Global biogeography of scaly tree ferns (Cyatheaceae): evidence for Gondwanan

Global biogeography of scaly tree ferns (Cyatheaceae): evidence for Gondwanan
Journal of Biogeography (J. Biogeogr.) (2014) 41, 402–413
ORIGINAL
ARTICLE
Global biogeography of scaly tree ferns
(Cyatheaceae): evidence for Gondwanan
vicariance and limited transoceanic
dispersal
Petra Korall1* and Kathleen M. Pryer2
1
Systematic Biology, Evolutionary Biology
Centre, Uppsala University, Norbyv€agen 18D,
SE-752 36, Uppsala, Sweden, 2Department of
Biology, Duke University, Durham, NC,
27708, USA
ABSTRACT
Aim Scaly tree ferns, Cyatheaceae, are a well-supported group of mostly treeforming ferns found throughout the tropics, the subtropics and the south-temperate zone. Fossil evidence shows that the lineage originated in the Late Jurassic period. We reconstructed large-scale historical biogeographical patterns of
Cyatheaceae and tested the hypothesis that some of the observed distribution
patterns are in fact compatible, in time and space, with a vicariance scenario
related to the break-up of Gondwana.
Location Tropics, subtropics and south-temperate areas of the world.
Methods The historical biogeography of Cyatheaceae was analysed in a maximum likelihood framework using Lagrange. The 78 ingroup taxa are representative of the geographical distribution of the entire family. The phylogenies
that served as a basis for the analyses were obtained by Bayesian inference
analyses of mainly previously published DNA sequence data using MrBayes.
Lineage divergence dates were estimated in a Bayesian Markov chain Monte
Carlo framework using beast.
*Correspondence: P. Korall, Systematic
Biology, Evolutionary Biology Centre, Uppsala
University, Norbyv€agen 18D, SE-752 36
Uppsala, Sweden.
E-mail: [email protected]
This is an open access article under the terms
of the Creative Commons
Attribution-NonCommercial License, which
permits use, distribution and reproduction in
any medium, provided the original work is
properly cited and is not used for commercial
purposes.
Results Cyatheaceae originated in the Late Jurassic in either South America or
Australasia. Following a range expansion, the ancestral distribution of the marginate-scaled clade included both these areas, whereas Sphaeropteris is reconstructed as having its origin only in Australasia. Within the marginate-scaled
clade, reconstructions of early divergences are hampered by the unresolved
relationships among the Alsophila, Cyathea and Gymnosphaera lineages. Nevertheless, it is clear that the occurrence of the Cyathea and Sphaeropteris lineages
in South America may be related to vicariance, whereas transoceanic dispersal
needs to be inferred for the range shifts seen in Alsophila and Gymnosphaera.
Main conclusions The evolutionary history of Cyatheaceae involves both
Gondwanan vicariance scenarios as well as long-distance dispersal events. The
number of transoceanic dispersals reconstructed for the family is rather few
when compared with other fern lineages. We suggest that a causal relationship
between reproductive mode (outcrossing) and dispersal limitations is the most
plausible explanation for the pattern observed.
Keywords
Cyatheaceae, Gondwana, historical biogeography, Lagrange, long-distance dispersal, scaly tree ferns, transoceanic dispersal, vicariance.
INTRODUCTION
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
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doi:10.1111/jbi.12222
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
ª 2013 The Authors Journal of Biogeography Published by
John Wiley & Sons Ltd
Biogeography of scaly tree ferns (Cyatheaceae)
& 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 southtemperate 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).
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.
MATERIALS AND METHODS
Nomenclature
Taxonomy within Cyatheaceae differs among authors. We
follow Korall et al. (2007) and use generic names corresponding to the major lineages within the family: Alsophila,
Cyathea and Sphaeropteris. Species within the Gymnosphaera +
C. capensis clade are all referred to as Cyathea and not
Alsophila as in Korall et al. (2007) to accommodate those
taxa new to this study for which Alsophila synonyms are not
available.
Taxon sampling
The ingroup comprised a total of 78 taxa (see Appendix S1
in Supporting Information) encompassing the geographical
(as well as morphological and taxonomic) variation within
Cyatheaceae and includes almost all taxa from Korall et al.
(2007), as well as 14 additional species from Janssen
et al. (2008). The Alsophila sinuata specimen used in Korall
et al. (2007) was excluded here and replaced by A. sinuata
from Janssen et al. (2008) because more sequence data were
available for the latter.
The outgroup includes 14 tree fern representatives from
three closely related families (Korall et al., 2006; Schuettpelz
& Pryer, 2007): 10 representatives from Dicksoniaceae,
three from Cibotiaceae, and Thyrsopteris elegans from the
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403
P. Korall and K. M. Pryer
monotypic Thyrsopteridaceae [family circumscriptions follow
Smith et al. (2006); Appendix S1]. This sampling covers the
entire geographical range for the outgroup. Representatives
from Metaxyaceae were excluded to avoid analytical problems potentially associated with the elevated substitution
rates previously reported for the family (Korall et al., 2006).
Molecular data
Almost all the DNA sequence data used in this study were
previously published in either Korall et al. (2007) or Janssen
et al. (2008). GenBank accession numbers are presented in
Appendix S1 for the five plastid regions analysed, comprising
the protein-coding rbcL gene, and four non-coding, intergenic spacer (IGS) regions: rbcL–accD [including 93 bases from
the rbcL gene (bases not included in the rbcL data set) and
779 from the accD gene], rbcL–atpB, trnG–trnR (trnG–R,
includes the trnG intron), and trnL–trnF (trnL–F, includes
the trnL intron). New sequence data is provided here for the
non-coding regions of Thyrsopteris elegans (GenBank accession numbers HG422547–HG422550). DNA isolation, amplification and sequencing of these regions were as described in
Korall et al. (2007). For a few taxa, sequences from all
regions were not available (one sequence was missing for
rbcL–atpB, two for rbcL–accD, one for trnG–R, and ten for
trnL–F; see Appendix S1); in these cases, the sequence data
were coded as missing.
Sequence alignments
Each region was aligned manually using MacClade 4.08
(Maddison & Maddison, 2005). Ambiguously aligned regions
were excluded from the analyses and no gap coding was performed. The sequence alignments are deposited in the Dryad
data repository (see Data Accessibility below).
Phylogenetic analyses
The five single-region data sets and the combined data set
were analysed with a Bayesian Markov chain Monte Carlo
(BMCMC) approach using the parallel version of MrBayes
3.1.1 (Huelsenbeck & Ronquist, 2001; Ronquist & Huelsenbeck, 2003). Nucleotide substitution models for each of the
regions were chosen using MrAIC 1.4 (Nylander, 2004) in
combination with phyml 2.4.4 (Guindon & Gascuel, 2003).
Model choice was based on the corrected Akaike information
criterion (AICc) and was the GTR+I+G model for the rbcL
and rbcL–accD regions, and the GTR+G model for rbcL–
atpB, trnG–R and trnL–F. Each analysis was run for 10 million generations, on six parallel chains, with the temperature
parameter (for heating the chains) set to 0.1. Four independent analyses of each region were run simultaneously. The
values sampled for different parameters were examined using
the program Tracer 1.5 (Rambaut & Drummond, 2009) to
determine whether the parameters had converged. We also
examined the standard deviation of the split frequencies
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among the independent runs as calculated by MrBayes. For
each analysis, every 1000th tree was sampled and, after
parameter values were analysed, 1000 initial trees were discarded as ‘burn-in’. Trees from each of the independent
analyses (except those discarded as burn-in) were pooled
before calculating a majority-rule consensus tree for each
region. All trees were rooted with the most distant outgroup
taxon, Thyrsopteris elegans (Korall et al., 2006).
Combinability of data sets
The combinability of data sets was evaluated by examining
the consensus topologies from each of the five single-region
analyses for potential conflicts. Incongruence supported by a
Bayesian posterior probability of 0.99 or higher was considered a conflict. A single conflict was found at the branch
tips, involving the position of Alsophila lastii; it was sister to
Alsophila dregei in the rbcL topology, and in a polytomy with
Alsophila hyacinthei and Alsophila glaucifolia in the trnG–R
topology. Given the very minimal conflict detected, the five
data sets were combined into a single concatenated data set
including a total of 5889 characters (not counting the
excluded characters).
Analyses of the combined data set
The BMCMC analysis of the combined data set was performed using a single partition for the entire data set (the
GTR+I+G model was selected as described above), and with
the same settings as for the single-region analyses. Previous
studies have shown that the relationships among Cyathea,
Alsophila s.s. and Gymnosphaera + C. capensis are unclear,
irrespective of analytical method used (maximum parsimony,
maximum likelihood or Bayesian inference; see Korall et al.,
2007, and references therein). The frequency of each of the
possible resolved topologies among these three groups was
calculated from the pool of trees (after removal of the burnin) resulting from the Bayesian analysis of the combined data
set by using the Perl script ‘seltrees.pl’ (written by T. Eriksson, University of Bergen, Norway).
Molecular dating
Estimates of lineage divergence times were calculated in a
BMCMC framework using beast 1.5.4 (Drummond & Rambaut, 2007). The combined data set was analysed as a single
partition using the GTR+I+G model with four rate categories
(model chosen as for the phylogenetic analyses above), an
uncorrelated, lognormal clock model, a Yule tree prior, and
the starting tree was randomly generated. The MCMC chains
were run for 10 million generations, and parameters were
sampled every 1000 generations.
Because the results of the phylogenetic analyses using
MrBayes (above) were inconclusive with respect to the relationships of the three groups of marginate-scaled taxa (Cyathea, Alsophila s.s., and Gymnosphaera including C. capensis),
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Biogeography of scaly tree ferns (Cyatheaceae)
three independent beast analyses were performed, each with
topological constraints reflecting the three possible sister
relationships (i.e. with Alsophila + Cyathea, Alsophila + Gymnosphaera, and Cyathea + Gymnosphaera). This allowed us to
investigate how each of the different topologies would affect
our biogeographical interpretations. All analyses were also
constrained so that Thyrsopteris elegans was designated sister
to all other taxa.
Three calibration points based on fossil evidence were
incorporated in each of the analyses. All three calibration
points were assigned a lognormal prior distribution to allow
for the possibility of very old ages, but with larger probability
closer to the estimated age of the fossil. The mean and standard deviation were set to 1, and the offset was conservatively
set to the estimated age (following Gradstein et al., 2004) of
the upper boundary of the strata where the fossil was found.
Fossils referred to the genus Cyathocaulis, such as Cyathocaulis naktogensis and Cyathocaulis yabei (Upper Jurassic) are
stem members of Cyatheaceae, as shown in a morphological
phylogenetic analysis based on trunk characters of extinct and
extant tree ferns (Lantz et al., 1999). The upper boundary of
the Upper Jurassic (145.5 Ma) therefore serves to calibrate
the most recent common ancestor of Cyatheaceae and its sister group, sensu Korall et al. (2006, 2007) and Smith et al.
(2006). The triporate spores of the fossil genus Kuylisporites
are similar to spores of extant species in the clades Cyathea
and Alsophila, but not to Sphaeropteris (Mohr & Lazarus,
1994; Collinson, 2001). Kuylisporites waterbolkii is sometimes
more specifically referred to ‘Cnemidaria’ taxa within Cyathea
(Mohr & Lazarus, 1994). However, this interpretation has
been questioned based on similar spores observed in Alsophila
decurrens (Collinson, 2001), suggesting a position along the
stem lineage of the marginate-scaled clade. We therefore conservatively use the first appearance of these fossils (Cenomanian, Upper Cretaceous, upper boundary of strata 93.5 Ma)
to calibrate the most recent common ancestor of the marginate-scaled clade + Sphaeropteris (i.e. Cyatheaceae). The fossils
Lophosoria cupulatus and Conantiopteris (both Aptian, Lower
Cretaceous) are, based on spore, leaf and/or trunk morphology, considered to be closely related to the extant species Lophosoria quadripinnata, either as stem or crown group
members (Cantrill, 1998; Lantz et al., 1999, includes a phylogenetic analysis based on trunk characters). We assign the age
of the upper boundary of the Aptian, Lower Cretaceous (112
Ma) to the most recent common ancestor of Dicksoniaceae,
taking into account the moderately supported relationships
among the three genera in the family (Calochlaena, Dicksonia
and Lophosoria; Korall et al., 2006).
Each beast analysis was repeated twice and the sampled
values were examined for convergence using the program
Tracer 1.5 (Rambaut & Drummond, 2009). After the
removal of the burn-in (1 million generations in each
analysis, corresponding to 10% of the samples) the remaining samples from the two runs were summarized as a
maximum clade credibility tree with mean divergence times
using TreeAnnotator (part of the beast package).
Biogeographical analyses
We performed biogeographical analyses in a maximum likelihood framework using Lagrange v.20100721 (Ree et al.,
2005; Ree & Smith, 2008). Three analyses were performed,
one for each of the three possible resolved topologies, and
based on the time-calibrated trees resulting from the beast
analyses. We defined eight biogeographical regions, mainly
following Sanmartın & Ronquist (2004), but with a few
exceptions where the aim of our study was better addressed
using more inclusive units. These regions are: (A) South
America (including Mexico and Central America); (B) Atlantic (including St. Helena); (C) Africa; (D) Madagascar, Comoros, Reunion and Mauritius; (E) India (including Sri
Lanka); (F) Southeast Asia (including the Malaysian Peninsula, Philippines, Sumatra and Borneo); (G) Australasia
including Australia, New Zealand, New Caledonia and New
Guinea, as well as Lord Howe and Norfolk Islands (this is the
greatest departure from Sanmartın & Ronquist (2004), who
treated this area as four separate regions); and (H) Southwest
Pacific (including Fiji and Hawaii). Information on the geographical distribution of the terminal taxa was gathered from
the literature (mainly Holttum, 1963, 1964, 1965; Conant,
1983; Conant et al., 1995; Conant & Stein, 2001; Large &
Braggins, 2004). The distribution ranges for each of the
extant taxa (the terminals) are almost always restricted to a
single region (86 out of 91 taxa, the remaining five taxa occur
in two regions, Appendix S1). Therefore, when running the
Lagrange analyses, the ancestral ranges were set to include a
maximum of two of the eight defined regions. All possible
combinations of regions were allowed. Because our aim was
to test whether vicariance could explain some of the patterns
observed, we used a conservative approach with no dispersal
constraints. The rate parameters were estimated. Analyses
were set up using the online configuration tool (http://www.
reelab.net/lagrange). The Python script ‘Output.py’ in
Lagrange was manually modified to report all relative
posterior probabilities (default setting is to report up to a
cumulative probability of 0.95). This allowed us to calculate,
for each daughter lineage, the sum of the relative probabilities
for each possible ancestral area; in each case, the area yielding
the highest relative probability is presented here.
RESULTS
Phylogenetic relationships
The phylogenetic analysis using MrBayes shows a robustly
supported topology (see Fig. S1 in Appendix S2). All relationships discussed below are supported by a posterior probability
(PP) of 0.99, unless otherwise stated. Within Cyatheaceae,
Sphaeropteris is sister to the marginate-scaled clade, which
includes three lineages: Cyathea, Alsophila s.s. and Gymnosphaera + C. capensis (the two latter groups are hereafter referred to
as Alsophila and Gymnosphaera, respectively). The relationships
among these groups, however, are unclear, with none of the
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P. Korall and K. M. Pryer
three possible topological resolutions receiving support above
PP 0.5. Our calculations suggest that the Alsophila + Gymnosphaera (‘AlGy’) topology is slightly more common (present in
39% of the pool of trees, after the removal of the burn-in) than
the other two topologies, Alsophila + Cyathea (‘AlCy’, present
in 31% of the trees) and Cyathea + Gymnosphaera (‘CyGy’,
present in 30% of the trees). All other ingroup relationships are
the same as in previously published studies (Korall et al., 2007;
Janssen et al., 2008). It should be noted that, besides the singlepartition analysis presented here, an analysis with five partitions
representing each of the different plastid regions was tried in
order to address differences in evolutionary constraints. The
analysis, however, failed to converge (data not shown). Because
the topology resulting from our single-partition analysis is congruent with a previous study that used a five-partition scheme
in the analysis (Korall et al., 2007), we consider it to be a reasonable estimate of the phylogeny. The topologies resulting
from the beast analyses were congruent with the MrBayes
analyses (except when certain nodes were constrained) (Fig. 1
and Figs S1–S4 in Appendix S2).
Divergence date estimates and biogeographical
patterns
Divergence date estimates calculated for the three differently
constrained topologies resulted in very similar estimates, and
differed by only a few million years across the analyses (Figs
S2–S4). These differences are minor when compared to the relative uncertainty inherent in each analysis, as reflected in the
95% highest posterior density of the age estimates (grey horizontal bars in Fig. 1e and Figs S2–S4). The data set is also
rather robust to different analytical schemes. Preliminary
analyses that changed the prior distributions of the fossil constraints or that analysed the data using penalized likelihood
(Sanderson, 2002) showed mostly very minor differences (data
not shown).
The estimated dates from the beast analysis of the most
frequent topology (i.e. with Alsophila + Gymnosphaera being
constrained as monophyletic) are presented below. Age estimates from the analyses based on the other two constrained
topologies are found in Figs S3 and S4. The results of the
three separate biogeographical analyses were congruent across
most nodes (detailed results, including all possible reconstructions, from the three analyses are deposited in the Dryad
data repository, see Data Accessibility). The major exceptions
are early divergences in the marginate-scaled clade – an obvious consequence of the three different topologies constrained
for the possible relationships among the Alsophila, Cyathea
and Gymnosphaera lineages (Fig. 1b–d). The phylogenetic
reconstruction of biogeographical patterns that is presented
below focuses mainly on large-scale, transoceanic events.
Origin of Cyatheaceae – the scaly tree ferns
Cyatheaceae diverged from its sister lineage Dicksoniaceae
150 Ma in the Late Jurassic (Fig. S2), with the crown group
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originating 96 Ma in the mid-Cretaceous (Fig. 1e, Fig. S2).
The geographical range of the ancestor to the crown group is
inconclusive, however, with two areas estimated as possible:
the areas that today are South America or Australasia
(Fig. 1b–d). Neither of these geographical alternatives
receives strong support under any of the three differently
constrained scenarios; the ancestral area assigned the highest
relative probability even differs among these topologies
(Fig. 1b–d).
Sphaeropteris
Within Cyatheaceae, the crown group of Sphaeropteris dates
back to 90 Ma in the Late Cretaceous (Fig. 1e, Fig. S2), with
a very high probability (0.95–0.98) of having originated in
Australasia (Fig. 1b–d). Within Sphaeropteris, range expansion into Southeast Asia occurred at three different time
intervals from the Late Cretaceous to the present, whereas its
expansion into South America was a single event in the
Eocene (43–36 Ma, range expansion event number 1 in
Fig. 1e). A single species, S. medullaris, occurs in Australasia
and the Southwest Pacific. Its range expansion occurred in
the stem lineage of the species, either in, or subsequent to,
the Oligocene.
Origin of the marginate-scaled clade
The ancestral range of the marginate-scaled clade probably
included both Australasia and South America. This is the
most likely scenario irrespective of topology (0.46/0.76/0.79
for the AlGy, AlCy, CyGy constrained topologies, respectively, see Fig. 1b–d). Therefore, despite the ambiguity of the
ancestral range of the family as a whole, we can deduce that
a range expansion occurred in the stem lineage of the family,
either from Australasia to South America or from South
America to Australasia (Fig. 1b–d, event 2 in Fig. 1e), in
the Late Jurassic–Early Cretaceous (150–96 Ma, Fig. S2).
The crown group age of the marginate-scaled clade is
estimated to be 82 Ma (Late Cretaceous; Fig. 1, Fig. S2) with
the three major lineages (Alsophila, Cyathea and Gymnosphaera) all having crown group origins around the Palaeocene–
Eocene boundary (54, 56 and 55 Ma, respectively; Fig. 1e,
Figs S2–S4).
Biogeographical patterns in the marginate-scaled clade:
when Alsophila and Gymnosphaera are constrained
to be sister-groups
Implementing the AlGy topological constraint (Fig. 1b,e)
reconstructs both the Cyathea and the Alsophila + Gymnosphaera lineages to have originated in Australasia (albeit with
low relative probability, 0.53 and 0.40, respectively); i.e. with
their common ancestor experiencing a local extinction in
South America. In Cyathea, this is followed by a range
expansion back into South America in its stem lineage (82–
54 Ma; event 3 in Fig. 1e, Fig. S2). Because the relative
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Biogeography of scaly tree ferns (Cyatheaceae)
probability for a local extinction in the common ancestor of
the marginate-scaled clade is rather low (0.53), it is possible
that Cyathea inherited the full ancestral range of the ancestor, including both South America and Australasia (relative
(a)
probability of 0.31). Within the crown group of Cyathea, a
basal split suggests a vicariance scenario, whereby the Australasian and South American species diverged sometime in
the Late Cretaceous–Palaeocene (82–54 Ma; Fig. 1e, Fig. S2).
(e)
(b)
(c)
(d)
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P. Korall and K. M. Pryer
A single colonization from Australasia to the Southwest Pacific is found in Cyathea alata.
Under the AlGy topological constraint, a range expansion
is also reconstructed for the common ancestor of Alsophila
and Gymnosphaera, but from Australasia into Africa, during
the Late Cretaceous (82–77 Ma; event 4a in Fig. 1e, Fig. S2).
The ancestor of Alsophila is reconstructed as having a wide
biogeographical range in both Australasia and Africa, a distribution inherited by the A. acutula–A. manniana group,
one of the two lineages resulting from the earliest divergence
within Alsophila (Fig. 1e). This lineage splits into one African
clade (A. lastii–A. manniana) that is later dispersed into
Madagascar, and into another clade (A. acutula–A. cunninghamii) that diverges into an Australasian group and an African/Madagascan–South American group. The colonization of
South America is consistently resolved as a single dispersal
event from Africa in the Eocene or possibly Oligocene (48–
31 Ma; event 5 in Fig. 1e, Fig. S2). The other lineage to
result from the basal split in Alsophila (Fig. 1e), A. walkerae–
A. stelligera, has an ancestral range that is restricted to
Australasia, with later colonization of India (during the Oligocene–Miocene, 34–19 Ma) and Southeast Asia (Miocene,
19–14 Ma).
The ancestral range of the Gymnosphaera lineage, under
the AlGy constraint, is restricted to Africa, with subsequent
range expansion into South America in the Eocene (55–33
Ma; event 6 in Fig. 1e, Fig. S2). This was followed by further
colonizations of Madagascar (from Africa) and Southeast
Asia (from South America) (Oligocene, 33–28 Ma; event 7 in
Fig. 1e, Fig. S2). Cyathea capensis occurs in both South
America and Africa and its range expansion from Africa is
estimated to have occurred along the terminal branch (55
Ma–present; event 8 in Fig. 1e, Fig. S2).
within the marginate-scaled clade. These results, however,
differ to some extent from those resulting from the AlGy
topology presented above.
The ancestral range for Cyathea is in both South America
and Australasia in the AlCy and CyGy topologies (Fig. 1c,d),
supporting the reconstruction with the next to highest relative probability in the AlGy topology (see above). From this,
it follows that range expansion event 3, which is invoked by
the AlGy topology (Fig. 1e), is absent in these reconstructions. Within the crown group of Cyathea, the ancestral
range reconstructions are identical under all three topological
constraints (described above, Fig. 1e), with minor differences
in relative probabilities.
Under either the AlCy or CyGy topological constraint, a
range expansion into Africa for the Alsophila and Gymnosphaera lineages is delayed when compared to the AlGy topology, where Africa was reconstructed as part of the ancestral
area for their common ancestor. In the AlCy and CyGy
topologies, colonizations of Africa are found in the stem
lineage of the Alsophila acutula–A. manniana group in Alsophila and in the stem lineage of the Cyathea boiviniiformis–
C. salvinii group in Gymnosphaera (event 4b in Fig. 1e). The
ancestral area reconstructed for the Alsophila stem lineage is
Australasia, whereas it is South America for the Gymnosphaera stem lineage (Fig. 1c,d).
As with the Cyathea crown group, the ancestral areas
reconstructed for the more derived divergences within the
Alsophila and Gymnosphaera crown groups are identical
under all three topological constraints, with the exception of
range expansion event 8 that is invoked for C. capensis (here
it is from South America to Africa, i.e. the reverse direction
that was reconstructed for the AlGy topology).
DISCUSSION
Biogeographical patterns in the marginate-scaled clade:
when Alsophila and Gymnosphaera are NOT constrained
to be sister-groups
Evolutionary history of scaly tree ferns
(Cyatheaceae)
Implementing either the AlCy or CyGy topological constraints (Fig. 1c,d) yields ancestral range reconstructions that
are identical to one another for the three major lineages
The phylogenetic relationships we obtained for scaly tree
ferns (Fig. 1, Figs S1–S4) are congruent with those from
recently published studies (Korall et al., 2007; Janssen et al.,
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.
408
Journal of Biogeography 41, 402–413
ª 2013 The Authors Journal of Biogeography Published by John Wiley & Sons Ltd
Biogeography of scaly tree ferns (Cyatheaceae)
2008). The conclusions we draw below regarding vicariance
versus long-distance dispersal take into account the uncertainty inherent in our age estimates.
Cyatheaceae originated 150 Ma in the Late Jurassic (Figs
S2–S4), a time marking the start of the rifting between the
western and eastern parts of Gondwana (McLoughlin, 2001,
and references therein). The ancestral geographical distribution of the family was in areas that today are either South
America or Australasia; our analyses failed to provide an
unambiguous reconstruction (Fig. 1b–d). When the Cyatheaceae crown group began to diversify 96 Ma in the midCretaceous (Figs S2–S4; this age estimate agrees with a study
including all leptosporangiate ferns: Schuettpelz & Pryer,
2009), South America and Australasia were still connected
via Antarctica, whereas the African, Madagascan and Indian
landmasses had already separated from the rest of Gondwana. The range expansion reconstructed for the group –
either from Australasia to South America, or from South
America into Australasia (event 2 in Fig. 1e) – is therefore
compatible with migration across the Gondwanan continent,
indicating that a transoceanic dispersal event does not need
to be inferred for either scenario. It is interesting that the
reconstructed ancestral distribution did not include Africa,
lending support to the notion that the expansion occurred
after Africa had drifted away.
In Sphaeropteris, although several range expansions are
reconstructed to have taken place from its ancestral distribution in Australasia into Southeast Asia, there is only a single
colonization of South America (43–36 Ma, during the
Eocene, event 1 in Fig. 1e, Figs S2–S4). The timing of this
range expansion coincides with the separation of South
America and Antarctica from Australia and New Guinea,
dated at 52–35 Ma (Sanmartın & Ronquist, 2004), suggesting
that the break-up of Gondwana may be responsible for this
vicariance pattern (albeit if the 95% highest posterior density
is taken into account, the timing may have post-dated the
separation of the continents).
Following the origin of the marginate-scaled clade crown
group in South America and Australasia during the Late Cretaceous, its divergence into three different lineages is estimated to have happened during a very short time frame of
approximately five million years (82–77 Ma, Fig. 1e). Such
rapid radiations are notoriously difficult to resolve (Whitfield
& Lockhart, 2007), and so the lack of support for the phylogenetic sequence in which these divergences took place is not
surprising. Unfortunately, as a consequence, our most
ambiguous biogeographical reconstructions relate to these
particular cladogenesis events.
For Cyathea, this ambiguity affects the overall size of the
ancestral distribution: was it restricted to Australasia (due to
an ancestral extinction event in South America; Fig. 1b), or
did it include South America as well (Fig. 1c,d)? Under the
first scenario, a range expansion into South America is
invoked in the stem lineage of Cyathea (event 3 in Fig. 1e).
The estimated timing of the split of the ancestral range into
South America and Australasia at the earliest divergence of
the group (82–54 Ma; Fig. 1e) is compatible with a vicariance interpretation, because the final separation of these
Gondwanan elements occurred later, around 50–35 Ma
(Sanmartın & Ronquist, 2004).
The inconclusive ancestral reconstructions of the biogeographical scenarios possible for the earliest lineages of Alsophila and Gymnosphaera can mainly be reduced to a single
question: when did the groups spread from Australasia to
Africa? The reconstructions suggest that this happened either
in the common ancestor of the two groups during the Late
Cretaceous (82–77 Ma; Fig. 1b, event 4a in Fig. 1e), or once
within each group in the early Eocene (56–52 Ma for Alsophila
and 55–33 Ma for Gymnosphaera, event 4b in Fig 1e). Irrespective of which scenario is correct, it is clear that the range
expansion(s) into Africa can only be explained by transoceanic dispersal(s), because the separation of Africa from Australasia (which was then still united with South America)
occurred some 135 Ma (Sanmartın & Ronquist, 2004), at least
50 Myr before any of the reconstructed range expansions.
A second transoceanic dispersal event is invoked within
Alsophila – a range expansion from Africa to South America
(48–31 Ma; event 5 in Fig. 1e). The paraphyly of the South
American taxa observed in this reconstruction could be
explained if the ancestor was in both Africa and South
America for a relatively short time prior to cladogenesis.
However, because the paraphyly is only moderately supported (PP = 0.89; Fig. S1), the New World species may
actually be monophyletic. The two African Alsophila lineages
include subsequent range expansions and cladogenesis events
in Madagascar and neighbouring islands (Fig. 1e). Within
the scaly tree ferns, most large-scale range expansions
occurred within Gymnosphaera, and all of these need to be
explained by long-distance dispersal: two from Africa to
South America or vice versa (events 6 and 8 in Fig. 1e) and
one from South America to Southeast Asia (event 7 in
Fig. 1e).
Our reconstruction results for scaly tree fern colonization
of Madagascar and neighbouring islands differ slightly from
those of Janssen et al. (2008), whose study was based on
mainly the same data set but with focus on Madagascan
diversification. Within the Alsophila lineage, both studies
show a range expansion from Africa to Madagascar in the A.
lastii–A. manniana lineage. However, within Alsophila, we
also reconstruct a second colonization of Madagascar from
Africa, contradicting the reconstruction of a New World origin by Janssen et al. (2008) for these same taxa. In their discussion, however, they refer to their result as ‘improbable’,
and suggest that an African origin is more likely (in line with
our results). The ambiguity we reveal regarding the ancestral
area of Gymnosphaera (Fig. 1) is further emphasized by Janssen et al. (2008). Their reconstruction supports an African
origin of the group (as in Fig. 1b in our study), but their
topology shows Alsophila and Cyathea as sister (corresponding to Fig. 1c in our study).
Journal of Biogeography 41, 402–413
ª 2013 The Authors Journal of Biogeography Published by John Wiley & Sons Ltd
409
P. Korall and K. M. Pryer
Vicariance and long-distance dispersal in ferns
The large-scale patterns that we reconstruct in our biogeographical analysis of the scaly tree ferns, support eight to ten
range expansions over areas that today are large oceans.
Three of these are expansions between Australasia and South
America: a range expansion in the stem lineage of the family
(event 2 in Fig. 1e), in the Sphaeropteris lineage (event 1 in
Fig. 1e), and a possible expansion in the Cyathea lineage
(event 3 in Fig. 1e). We estimate the timing of these events
to have occurred when South America, Antarctica and Australasia were still interconnected, but the African, Madagascan and Indian landmasses had already separated. This
suggests that migration from Australasia to South America
via Antarctica was possible and, therefore, transoceanic dispersal does not need to be invoked. In addition, the ancestral
distribution patterns that followed the range expansions in
Cyathea and Sphaeropteris are compatible with vicariance due
to the timing of the break-up of the South American and
Australasian landmasses. All other range expansions (in Alsophila and Gymnosphaera) are too young to correspond to a
Gondwanan break-up scenario, and long-distance, transoceanic dispersals are most probably responsible for the patterns
seen.
Because the early divergences among the major lineages of
ferns occurred prior to the break-up of the Gondwanan continent (Schuettpelz & Pryer, 2009), one might expect that the
resulting distribution patterns could be explained by vicariance. The timing of at least some of the early divergences
and distribution patterns in these lineages are compatible
with a Gondwanan vicariance scenario (Dubuisson et al.,
2003; Hennequin et al., 2008), but more recent long-distance
dispersal events post-dating the break-up of Gondwana have
most probably also affected the distributions we see today
(Perrie et al., 2007).
Polypods, which include some 80% of extant fern species
diversity (Pryer et al., 2004), diverged from their closest relatives in the Triassic; however, many subgroups did not
diversify until the Eocene or later (Schuettpelz & Pryer,
2009). In other words, these radiations occurred after the
Gondwanan break-up, requiring long-distance dispersal to
be invoked for all transoceanic dispersals. Extrapolating from
some studies that either analyse, discuss and/or map distribution patterns without explicitly analysing the biogeographical data (Ranker et al., 2004: grammitids, Polypodiaceae;
Rouhan et al., 2004: Elaphoglossum, Dryopteridaceae; Schneider et al., 2004: Aspleniaceae; Kreier & Schneider, 2006:
Platycerium, Polypodiaceae; Kirkpatrick, 2007: cheilanthoids,
Pteridaceae; Rouhan et al., 2007: Lomariopsis, Lomariopsidaceae; Kreier et al., 2008: microsoroids, Polypodiaceae; Hennequin et al., 2010: Nephrolepis, Lomariopsidaceae) as well
from some studies on early diverging lineages (Wikstr€
om
et al., 2002: Schizaeaceae; Nagalingum et al., 2007: Marsilea,
Marsileaceae; Hennequin et al., 2008: Hymenophyllaceae),
we note that the number of transoceanic dispersal events in
scaly tree ferns (calculated as the number of long-distance
410
dispersals per species and million years) are found at the
lowest end of the scale. Being very conservative in the estimates, we can conclude that a few of these other fern
groups show similar numbers, whereas most of the groups
show rates of transoceanic dispersals that are 1.5–3.5 times
higher, with extremes showing rates that are 20 times (or
more) higher than what we observed for the scaly tree ferns
in this study.
Scaly tree fern dispersability
The biological factor with the greatest impact on a fern’s
ability to disperse is most likely to be the mode of reproduction. Some diploid ferns are extreme outcrossers (see
e.g. Soltis & Soltis, 1992); therefore, for a dispersal event to
be successful, haploid spores from two different sporophytic
individuals would have to disperse to the exact same place
to effect fertilization (sporophytic outcrossing; K.M. Pryer
et al., in prep.). Polyploids, on the other hand, often seem
to reproduce by gametophytic selfing (empirical studies are
rather few, but see e.g. Masuyama & Watano, 1990; Soltis &
Soltis, 1990; Chiou et al., 2002; Haufler, 2007; K.M. Pryer
et al., in prep.), an extreme form of inbreeding where the
sperm and egg come from the same haploid gametophyte
(i.e. the progeny are homozygous at all loci). For species
capable of gametophytic selfing, only a single spore is
needed to effect a successful fertilization event, which
should strongly increase its chances for dispersal (see empirical examples in e.g. Trewick et al., 2002; Wubs et al.,
2010).
Scaly tree ferns are mostly functionally diploid. Although
the group has a haploid chromosome number of n = 69, it
is probably of an ancient polyploid origin. Only three polyploid tree ferns are known (Nakato, 1989; Conant et al.,
1994). The breeding system in scaly tree ferns has only been
investigated in two species, as far as we are aware: Alsophila
firma and Cyathea stipularis. Both taxa were included in this
study, and have been shown to be predominantly outcrossers
(Soltis et al., 1991). Although the data are sparse, this suggests that sporophytic outcrossing may be the more common
breeding system within the group. The relatively few transoceanic dispersal events that we reconstructed here are likely
to be due to the reproductive limitations posed by outcrossing in scaly tree ferns.
Our hypothesis of a causal relationship between breeding
system (and indirectly ploidy level) and dispersability success
in scaly tree ferns is further strengthened by a comparison
among lineages within the scaly tree ferns. We observe seemingly different rates of dispersal success within the family.
Three out of the four to six long-distance dispersal events
inferred are in the Gymnosphaera group, yet this lineage has
the fewest taxa (approximately 30 species; Holttum, 1963,
1964, 1981; Janssen, 2007). The fact that two of the three
known tetraploids in the family, Cyathea metteniana (Hance)
C. Chr. & Tard.-Blot. and Cyathea hancockii Copel., belong
to this group (Holttum, 1965; Nakato, 1989) indicates that
Journal of Biogeography 41, 402–413
ª 2013 The Authors Journal of Biogeography Published by John Wiley & Sons Ltd
Biogeography of scaly tree ferns (Cyatheaceae)
the cause of this potential shift in the dispersal trend may be
explained by a shift in reproduction mode, from sporophytic
outcrossing to gametophytic selfing. To further test this
hypothesis a better understanding of the biology, and particularly the breeding system(s), of scaly tree ferns is critical.
This is also true in the broader context of the evolutionary
history of all ferns.
ACKNOWLEDGEMENTS
This work was supported by grants from the Swedish
Research Council (2003-2724) and the Swedish Research
Council for Environment, Agricultural Sciences and Spatial
Planning (Formas) to P.K. (2006-429 and 2010-585) and a
National Science Foundation CAREER award (DEB-0347840)
to K.M.P. All analyses were performed on resources provided
by the Swedish National Infrastructure for Computing
(SNIC) through Uppsala Multidisciplinary Center for
Advanced Computational Science (UPPMAX) under Project
p2009050. We are grateful to R. Ree for advice on calculating
relative probabilities in Lagrange and S. Bolinder, A. Larsson,
A. Rydberg, C. Rydin, S. Weststrand and three referees for
comments on the manuscript.
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SUPPORTING INFORMATION
Additional Supporting Information may be found in the
online version of this article:
Appendix S1 Taxa examined in this study: collection locality, voucher information, fern DNA database numbers and
GenBank accession numbers for each sequenced region.
Appendix S2 Additional figures showing phylogenetic relationships and lineage divergence times based on Bayesian
Markov chain Monte Carlo (BMCMC) analyses (Figs S1–S4).
DATA ACCESSIBILITY
The combined (rbcL, rbcL–accD IGS, rbcL–atpB IGS, trnG–R
and trnL–F) dataset and detailed results, including all possible reconstructions, from the three separate biogeographical
analyses using Lagrange are deposited in the Dryad data
repository (doi: 10.5061/dryad.0q08k).
BIOSKETCHES
Petra Korall is an associate professor at Uppsala University,
Sweden. Her research interests include the evolutionary history of ferns and lycopods, with a focus on systematics and
historical biogeography.
Kathleen M. Pryer is a professor of biology at Duke University, NC, USA. Her research focuses on integrated molecular and morphological phylogenetic studies of land plant
diversity and evolution, especially ferns (http://pryerlab.
biology.duke.edu/).
Editor: Aristeidis Parmakelis
Journal of Biogeography 41, 402–413
ª 2013 The Authors Journal of Biogeography Published by John Wiley & Sons Ltd
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