics are relict elements of a flora that extended along the Mediterranean basin during the Tertiary period or are recent derivatives from continental ancestors (Carlquist 1962, 1974; Bramwell 1972, 1975, 1976; Sunding 1979; Böhle et al. 1994).
Resolution of phylogenetic relationships within the alliance has been a difficult challenge, in part because of a lack of genetic variation in nrDNA sequences. In addition, despite the long cpDNA(>4000bp), the phylogeny was highly unresolved and weakly supported( Lee et al. 2005). Therefore, additional molecular characters are needed to accurately estimate phylogenetic relationships within the alliance. Phylogenetic analyses of plant taxa have been used to test for various evolutionary processes including reticulation. The examination of biparentally and uniparentally inherited markers in the same taxa is a sensitive means for detecting footprints of past reticulations. Previous investigations in the Hawaiian plants have concluded that hybridization has been of profound evolutionary significance in this flora (e.g., the silversword alliance; Baldwin et al. 1990). However, the role of hybridization in the Macaronesian flora is unclear (Francisco-Ortega et al. 1996; Brochmann et al. 2000).
Single- or low-copy nuclear genes, especially their introns, can be very useful in phylogenetic reconstruction of closely related plant species (Sang 2002). Although there are many theoretical and practical questions concerning the phylogenetic utility of low-copy nuclear genes, recent studies have demonstrated that rapidly evolving introns of low-copy nuclear genes can provide sufficient phylogenetic information to resolve interspecific relationships previously unresolved, or poorly resolved, by cpDNA or nrDNA (e.g., Doyle et al. 1996; Sang et al. 1997; Small et al. 1998; Emshwiller and Doyle 1999; Tank and Sang 2001).
In this study, we used three low-copy nuclear genes to the species of woody Sonchus alliance to investigate the phylogeny as well as to elucidate the extent to which hybridization has contributed to the radiation of woody Sonchus alliance.
Materials and Methods