Livistona palms in Australia: Ancient relics or opportunistic immigrants?
Introduction
Wallace’s Line is one of the longest recognized biogeographical boundaries on Earth (Whitmore, 1982). It is thought to represent an ancient divide between the biotas of Laurasia and Gondwana, which were separated by continental drift (vicariance) 180–160 Ma, then came into proximity again only recently (<25 Ma) as the Australian plate drifted north to collide with southeast (SE) Asia (Hall, 2002, McLoughlin, 2001, Scotese, 2004). Despite the marine barrier—there has been deep water between the continental shelves of Sundaland and Australia-New Guinea for >150 Ma (Hall, 2002, McLoughlin, 2001, Scotese, 2004)—some organisms are thought to have crossed Wallace’s Line and established sister taxa on either side of the barrier by allopatric speciation (Barlow and Hyland, 1988, Metcalfe et al., 2001, Whitmore, 1987). However, it remains unclear how much mixing there has been between the two continental biotas (Bowman et al., 2009a, Bowman et al., 2009b, Truswell et al., 1987). Vicariance and long-distance colonization hypotheses make contrasting predictions about the timing of divergences between sister taxa on either side of oceanic gaps (Crisp et al., 2009, Poux et al., 2005), such as Wallace’s Line. Vicariant taxa on either side of Wallace’s Line should have diverged more than 150 Ma, whereas divergences resulting from dispersal should be younger and increasing in frequency as the two continental plates came into proximity.
The fan palm genus Livistona makes an excellent case study for testing these alternative hypotheses because it has 36 species distributed on both sides of Wallace’s Line—mainly in SE Asia, Australia, New Guinea and Melanesia, with a single outlying species (L. carinensis) in Somalia, Yemen and Djibouti (Dowe, 2009, Rodd, 1998). Two Australian species (L. benthamii and L. muelleri) are shared with southern New Guinea but neither crosses Wallace’s Line; nor does any other species occurring in Papuasia, except L. rotundifolia (Baker and Dransfield, 2006, Dowe, 2009). Both vicariance (Rodd, 1998) and long-distance colonization (Dransfield, 1987) have been suggested to explain the disjunction of Livistona across Wallace’s Line. Dransfield (1987) argued that palms originated in Pangea and have both Laurasian and Gondwanan descendant lineages but that Livistona migrated from Sundaland (in Malesia, on the Laurasian side of Wallace’s Line) and is not a Gondwanan relic. Rodd (1998) speculated that “proto-Livistona had already emerged by the time Laurasia and Gondwana began to separate, and its progeny evolved independently but extremely slowly (by flowering-plant standards) in both landmasses and their daughter lands”. The Australian fossil record does not show when Livistona reached Australia, lacking Livistona or even unequivocal representatives of its subfamily (Coryphoideae), except possibly Palmoxylon queenslandicum (Conran and Rozefelds, 2003), whose affinity is unclear. The first aim of this study was to use phylogenetic molecular dating to determine whether the distribution of Livistona is the result of vicariance or more recent dispersal across Wallace’s Line.
The geographic pattern and timing of diversification of Livistona within Australia is the second major focus of this study. Whereas Livistona in Asia occurs mostly in aseasonal wet communities, Australian taxa are found primarily in the monsoonal tropics, ranging east from the Kimberley to Cape York Peninsula and south along the coast and adjacent ranges of Queensland (Table 1; Fig. 1). Did the monsoonal tropics provide an ecological opportunity for an immigrant ancestor to radiate? Additionally, two Australian Livistona species occur as outliers in the arid zone (Fig. 1): L. alfredii in the Pilbara, Western Australia, and L. mariae, in Central Australia, often described as ‘relictual’ (Greenwood and Conran, 2000, Latz, 1975, Rodd, 1998, White, 1988). Are the species in the arid zone remnants of once widespread species whose ranges have subsequently contracted with the aridification of Australia (from 15 Ma, Macphail, 1997, Martin, 2006), such that they are now restricted to small refugia (Dowe, 1995, Dransfield, 1987)?
To summarize, the aims of this study were to use phylogenetic molecular dating to (1) test the Gondwanan relic hypothesis versus that of recent immigration from Asia, and (2) reconstruct the timing and direction of habitat transitions in relation to likely driving events in geological and climatic history.
Section snippets
Sampling
All 18 Australian Livistona species (after Dowe, 2009, Rodd, 1998) were sampled, as well as six of their 18 congeners from elsewhere, representing the geographic and ecological range of the genus (Table 1). Outgroup sampling (Appendix 1) was based on Asmussen et al. (2006), included all five subfamilies of Arecaceae and aimed to also include (1) genera closely related to Livistona, especially within the tribe Trachycarpeae and subfamily Coryphoideae, and (2) groups across the Arecaceae with
DNA sequences
All new sequences have been deposited in GenBank (Accession Nos. AB521750–521753, AB522003–522084, AB522432–522654, and AB523393–523396). The cpDNA sequences showed little length variation within the Trachycarpeae: psbB-psbF, 678–683 bp; rpl20-rps12, 562–575; trnT-trnD, 736–738; psbZ, 503–548; trnL intron + trnL-trnF, 880–925. Their base composition was AT-biased (64%) but stationary among taxa (P = 1.0000). The MS sequences were compositionally unbiased and stationary among taxa (P = 1.0000). They
Origin of Livistona in Australia
The Australian clade of Livistona originated from SE Asia, according to both parsimony and ML optimisation. Our best estimates of when this lineage diverged from its Asian sister group are 5.9–16.8 Ma (=minimum crown age to maximum stem age). This timing decisively rejects a Laurasian origin (Rodd, 1998) in favour of the hypothesis that Livistona dispersed across Wallace’s Line as Australia approached SE Asia from the south (Dransfield, 1987).
Our results also suggest that Papuasia was colonised
Conclusion
If, as the geological record suggests, the monsoonal biome existed in Australia from the late Eocene, we would predict that modern-day descendants of early lineages might still occupy that biome. We know that adaptation to biomes has been conserved over such time periods (Crisp et al., 2009). Livistona was thought to be an ancient and relictual lineage in the Australian tropics but our data show that it is a more recent immigrant – most likely around 15 Ma and unlikely before the Miocene. Little
Acknowledgments
This research was partly supported by grants from the Ministry of Education, Science, Sports and Culture of Japan (14405006) and the Australian Research Council (DP0665253). We thank John Dowe and Stephen van Leeuwen for assistance with the field work and for providing background material; Dan Rosauer for drafting the base map of Fig. 1; Andrew Thornhill for assistance with collecting; Melita Baum and David Morris for assistance with the lab work; Bill Baker, Bee Gunn and Madeleine Harley for
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