Introduction

Geologic and climatic oscillation events are likely to have profound effects on both rates of speciation and current distributions of taxa. The processes and timelines involved have long intrigued biologists [1] [2] [3] [4]. Moreover, there has been a long-standing attempt to explain and understand the high species richness of the tropical rainforests [5] [6] [7] [8] and whether the rainforests act as a museum or cradle for biological diversity [9] [10] [11] [12]. More generally, there is growing appreciation of how long-term processes of landscape change and phyletic evolution shape regional species pools and geographically structured diversity [11] [13]. Explanations for the high biological diversity of the tropics and the current distributions of species have included biotic and abiotic factors with no single variable accounting for the patterns we observe. One suite of related factors that have been proposed to explain some of the observed patterns are the effects of Pleistocene or older climate change in the tropics, the resulting refugia, and the impacts of these events on speciation [14] [15], though such refuge-based models have proved controversial [16] [17]. In this paper we explore whether climate-driven vicariance, as measured by biogeographic structuring (ancestral ranges) of sister clades within the phylogeny, has shaped long-term speciation in a system with a well-characterized history of late Quaternary contraction and refugia: The rainforests of the Australian Wet Tropics.

The Australian Wet Tropics (AWT) are the largest surviving remnant of Gondwanan rainforest, which once dominated the continent of Australia [18]. Climatic modeling and paleoecological data [19] [20] [21] [22] [23] and previous research on montane taxa [24] [25] [26] have shown that the currently near continuous rainforest was sundered into disconnected refugia as a result of late Quaternary climatic processes. These late Quaternary processes, especially in the mid- late Pleistocene [20] [27], had profound effects on current patterns of species richness and phylogeographic diversity [21] [25] [26] [28]. Based on the growing body of research from this region, there is support for the hypothesis that these refugia are the result of late Quaternary climate changes [24] [29] [30], perhaps undergoing extreme contraction, and for many of the groups that are found in these regions, large-scale extinction. Not only have phylogeographic studies provided evidence of Pleistocene climatic events leaving discrete genetic signatures on modern populations, there is mounting evidence that the major “breaks” in genetic diversity (including across the Black Mountain Corridor) typically predate the Last Glacial Maximum ([31]: late-Miocene or early Pliocene; [32]: Pliocene; [33]: pre-Pleistocene; [28]: Late Pliocene or Early Pleistocene; [34]: pre-Pleistocene; [26]: Pliocene 2.3–4.6 Ma). Interestingly, historic biogeographic features within the AWT may not have the same impact on related taxa, as was found for two species of closely related rainforest trees with overlapping habitats and differing signature of the impact of the geographic features and genetic diversity [34]. These previous studies highlight variation among species in the spatial and temporal scale of response.

The habitat contractions of the AWT are thought to be due to the long-term aridification of the Australian continent, which began during the mid-Miocene, through the Pliocene and continued with the high amplitude oscillation of the mid-late Pleistocene [18] [35]. The resulting isolation and local reduction of rainforest tracts doubtlessly had major effects on the biological diversity of this region. The contractions of the forests would clearly have cascading effects for taxa that had close ecological and evolutionary relationships with the rainforest themselves [36] [37] [38]. The escarpments and montane areas that harbor the endemic fauna of the Australian Wet Tropics are, in general, an ancient land formation. In this context, it is possible that rainforest contractions much deeper in time (e.g. mid-Miocene) have shaped current patterns of diversity. If true, how does this affect deeper phylogenetic splits? If climate driven contractions of rainforest over Plio-Pleistocene (or even mid- late Miocene or older) affected speciation processes, then phylogenies of species-rich endemic genera should show geographic structure at these deeper evolutionary scales analogous to phylogeographic studies observed within species. For most vertebrate genera unraveling the geography of speciation associated with longer time scales has not been possible because few, closely related clades of species are found within the AWT and most genera have low species diversity [39]. A prominent exception is microhylid frogs in the genus Cophixalus, for which spatial phylogenetic pattern is congruent with geography [40].

Earthworms have been largely overlooked in studies of historical biogeography and phylogeography and have rarely been subjected to molecular analysis until recently [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53]. As a diverse, Australian Wet Tropics endemic and rainforest-restricted group, earthworms of the genus Terrisswalkerius provide a promising system with which to address questions of long-term historical processes in the AWT (Fig 1). Like terrestrial snails that have been highly informative for phylogeography [25] [54], the earthworms are expected to have limited dispersal ability and to have been less prone to late Pleistocene local extinctions because of their large overall distributions and diversity. As inhabitants of moist soils and rotten logs with presumed environmental buffering, their biogeographic history may provide a rich view of the long-term historical processes that have shaped the region. Terrisswalkerius earthworms are striking due to their size (ranging from 24mm- 317mm) and often bright colors. The 25 species of Terrisswalkerius are distributed in the wet tropical rainforests of eastern North Queensland, from the Paluma Range, near Townsville north to the isolated rainforest of the McIlwraith Range in the Cape York Peninsula. Jamieson [42] reviewed the biology, distribution, and morphology of all known species of Terrisswalkerius and wide-scale molecular phylogenetic analyses [44] [53] confirmed that the genus is a member of the annelid family Megascolecidae. An unpublished new species is included in the present study and a further species, T. leichhardti, was described by Jamieson et al. [55].

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Fig 1. Distribution data for species of Terrisswalkerius earthworms within the Australian Wet Tropics with biogeographic subregions after Williams et al. [82].

The AWT are found between latitudes 15°39' south and 19°17' south, and longitudes 144°58' east and 146°27' east. A. Map showing sample collection locations for species of Terrisswalkerius. B. Species presence within each subregion for all known collections of Terrisswalkerius.

https://doi.org/10.1371/journal.pone.0136943.g001

As these endemic earthworm species are intimately associated with forest environments, they may be expected to track this long continuous history of decimation of the Gondwanan mesothermal rainforests that once dominated Australia, of which the Wet Tropics in considered to be the largest surviving remnant. Thus broadly, the current endemic species distributions of Terrisswalkerius may reflect a balance of 1) re-structuring of old, geographically dispersed diversity into persistent and much reduced regions; and 2) biogeographic speciation among regions. This balance might be weighed by assessing (by ancestral range reconstruction) the signal of vicariant speciation consistent with Australian Wet Tropics biogeography, compared to more complex idiosyncratic patterns expected from restructuring of ancestral diversity.

Here we estimate the phylogenetic structure of the AWT endemic earthworm lineage, Terrisswalkerius, overlaying this with current species distributions and ancestral range reconstruction to infer the effect of known barriers deduced from previous phylogeographic studies on long-term speciation processes. We also provide a provisional estimate for the timing of the diversification of the genus and the results are compared to recent phylogenetic analyses of other endemic but lower diversity taxa.

Materials and Methods

We amplified and sequenced DNA fragments from 16 species of Terrisswalkerius (34 specimens) and an additional 34 megascolecid outgroup taxa (68 specimens total). The nine species of Terrisswalkerius that were not included in this study are known only from the type collections and were not preserved in a manner likely to yield useful DNA sequences, excepting T. oculatus, although known from three localities, was not obtainable for sequencing. All collections were made under the auspices of the Department of the Environment (Queensland, Australia) in which one of the authors (KRM) was an officer and no additional permissions were required. The species studied are not on any threatened or endangered lists but we hope that this paper will encourage their protection. All worms were humanely euthanized in dilute ethanol following all protocols of the Department of the Environment (Queensland, Australia). All included specimens were sequenced for the mitochondrial (mt) 12S rRNA gene, with nested coverage for the following additional genes: mt 16S rRNA, mt DNA COII, and nuclear 28S rRNA [see S1 File for sequencing details and S1 Table for GenBank accession numbers].

Results

All phylogenetic methods recovered similar topologies for the relationships within Terrisswalkerius and we present the BEAST consensus tree with ML and Bayesian support values in Fig 2 (see S1 Fig for the full tree with all distant outgroup taxa included). The major framework of the tree (Fig 2) is generally congruent between mitochondrial and nuclear gene data (see, however, [44], where 28S rDNA showed poor resolution compared with the combined nuclear and mitochondrial analysis; S3 and S4 Figs). Terrisswalkerius is recovered as monophyletic with the exception of T. athertonensis, the inclusion of this species being rejected by the SH test (delta lnL = 36.4, p = 0.0057). The position of Didymogaster Fletcher 1886 is somewhat equivocal with mtDNA being responsible for placement within Terrisswalkerius sensu lato, where T. athertonensis is included. All data unequivocally support Fletcherodrilus Michlaelsen 1891 as the sister group of Terrisswalkerius, if T. athertonensis is excluded, as also shown in the maximum likelihood analysis of Jamieson et al. [44]. The present and previously published results [42] [44] suggest that T. athertonensis should be placed in a separate, cryptic genus. In a morphological cladistic analysis [73] it grouped with T. oculatus, embedded among Terrisswalkerius species but the morphological tree shows little agreement with molecular analyses. Terrisswalkerius Jamieson 1994 remains current pending further resolution and confirmation of the separate generic position of T. athertonensis. Terrisswalkerius s.s. is mostly well resolved with the exception of basal relationships involving T. macilwraithi and the T. nashi-liber lineage.

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Fig 2. BEAST relaxed-clock maximum clade credibility phylogeny, showing BEAST posterior probabilities followed by RAxML bootstrap supports (PP/BS as percent; some terminal nodes show only PP to avoid clutter) based on the combined data matrix.

Terrisswalkerius taxa north of the Black Mountain Corridor (BMC) in grey. Node circles indicate biogeographic analyses: grey and black filled circles indicate inferred ‘vicariance’ by respectively either or both DEC and DIVA methods. Key dating nodes indicated. Divergence scale in Ma. See Jamieson et al. [44] and Buckley et al. [49] for more details on higher classification. Full tree with distant outgroups shown in S1 Fig.

https://doi.org/10.1371/journal.pone.0136943.g002

Although, in general, morphologically defined species are recovered as monophyletic, we do recover two paraphyletic species (T. canaliculatus and T. nashi), which may suggest that further taxonomic revision is needed in this diverse and understudied group. However, as implied by Jamieson [42], T. nashi would be monophyletic if T. liber, differing chiefly in approximation of genital pores, is included within T. nashi.

Absolute and relative divergence dating

Relaxed-clock dating estimated the age of monophyletic members of Terrisswalkerius (labeled “A” in Fig 2; S1 Fig) to be 52 Ma (with 95% CI of 31–84 Ma), the separation of the Cape York species, T. mcilwraithi at 40 Ma (21–58 Ma), the North-South Queensland split in Fletcherodrilus at 36 Ma (12–46) and the Qld-Tasmania split in the Diporochaeta group at 36 Ma (19–62 Ma). There is considerable uncertainty in these estimates due to limited calibration information and high level of molecular change in the mitochondrial genes. However, relative dating should be less sensitive to these limitations [49] [75]. Significantly, the Cape York T. mcilwraithi, North-South Queensland Fletcherodrilus and the Qld-Tasmania Diporochaeta splits are all younger than Terrisswalkerius s.s. (node A), and this pattern is consistent considering mitochondrial or nuclear data alone (Fig 3). It is also worth noting that the clade is around five times older than typical intra-specific phylogeography (Fig 2).

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Fig 3. Ages of various groups relative to monophyletic Terriwalkerius (node A).

Quartile and 95% CI BEAST posterior distributions. Digaster-Diporochaeta (node D) represents taxa spread from SE Queensland to Tasmania; Fletcherodrilus (node C) represents taxa spread across the Burdekin and St. Lawrence Gaps from the AWT to SE Qld; Cape York T. macillwraithi represents the Laura Gap. Estimates use three different data matrices: mtDNA only, 28SrDNA only, and combined (all) (see SI for further details).

https://doi.org/10.1371/journal.pone.0136943.g003

This inference places extant monophyletic Terrisswalkerius well into the Paleogene era of isolation of mesothermal Gondwanian Australia [19] [71] [76], and even the lower confidence interval (31 Ma) places a substantial proportion of the Australian Wet Tropics endemic diversity pre-dating the mid-Miocene major shift towards fragmentation and aridification [20] [71]. This can be further put in context by considering the relative age of phylogenetic splits spanning deep Austral biogeography (Fig 2). While each of these splits may not directly relate to historically driven eco-geographic divergence processes, they set a maximum (e.g. an overestimation due to extinction or under-sampling) and therefore emphasize the age of the AWT endemic Terrisswalkerius diversity as likely pre-dating the mid-Miocene major shift towards fragmentation and aridification. This can also explain their apparent absence from any New Guinean landmass, which (at ca. 10–15 Ma; [72]) likely post-dates these major historical eco-geographic divisions.

Discussion

Geologic and climatic oscillation events are expected to have profound effects including attenuation and extinction on less mobile organisms; the genetic signatures of these events are apparent in the endemic Australian Wet Tropics earthworms. The substantial component of geographical speciation between regions–vicariance–estimated across the phylogeny as a whole and fairly apparent in sister tip lineage phylogeography (Fig 2), indicates that the biogeographical structure of the AWT rainforests has played a significant role in diversification of Terrisswalkerius.

In the case of these earthworms, morphological conservatism hides the deep evolutionary history of the lineage, as there are few morphological characters to distinguish these species, which lack the genital markings that show high specificity in other megascolecid genera. Although we infer a middle Paleogene age (ca. 52 Ma) for the genus Terrisswalkerius, dating divergence times for lineages can be challenging in the absence of fossil or biogeographic information, but clearly molecular divergences are high. Relatively, Terriwalkerius is five times older than typical intra-specific phylogeography (Fig 2) and older than major biogeographic divisions (Fig 2; S2 Fig; S3 Table), consistent with it being of at least Olio-Miocene age.

Our finding of the signature of an ancient north-south division in Terrisswalkerius suggests the ‘Black Mountain Corridor’ may be a much older biogeographic barrier than had been previously proposed and played a role in AWT diversification prior to the Pleistocene-era phylogeography of previous vertebrate studies. There is now mounting evidence from phylogenetic analyses that the BMC barrier predates the Last Glacial Maximum [32] [33] [74] and our findings significantly push back the date for this barrier and add to this growing body of work. The distantly isolated Cape York species, the complex biogeographic history of the ‘northern’ lineage, and the sheer age of the whole group, all suggest a complex history coinciding with the drying of the Australian continent, which is thought to have begun with the major ice-sheet expansion of the late-Paleogene [76] and continued from the mid-Miocene [18]. Thus for Terrisswalkerius, in situ AWT endemic processes continued all the while the greater mesic mesothermal Gondwanian world shrank and fragmented around it, leaving the isolated ‘island world' of the Australian Wet Tropics.

Although the AWT are a very well studied region, the area to the north is less explored and the effect of long-term aridification and Pleistocene cycles on the isolated rainforest patches of the Cape York peninsular region are not well known. In addition the biological diversity of the Cape York (CY) area is understudied, but available evidence suggests that this region contains phylogenetic relics i.e. deep branch taxa [39]; this is the case in Cophixalus microhylid frogs [40], leaf-tailed geckos [77], camaenid snails [54], dung beetles [78] and now within the Terrisswalkerius earthworms (T. mcilwraithi: Fig 2). Our data recover the northern Cape York species, T. mcilwraithi, as an early lineage separating at ~ 26 Ma, within the primarily northern clade which is suggestive of long-term isolation and/or extinction of species within this clade. The hiatus in occurrence of Terrisswalkerius between CL and CY appears to be real as extensive collecting by KRM in the region between CL and T. mcilwraithi at CY yielded no specimens of the genus. The type-locality of T. mcilwraithi is, thus, almost two degrees of latitude north of previous records for the genus. Collection records by Dyne and Wallace [79] did not extend North of Cooktown. Biological diversity of the Cape York Peninsula is understudied and our results suggest that biologists interested in understanding phylogenetic, biogeographic, and evolutionary patterns of taxa of the AWT should include taxa from this region when they are known.

There is mounting evidence for a pre-Pleistocene climatic effect on flora and fauna across the Black Mountain Corridor of the Australian Wet Tropics but we are still in the early stages of understanding the factors that may have shaped this pattern. Aridification of the Australian continent undoubtedly led to contractions of the forest and may have driven species into relictual habitats thus resulting in contracted ranges and/or massive extinctions. Diversification in isolation would have been possible during these periods of forest contractions and our data are suggestive of this. Studies of species across many taxonomic scales in this region suggests that vicariance events may be geographically, but not necessarily temporally congruent, which poses the question how common is this in other geographic regions?

Our data show that for the endemic Terrisswalkerius earthworms, the rainforests of the AWT have acted not only as a museum to preserve biological diversity, but also have promoted continued and rapid diversification of the genus. Interestingly we see deep divergences among species of the endemic Terrisswalkerius earthworms, with many lineages restricted to distinct biogeographic regions (Fig 1), but we also recover paraphyletic species suggesting recent separation without sufficient evolutionary time for ancestral polymorphisms to sort. Hybridization cannot be ruled out for the two pairs of paraphyletic species [T. canaliculatus and T. terrareginae: overlap in their distributional ranges in the Atherton Uplands (AU) and Bellenden-Ker/Bartle-Frere (BK); T. nashi and T. liber overlap in their distributional range in the Mount Finnigan Uplands (FU)]. However, it is more probable that T. liber has evolved within T. nashi by reduction of the distance between the male pores and that reproductive isolation is not complete. In other lineages with overlapping distributions we see clear genetic differentiation such as in the Carbine Uplands (CU) where eight species are found co-occurring and the Atherton Uplands (AU) with twelve species. The results presented here show that for the Terrisswalkerius earthworms, the AWT are the center for ancient and ongoing diversification.

Although many vertebrate and invertebrate taxa have been examined from the Australian Wet Tropics UNESCO World Heritage Site, the Terrisswalkerius earthworms are the most diverse radiation that has been examined to date and the results presented here highlight the need for continued investigations across diverse groups of species and with varying depths of evolutionary history in the AWT, but also in other geographic arenas. Generally, vertebrates requiring large areas may have been subject to extinction, but when smaller vertebrates (i. e. frogs: [40] [80] [81] and invertebrate radiations have been examined [25] [28] [33] [54] [78], these data provide a richer picture of the evolutionary history of this region and suggest that these taxa had limited dispersal abilities and/or were buffered in smaller areas. The Australian Wet Tropics, which are biologically and topologically complex [82], provide an exceptional opportunity to study not only short-term evolutionary processes resulting from the Last Glacial Maximum (LGM), but also long-term processes, which affected the diversification and redistribution of biodiversity on a larger timescale. This is not unique to the AWT, with parallel questions being asked in other biologically diverse and geographically old regions such as the African tropics [83], Neotropics [84], and Madagascar [85]. Our results highlight the need for continuing evolutionary studies in this and other geographically old regions to include data from multiple levels, from individuals to higher taxonomic scales, across diverse groups of organisms thus allowing independent replicates of evolution in order to more fully unravel the complex history of the planet’s biological diversity.

Supporting Information

Acknowledgments

We thank Richard H. Ree for advice on DEC-LAGRANGE analysis and Juan L. Parra with assistance with creating the distribution map figure. We are grateful to Simon and Annie Tillier for previous work in providing sequences and Edmund Ling for early sequencing and molecular analysis of Terrisswalkerius. B.G.M.J. thanks the Australian Research Council for grants supporting collection of Wet Tropics earthworms and K.R.M. for making this possible. C.S.M. would like to thank the Department of Science and Education, Field Museum of Natural History and the Miller Institute for Basic Research in Science, University of California Berkeley for support during the analyzing and writing of this research.