- Top of page
- Materials and methods
- Data Accessibility
- Supporting Information
The dispersal units of ferns are haploid spores, whereas in seed plants they are diploid seeds. Spores are minute, relative to most seeds, and easily transported by wind, suggesting that successful long-distance dispersal of ferns should be both easy and common. This idea, however, has been challenged by recent studies showing that the reproductive biology of ferns is more complex than previously assumed. Most diploid ferns are predominantly obligate outcrossers (Soltis & Soltis, 1987; Haufler, 2007), requiring that the male and female gametes originate from different gametophytes that, in turn, each originated from spores from different sporophytic individuals, termed intergametophytic crossing (Klekowski, 1973), or now simply referred to as outcrossing (K.M. Pryer et al., in prep.). Therefore, for a fern to migrate into a new environment, not one, but two, spores – from different individuals – are usually needed. Furthermore, the spores must land in such close proximity that the male gamete is able to swim to the female gamete in a thin film of water. This additional biological complexity confounds the presumed potential of small spore size for yielding a successful dispersal event. Hence, our understanding of fern biogeography is undergoing a paradigm shift where vicariance versus long-distance dispersal scenarios both need to be carefully evaluated in analysing the global distribution patterns of ferns (Wolf et al., 2001; Haufler, 2007).
Despite the reported bias of ferns towards outcrossing, however, there are studies showing that some ferns appear to be more easily dispersed than their sister group, the seed plants. For example, the proportion of fern species on islands, relative to angiosperms, has been shown to be higher than expected when compared with mainland diversity (Kreft et al., 2010; see also early work by Tryon, 1970; Smith, 1972). Furthermore, the level of fern endemism on islands is lower than that of angiosperms (Smith, 1972; Ranker et al., 1994), suggesting that the dispersal of fern individuals to islands occurs frequently enough to prevent those populations from becoming genetically distinct (Ranker et al., 1994).
What is the impact of long-distance dispersal in shaping the biogeography of ferns? To what extent are vicariance scenarios responsible for these patterns? Ultimately, to fully address these and other questions on fern distribution patterns from an evolutionary perspective, several comparable studies are needed across many clades of ferns. Although geography is often discussed in fern phylogenetic studies, more studies are needed that explicitly analyse the historical biogeography of a group (see e.g. Kreier & Schneider, 2006; Janssen et al., 2007, 2008; Perrie et al., 2007; Kreier et al., 2008; Hennequin et al., 2010), and that test hypotheses of vicariance versus long-distance dispersal by incorporating geological time.
Scaly tree ferns, Cyatheaceae, are a well-supported group of mostly tree-forming ferns and include approximately 500 species (Conant et al., 1995). Members of Cyatheaceae are found throughout the tropics, subtropics and the south-temperate zone (Kramer, 1990), with the greatest species diversity in tropical areas of America and Malesia (Conant et al., 1995). Recently, the phylogeny of scaly tree ferns received considerable attention (Conant et al., 1994, 1995; Conant & Stein, 2001; Korall et al., 2006, 2007; Janssen et al., 2008), such that we now have a robust understanding of the broader relationships within the group. Members of the family consistently fall into four major (genus-level) groups, with Sphaeropteris sister to the other three, Cyathea, Alsophila s.s. and Gymnosphaera + Cyathea capensis (Korall et al., 2007). The basal dichotomy is supported by scale morphology, with Sphaeropteris having conform scales and the others marginate scales (see fig. 1 in Korall et al., 2007).
Figure 1. Global biogeographical patterns for scaly tree ferns, Cyatheaceae. (a) Map showing eight biogeographical regions as defined in this study: South America (green), Africa (blue), Madagascar and neighbouring islands (pink), India and Sri Lanka (red), Southeast Asia (orange), Australasia (yellow), and Southwest Pacific (black) (the Atlantic region has been omitted in this figure since no ingroup taxa occur in the region). Map modified from http://commons.wikimedia.org/wiki/File:Blank_map_of_world_no_country_borders.PNG under the terms of the GNU Free Documentation License, version 1.2. (b–d) Schematic cladograms showing the three possible topologies among the marginate-scaled groups, and how these different topologies impact biogeographical reconstructions in early divergences of the family. (b) AlGy topology, i.e. with Alsophila and Gymnosphaera constrained as monophyletic. (c) AlCy topology, i.e. with Alsophila and Cyathea constrained as monophyletic. (d) CyGy topology, i.e. with Cyathea and Gymnosphaera constrained as monophyletic. (e) Full historical biogeographical reconstruction (using Lagrange) on the AlGy topology, i.e. the most common topology. Divergence dates were estimated using beast. Grey bars indicate 95% highest posterior density of the age estimates. Coloured squares indicate reconstructed ancestral ranges and mirror map colours in (a). Two-coloured squares denote ancestral ranges that include two of the regions defined in (a). Numbers adjacent to squares denote the relative probability of the ancestor having that specific ancestral range. The two squares at the root nodes denote the two scenarios with the highest relative probability. Hexagons (1–8) denote range expansion events.
Download figure to PowerPoint
Relationships among the three groups with marginate scales are unclear, with DNA sequence data not strongly supporting any of the possible resolved topologies. All four groups occur in both South America and Australasia, whereas only Alsophila s.s. and Gymnosphaera + C. capensis are represented in Africa. Fossils that can be referred to stem lineages of Cyatheaceae and the marginate-scaled clade show that the family had its origin at least as early as in the Late Jurassic (Lantz et al., 1999) and diversified in the Late Cretaceous (Mohr & Lazarus, 1994).
Given its robust phylogeny and a fossil record that suggests a Mesozoic origin for the group (i.e. dating back to the supercontinent Gondwana), the scaly tree ferns are an ideal model group with which to study fern dispersal in an evolutionary context (Salvo et al., 2010). In this study we analysed the large-scale historical biogeography of Cyatheaceae by using a phylogeny that covered the broad geographical distribution of the group. We investigated the possible impact of transoceanic dispersals in shaping the biogeography of scaly tree ferns, and tested the hypothesis that some of the observed distribution patterns were in fact compatible, in time and space, with a vicariance scenario related to the break-up of Gondwana.