Gene flow creates a mirage of cryptic species in a Southeast Asian spotted stream frog complex

Species delimitation with limited sampling: An example from rare trapdoor spider genus Cyclocosmia (Mygalomorphae, Halonoproctidae)

The outcome of species delimitation depends on many factors, including conceptual framework, study design, data availability, methodology employed and subjective decision making. Obtaining sufficient taxon sampling in endangered or rare taxa might be difficult, particularly when non-lethal tissue collection cannot be utilized. The need to avoid overexploitation of the natural populations may thus limit methodological framework available for downstream data analyses and bias the results. We test species boundaries in rare North American trapdoor spider genus Cyclocosmia Ausserer (1871) inhabiting the Southern Coastal Plain biodiversity hotspot with the use of genomic data and two multispecies coalescent model methods. We evaluate the performance of each methodology within a limited sampling framework. To mitigate the risk of species over splitting, common in taxa with highly structured populations, we subsequently implement a species validation step via genealogical diversification index (gdi), which accounts for both genetic isolation and gene flow. We delimited eight geographically restricted lineages within sampled North American Cyclocosmia, suggesting that major river drainages in the region are likely barriers to dispersal. Our results suggest that utilizing BPP in the species discovery step might be a good option for datasets comprising hundreds of loci, but fewer individuals, which may be a common scenario for rare taxa. However, we also show that such results should be validated via gdi, in order to avoid over splitting.

The natural and human-mediated expansion of a human-commensal lizard into the fringes of Southeast Asia

BACKGROUND

Human-commensal species often display deep ancestral genetic structure within their native range and founder-effects and/or evidence of multiple introductions and admixture in newly established areas. We investigated the phylogeography of Eutropis multifasciata, an abundant human-commensal scincid lizard that occurs across Southeast Asia, to determine the extent of its native range and to assess the sources and signatures of human introduction outside of the native range. We sequenced over 350 samples of E. multifasciata for the mitochondrial ND2 gene and reanalyzed a previous RADseq population genetic dataset in a phylogenetic framework.

RESULTS

Nuclear and mitochondrial trees are concordant and show that E. multifasciata has retained high levels of genetic structure across Southeast Asia despite being frequently moved by humans. Lineage boundaries in the native range roughly correspond to several major biogeographic barriers, including Wallace's Line and the Isthmus of Kra. Islands at the outer fringe of the range show evidence of founder-effects and multiple introductions.

CONCLUSIONS

Most of enormous range of E. multifasciata across Southeast Asia is native and it only displays signs of human-introduction or recent expansion along the eastern and northern fringe of its range. There were at least three events of human-introductions to Taiwan and offshore islands, and several oceanic islands in eastern Indonesia show a similar pattern. In Myanmar and Hainan, there is a founder-effect consistent with post-warming expansion after the last glacial maxima or human introduction.

The Artefactual Branch Effect and Phylogenetic Conflict: Species Delimitation with Gene Flow in Mangrove Pit Vipers (Trimeresurus purpureomaculatus-erythrurus Complex)

Mangrove pit vipers of the Trimeresurus purpureomaculatus-erythrurus complex are the only species of viper known to naturally inhabit mangroves. Despite serving integral ecological functions in mangrove ecosystems, the evolutionary history, distribution, and species boundaries of mangrove pit vipers remain poorly understood, partly due to overlapping distributions, confusing phenotypic variations, and the lack of focused studies. Here, we present the first genomic study on mangrove pit vipers and introduce a robust hypothesis-driven species delimitation framework that considers gene flow and phylogenetic uncertainty in conjunction with a novel application of a new class of speciation-based delimitation model implemented through the program Delineate. Our results showed that gene flow produced phylogenetic conflict in our focal species and substantiates the artefactual branch effect where highly admixed populations appear as divergent nonmonophyletic lineages arranged in a stepwise manner at the basal position of clades. Despite the confounding effects of gene flow, we were able to obtain unequivocal support for the recognition of a new species based on the intersection and congruence of multiple lines of evidence. This study demonstrates that an integrative hypothesis-driven approach predicated on the consideration of multiple plausible evolutionary histories, population structure/differentiation, gene flow, and the implementation of a speciation-based delimitation model can effectively delimit species in the presence of gene flow and phylogenetic conflict.

Unsupervised machine learning for species delimitation, integrative taxonomy, and biodiversity conservation

Integrative taxonomy, combining data from multiple axes of biologically relevant variation, is a major goal of systematics. Ideally, such taxonomies will derive from similarly integrative species-delimitation analyses. Yet, most current methods rely solely or primarily on molecular data, with other layers often incorporated only in a post hoc qualitative or comparative manner. A major limitation is the difficulty of devising quantitative parametric models linking different datasets in a unified ecological and evolutionary framework. Machine Learning (ML) methods offer flexibility in this arena by easily learning high-dimensional associations between observations (e.g., individual specimens) across a wide array of input features (e.g., genetics, geography, environment, and phenotype) to delimit statistically meaningful clusters. Here, I implement an unsupervised method using Self-Organizing (or "Kohonen") Maps (SOMs) for such purposes. Recent extensions called "SuperSOMs" can integrate multiple layers, each of which exerts independent influence on a two-dimensional output grid via empirically estimated weights. The grid cells are then delimited into K distinct units that can be interpreted as species or other entities. I show empirical examples in salamanders (Desmognathus) and snakes (Storeria) with layers representing alleles, space, climate, and traits. Simulations reveal that the SuperSOM approach can detect K = 1, tends not to over-split, reflects contributions from all layers, and limits large layers (e.g., genetic matrices) from overwhelming other datasets, desirable properties addressing major concerns from previous studies. Finally, I suggest that these and similar methods could integrate conservation-relevant layers such as population trends and human encroachment to delimit management units from an explicitly quantitative framework grounded in the ecology and evolution of species limits and boundaries.

Scrutinising an inscrutable bark-nesting ant: Exploring cryptic diversity in the Rhopalomastix javana (Hymenoptera: Formicidae) complex using DNA barcodes, genome-wide MIG-seq and geometric morphometrics

Overlooking cryptic species diversity has grave implications on assessments of climate change impacts on biodiversity, ecosystems and organismal populations. Discriminating between cryptic species has long been challenging even for seasoned taxonomists, as interspecies morphological differences are often indiscernible by visual observation. Multi-disciplinary methods involving genetic analyses in conjunction with quantitative morphological data, should therefore be used to investigate boundaries between cryptic species. We adopted an integrated approach combining analyses of mitochondrial COI barcodes, a genome-wide dataset obtained via multiplexed inter-simple sequence repeats (ISSRs) genotyping by sequencing (MIG-seq), and geometric morphometrics to investigate species divergences in the inscrutable Rhopalomastix javana species complex. Objective clustering of COI suggested five putative molecular species units divergent from each other by thresholds within 4.2-10.6% uncorrected pairwise distance. Phylogenetic analyses based on concatenated MIG-seq data also recovered and strongly supported the monophyly of five major lineages in agreement with COI clusters. Co-ancestry analyses based on MIG-seq data using fineRADstructure resolved variable patterns of admixture linked to geography, and potential genetic drift within some putative species. Geometric morphometric analyses of specimen images further detected statistically significant differences in at least one of three anatomical aspects (Head, Meso, Profile) between all pairs of putative species. Head shape (full-face view) was determined to be the most informative character for species diagnosis, with relatively high classification accuracy. Thin-plate spline deformation grids highlighted areas of high variation between species in each shape for deeper taxonomic scrutiny. The presence of species from multiple distinct lineages existing in near-sympatry firmly demonstrates that R. javana comprises more than one closely-related species, but exact species boundaries are difficult to ascertain. Differences in elevation and its associated abiotic effects on ant adaptations and reproductive phenology may contribute to restricting gene flow and maintaining species boundaries between sympatric populations of the R. javana complex. We further assess the advantages and limitations of geometric morphometrics as a taxonomic tool. Despite its drawbacks, our combined approach has helped draw important insights on cryptic diversity in R. javana, and also identified gaps of knowledge that await address. Results from this study will inform and prime future in-depth taxonomic investigation on the R. javana complex, including formal descriptions and establishment of the five putative species.

Interspecific gene flow obscures phylogenetic relationships in an important insect pest species complex

As genomic data proliferates, the prevalence of post-speciation gene flow is making species boundaries and relationships increasingly ambiguous. Although current approaches inferring fully bifurcating phylogenies based on concatenated datasets provide simple and robust answers to many species relationships, they may be inaccurate because the models ignore inter-specific gene flow and incomplete lineage sorting. To examine the potential error resulting from ignoring gene flow, we generated both a RAD-seq and a 500 protein-coding loci highly multiplexed amplicon (HiMAP) dataset for a monophyletic group of 12 species defined as the Bactrocera dorsalis sensu lato clade. With some of the world's worst agricultural pests, the taxonomy of the B. dorsalis s.l. clade is important for trade and quarantines. However, taxonomic confusion confounds resolution due to intra- and interspecific phenotypic variation and convergence, mitochondrial introgression across half of the species, and viable hybrids. We compared the topological convergence of our datasets using concatenated phylogenetic and various multispecies coalescent approaches, some of which account for gene flow. All analyses agreed on species delimitation, but there was incongruence between species relationships. Under concatenation, both datasets suggest identical species relationships with mostly high statistical support. However, multispecies coalescent and multispecies network approaches suggest markedly different hypotheses and detected significant gene flow. We suggest that the network approaches are likely more accurate because gene flow violates the assumptions of the concatenated phylogenetic analyses, but the data-reductive requirements of network approaches resulted in reduced statistical support and could not unambiguously resolve gene flow directions. Our study highlights the importance of testing for gene flow, particularly with phylogenomic datasets, even when concatenated approaches receive high statistical support.

Species Delimitation, Phylogenomics, and Biogeography of Sulawesi Flying Lizards: A Diversification History Complicated by Ancient Hybridization, Cryptic Species, and Arrested Speciation

The biota of Sulawesi is noted for its high degree of endemism and for its substantial levels of in situ biological diversification. While the island's long period of isolation and dynamic tectonic history have been implicated as drivers of the regional diversification, this has rarely been tested in the context of an explicit geological framework. Here, we provide a tectonically informed biogeographical framework that we use to explore the diversification history of Sulawesi flying lizards (the Draco lineatus Group), a radiation that is endemic to Sulawesi and its surrounding islands. We employ a framework for inferring cryptic speciation that involves phylogeographic and genetic clustering analyses as a means of identifying potential species followed by population demographic assessment of divergence-timing and rates of bi-directional migration as means of confirming lineage independence (and thus species status). Using this approach, phylogenetic and population genetic analyses of mitochondrial sequence data obtained for 613 samples, a 50-SNP data set for 370 samples, and a 1249-locus exon-capture data set for 106 samples indicate that the current taxonomy substantially understates the true number of Sulawesi Draco species, that both cryptic and arrested speciations have taken place, and that ancient hybridization confounds phylogenetic analyses that do not explicitly account for reticulation. The Draco lineatus Group appears to comprise 15 species-9 on Sulawesi proper and 6 on peripheral islands. The common ancestor of this group colonized Sulawesi ~11 Ma when proto-Sulawesi was likely composed of two ancestral islands, and began to radiate ~6 Ma as new islands formed and were colonized via overwater dispersal. The enlargement and amalgamation of many of these proto-islands into modern Sulawesi, especially during the past 3 Ma, set in motion dynamic species interactions as once-isolated lineages came into secondary contact, some of which resulted in lineage merger, and others surviving to the present. [Genomics; Indonesia; introgression; mitochondria; phylogenetics; phylogeography; population genetics; reptiles.].

Ghost introgression facilitates genomic divergence of a sympatric cryptic lineage in Cycas revoluta

A cryptic lineage is a genetically diverged but morphologically unrecognized variant of a known species. Clarifying cryptic lineage evolution is essential for quantifying species diversity. In sympatric cryptic lineage divergence compared with allopatric divergence, the forces of divergent selection and mating patterns override geographical isolation. Introgression, by supplying preadapted or neutral standing genetic variations, can promote sympatric cryptic lineage divergence via selection. However, most studies concentrated on extant species introgression, ignoring the genetic legacy of introgression from extinct or unsampled lineages ("ghost introgression"). Cycads are an ideal plant for studying the influence of ghost introgression because of their common interspecific gene flow and past high extinction rate. Here, we utilized reference-based ddRADseq to clarify the role of ghost introgression in the evolution of a previously identified sympatric cryptic lineage in Cycas revoluta. After re-evaluating the evolutionary independency of cryptic lineages, the group-wise diverged single-nucleotide polymorphisms among sympatric and allopatric lineages were compared and functionally annotated. Next, we employed an approximate Bayesian computation method for hypothesis testing to clarify the cryptic lineage evolution and ghost introgression effect. SNPs with the genomic signatures of ghost introgression were further annotated. Our results reconfirmed the evolutionary independency of cryptic lineage among C. revoluta and demonstrated that ghost introgression to the noncryptic lineage facilitated their divergence. Gene function related to heat stress and disease resistance implied ecological adaptation of the main extant populations of C. revoluta.

Systematic revision of the Spotted and Northern Dusky Salamanders (Plethodontidae: Desmognathus conanti and D. fuscus), with six new species from the eastern United States

Spotted and Northern Dusky Salamanders (Desmognathus conanti and D. fuscus) have a long and complex taxonomic history. At least 10 other currently recognized species in the genus were either described from populations previously considered D. fuscus, described as or later considered subspecies thereof, or later considered synonyms thereof, before ultimately being recognized as distinct. Recent molecular analyses have also revealed extensive cryptic diversity within both species, which are polyphyletic assemblages of 13 distinct mitochondrial lineages with 5.7-10.3% uncorrected 'p' distances in the COI barcode locus. Based on phylogenomic data and population-clustering analyses considering admixture between lineages, 11 candidate species were circumscribed by recent authors. Those within D. conanti are also ecomorphologically variable, comprising both large, robust, keel-tailed populations, and small, gracile, round-tailed forms. Evaluating their distinctiveness based on genetic, geographic, and morphological evidence, we conclude that six of the candidates represent new species: Desmognathus anicetus sp. nov., D. bairdi sp. nov., D. campi sp. nov., D. catahoula sp. nov., D. lycos sp. nov., and D. tilleyi sp. nov. Consequently, we recognize eight total species from populations formerly associated with the nominal species D. conanti and D. fuscus, the re-delimited concepts of which also contain additional phylogeographic lineage diversity that may represent further distinct species. In addition to existing mitochondrial and nuclear phylogenetic, network, and clustering results, we present preliminary analyses of linear morphometrics to bolster diagnostic specificity based on phenotypic characteristics. These changes stabilize the previously paraphyletic taxonomy of species-level lineages within Desmognathus, though additional cryptic diversity may exist both within the species considered here, and elsewhere in the genus.

New Insights into the Variation and Admixture of the Cave-Dwelling Spider Trogloneta yunnanensis in South China Karst

Subterranean karst caves can contain unexpected biodiversity, but few studies related to spider population genetics have been conducted in the karst area of Southern China. In this study, we investigated the population genetic structure of Trogloneta yunnanensis (Song & Zhu, 1994) based on 73 spider samples from six underground populations in South China Karst. Population genetic structure analysis showed a clear divergence (FST > 0.9 and Nm < 0.05) among populations according to mitochondrial genes. The phylogenetic gene tree constructed by BI and ML methods recovered six geographic clades. Divergence time estimation indicated that the divergence of these six populations can be traced back to the late Pleistocene. We supposed that the geographic isolation led to the extreme population structure. According to this study and previous studies about troglobites living in this region, the subterranean habitats of the Yunnan-Guizhou Plateau may contain many organisms with similar genetic structures. The subterranean biodiversity in the karst area of Southern China needs to be re-evaluated and protected.

A decade of amphibian studies (Animalia, Amphibia) at Sekayu lowland forest, Hulu Terengganu, Peninsular Malaysia

Amphibians of Sekayu lowland forest have been studied more than a decade, with discoveries of new records of species showing no sign of abating between the years 2003 to 2020, indicating the remarkably rich diversity of anurans in this forest. Despite ceaseless anthropogenic activities in this area, this study successfully recorded 52 species of amphibians from 32 genera in the lowland forest of Sekayu. The species composition consisted of a single species from the family Ichthyophiidae and 51 species of anurans of 31 genera and six families. The number of species recorded has steadily increased especially during more recent surveys from 2015 to 2020. This study augments the total number of amphibian species recorded from Hulu Terengganu by ten additional species, increasing the total to 70 species for the district.

Comparative phylogeography reveals widespread cryptic diversity driven by ecology in Panamanian birds

Widespread species often harbor unrecognized genetic diversity, and investigating the factors associated with such cryptic variation can help us better understand the forces driving diversification. Here, we identify potential cryptic species based on a comprehensive dataset of COI mitochondrial DNA barcodes from 2,333 individual Panamanian birds across 429 species, representing 391 (59%) of the 659 resident landbird species of the country, as well as opportunistically sampled waterbirds. We complement this dataset with additional publicly available mitochondrial loci, such as ND2 and cytochrome b, obtained from whole mitochondrial genomes from 20 taxa. Using barcode identification numbers (BINs), we find putative cryptic species in 19% of landbird species, highlighting hidden diversity in the relatively well-described avifauna of Panama. Whereas some of these mitochondrial divergence events corresponded with recognized geographic features that likely isolated populations, such as the Cordillera Central highlands, the majority (74%) of lowland splits were between eastern and western populations. The timing of these splits are not temporally coincident across taxa, suggesting that historical events, such as the formation of the Isthmus of Panama and Pleistocene climatic cycles, were not the primary drivers of cryptic diversification. Rather, we observed that forest species, understory species, insectivores, and strongly territorial species-all traits associated with lower dispersal ability-were all more likely to have multiple BINs in Panama, suggesting strong ecological associations with cryptic divergence. Additionally, hand-wing index, a proxy for dispersal capability, was significantly lower in species with multiple BINs, indicating that dispersal ability plays an important role in generating diversity in Neotropical birds. Together, these results underscore the need for evolutionary studies of tropical bird communities to consider ecological factors along with geographic explanations, and that even in areas with well-known avifauna, avian diversity may be substantially underestimated.

Species delimitation using genomic data to resolve taxonomic uncertainties in a speciation continuum of pelagic seabirds

Speciation is a continuous and complex process shaped by the interaction of numerous evolutionary forces. Despite the continuous nature of the speciation process, the implementation of conservation policies relies on the delimitation of species and evolutionary significant units (ESUs). Puffinus shearwaters are globally distributed and threatened pelagic seabirds. Due to remarkable morphological status the group has been under intense taxonomic debate for the past three decades. Here, we use double digest Restriction-Site Associated DNA sequencing (ddRAD-Seq) to genotype species and subspecies of North Atlantic and Mediterranean Puffinus shearwaters across their entire geographical range. We assess the phylogenetic relationships and population structure among and within the group, evaluate species boundaries, and characterise the genomic landscape of divergence. We find that current taxonomies are not supported by genomic data and propose a more accurate taxonomy by integrating genomic information with other sources of evidence. Our results show that several taxon pairs are at different stages of a speciation continuum. Our study emphasises the potential of genomic data to resolve taxonomic uncertainties, which can help to focus management actions on relevant taxa, even if they do not necessarily coincide with the taxonomic rank of species.

Integrative species delimitation reveals fine-scale allopatric speciation in a good-flying insect: a case study on

Alpha taxonomy is fundamental for many biological fields. Delineation of the species boundary, however, can be challenging in a species complex, where different species share a similar morphology and diagnostic characters may not be available. In this context, integrative approaches that incorporate molecular and morphological data sets, and account for speciation history can be helpful to alpha taxonomy. Different approaches to species delimitation based on different assumptions are complementary and by integrating the results from multiple approaches we can generate a more reliable and objective taxonomic decision. In this study, we applied three molecular approaches to species delimitation and inferred the demographic history based on an isolation with migration model to test a morphologically based taxonomic hypothesis for the Cylindera pseudocylindriformis complex. We discuss the association between genetic divergence and microhabitat specialisation, and further corroborate that C. subtilis sp. nov. is a valid new species by integrating the results from model-based species delimitation and the genealogical divergence index. We argue that genetic endemism can occur at a small geographic scale, even in a winged insect like tiger beetles. Our results also indicated that there may still be undocumented species diversity of Taiwanese Cylindera remaining to be discovered. ZooBank LSID: urn:lsid:zoobank.org:pub:9DEC1432-365C-4872-8D06-73B95F30624F

Old specimens for old branches: Assessing effects of sample age in resolving a rapid Neotropical radiation of squirrels

Ultraconserved Elements (UCEs) have been useful to resolve challenging phylogenies of non-model clades, unpuzzling long-conflicted relationships in key branches of the Tree of Life at both deep and shallow levels. UCEs are often reliably recovered from historical samples, unlocking a vast number of preserved natural history specimens for analysis. However, the extent to which sample age and preservation method impact UCE recovery as well as downstream inferences remains unclear. Furthermore, there is an ongoing debate on how to curate, filter, and properly analyze UCE data when locus recovery is uneven across sample age and quality. In the present study we address these questions with an empirical dataset composed of over 3800 UCE loci from 219 historical and modern samples of Sciuridae, a globally distributed and ecologically important family of rodents. We provide a genome-scale phylogeny of two squirrel subfamilies (Sciurillinae and Sciurinae: Sciurini) and investigate their placement within Sciuridae. For historical specimens, recovery of UCE loci and mean length per locus were inversely related to sample age; deeper sequencing improved the number of UCE loci recovered but not locus length. Most of our phylogenetic inferences-performed on six datasets with alternative data-filtering strategies, and using three distinct optimality criteria-resulted in distinct topologies. Datasets containing more loci (40% and 50% taxa representativeness matrices) yielded more concordant topologies and higher support values than strictly filtered datasets (60% matrices) particularly with IQ-Tree and SVDquartets, while filtering based on information content provided better topological resolution for inferences with the coalescent gene-tree based approach in ASTRAL-III. We resolved deep relationships in Sciuridae (including among the five currently recognized subfamilies) and relationships among the deepest branches of Sciurini, but conflicting relationships remain at both genus- and species-levels for the rapid Neotropical tree squirrel radiation. Our results suggest that phylogenomic consensus can be difficult and heavily influenced by the age of available samples and the filtering steps used to optimize dataset properties.

Fishing for DNA? Designing baits for population genetics in target enrichment experiments: Guidelines, considerations and the new tool supeRbaits

Targeted sequencing is an increasingly popular next-generation sequencing (NGS) approach for studying populations that involves focusing sequencing efforts on specific parts of the genome of a species of interest. Methodologies and tools for designing targeted baits are scarce but in high demand. Here, we present specific guidelines and considerations for designing capture sequencing experiments for population genetics for both neutral genomic regions and regions subject to selection. We describe the bait design process for three diverse fish species: Atlantic salmon, Atlantic cod and tiger shark, which was carried out in our research group, and provide an evaluation of the performance of our approach across both historical and modern samples. The workflow used for designing these three bait sets has been implemented in the R-package supeRbaits, which encompasses our considerations and guidelines for bait design for the benefit of researchers and practitioners. The supeRbaits R-package is user-friendly and versatile. It is written in C++ and implemented in R. supeRbaits and its manual are available from Github: https://github.com/BelenJM/supeRbaits.

Combining Species Delimitation, Species Trees, and Tests for Gene Flow Clarifies Complex Speciation in Scrub-Jays

Complex speciation, involving rapid divergence and multiple bouts of post-divergence gene flow, can obfuscate phylogenetic relationships and species limits. In North America, cases of complex speciation are common, due at least in part to the cyclical Pleistocene glacial history of the continent. Scrub-jays in the genus Aphelocoma provide a useful case study in complex speciation because their range throughout North America is structured by phylogeographic barriers with multiple cases of secondary contact between divergent lineages. Here, we show that a comprehensive approach to genomic reconstruction of evolutionary history, i.e., synthesizing results from species delimitation, species tree reconstruction, demographic model testing, and tests for gene flow, is capable of clarifying evolutionary history despite complex speciation. We find concordant evidence across all statistical approaches for the distinctiveness of an endemic southern Mexico lineage (A. w. sumichrasti), culminating in support for the species status of this lineage under any commonly applied species concept. We also find novel genomic evidence for the species status of a Texas endemic lineage A. w. texana, for which equivocal species delimitation results were clarified by demographic modeling and spatially explicit models of gene flow. Finally, we find that complex signatures of both ancient and modern gene flow between the non-sister California Scrub-Jay (A. californica) and Woodhouse's Scrub-Jay (A. woodhouseii), result in discordant gene trees throughout the species' genomes despite clear support for their overall isolation and species status. In sum, we find that a multi-faceted approach to genomic analysis can increase our understanding of complex speciation histories, even in well-studied groups. Given the emerging recognition that complex speciation is relatively commonplace, the comprehensive framework that we demonstrate for interrogation of species limits and evolutionary history using genomic data can provide a necessary roadmap for disentangling the impacts of gene flow and incomplete lineage sorting to better understand the systematics of other groups with similarly complex evolutionary histories.

FrogCap: A modular sequence capture probe-set for phylogenomics and population genetics for all frogs, assessed across multiple phylogenetic scales

Despite the prevalence of high-throughput sequencing in phylogenetics, many relationships remain difficult to resolve because of conflicting signal among genomic regions. Selection of different types of molecular markers from different genomic regions is required to overcome these challenges. For evolutionary studies in frogs, we introduce the publicly available FrogCap suite of genomic resources, which is a large collection of ~15,000 markers that unifies previous genetic sequencing efforts. FrogCap is designed to be modular, such that subsets of markers and SNPs can be selected based on the desired phylogenetic scale. FrogCap uses a variety of marker types that include exons and introns, ultraconserved elements, and previously sequenced Sanger markers, which span up to 10,000 bp in alignment lengths; in addition, we demonstrate potential for SNP-based analyses. We tested FrogCap using 121 samples distributed across five phylogenetic scales, comparing probes designed using a consensus- or exemplar genome-based approach. Using the consensus design is more resilient to issues with sensitivity, specificity, and missing data than picking an exemplar genome sequence. We also tested the impact of different bait kit sizes (20,020 vs. 40,040) on depth of coverage and found triple the depth for the 20,020 bait  kit. We observed sequence capture success (i.e., missing data, sequenced markers/bases, marker length, and informative sites) across phylogenetic scales. The incorporation of different marker types is effective for deep phylogenetic relationships and shallow population genetics studies. Having demonstrated FrogCap's utility and modularity, we conclude that these new resources are efficacious for high-throughput sequencing projects across variable timescales.

How to Accurately Delineate Morphologically Conserved Taxa and Diagnose Their Phenotypic Disparities: Species Delimitation in Cryptic Rhinolophidae (Chiroptera)

Systematics and taxonomy are the backbone of all components of biology and ecology, yet cryptic species present a major challenge for accurate species identification. This is especially problematic as they represent a substantial portion of undiscovered biodiversity, and have implications for not only species conservation, but even assaying potential risk of zoonotic spillover. Here, we use integrative approaches to delineate potential cryptic species in horseshoe bats (Rhinolophidae), evaluate the phenotypic disparities between cryptic species, and identify key traits for their identification. We tested the use of multispecies coalescent models (MSC) using Bayesian Phylogenetic and Phylogeography (BPP) and found that BPP was useful in delineating potential cryptic species, and consistent with acoustic traits. Our results show that around 40% of Asian rhinolophid species are potentially cryptic and have not been formally described. In order to avoid potential misidentification and allow species to be accurately identified, we identified quantitative noseleaf sella and acoustic characters as the most informative traits in delineating between potential cryptic species in Rhinolophidae. This highlights the physical differences between cryptic species that are apparent in noseleaf traits which often only qualitatively described but rarely measured. Each part of the noseleaf including the sella, lateral lappets, and lancet furrows, play roles in focusing acoustic beams and thus, provide useful characteristics to identify cryptic Rhinolophus species. Finally, species delimitation for cryptic species cannot rely on genetic data alone, but such data should be complemented by other evidence, including phenotypic, acoustic data, and geographic distributions to ensure accurate species identification and delineation.

Widely used, short 16S rRNA mitochondrial gene fragments yield poor and erratic results in phylogenetic estimation and species delimitation of amphibians

Background

The 16S mitochondrial rRNA gene is the most widely sequenced molecular marker in amphibian systematic studies, making it comparable to the universal CO1 barcode that is more commonly used in other animal groups. However, studies employ different primer combinations that target different lengths/regions of the 16S gene ranging from complete gene sequences (~ 1500 bp) to short fragments (~ 500 bp), the latter of which is the most ubiquitously used. Sequences of different lengths are often concatenated, compared, and/or jointly analyzed to infer phylogenetic relationships, estimate genetic divergence (p-distances), and justify the recognition of new species (species delimitation), making the 16S gene region, by far, the most influential molecular marker in amphibian systematics. Despite their ubiquitous and multifarious use, no studies have ever been conducted to evaluate the congruence and performance among the different fragment lengths.

Results

Using empirical data derived from both Sanger-based and genomic approaches, we show that full-length 16S sequences recover the most accurate phylogenetic relationships, highest branch support, lowest variation in genetic distances (pairwise p-distances), and best-scoring species delimitation partitions. In contrast, widely used short fragments produce inaccurate phylogenetic reconstructions, lower and more variable branch support, erratic genetic distances, and low-scoring species delimitation partitions, the numbers of which are vastly overestimated. The relatively poor performance of short 16S fragments is likely due to insufficient phylogenetic information content.

Conclusions

Taken together, our results demonstrate that short 16S fragments are unable to match the efficacy achieved by full-length sequences in terms of topological accuracy, heuristic branch support, genetic divergences, and species delimitation partitions, and thus, phylogenetic and taxonomic inferences that are predicated on short 16S fragments should be interpreted with caution. However, short 16S fragments can still be useful for species identification, rapid assessments, or definitively coupling complex life stages in natural history studies and faunal inventories. While the full 16S sequence performs best, it requires the use of several primer pairs that increases cost, time, and effort. As a compromise, our results demonstrate that practitioners should utilize medium-length primers in favor of the short-fragment primers because they have the potential to markedly improve phylogenetic inference and species delimitation without additional cost.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01994-y.

Candidate‐species delimitation in Desmognathus salamanders reveals gene flow across lineage boundaries, confounding phylogenetic estimation and clarifying hybrid zones

Dusky Salamanders (genus Desmognathus) currently comprise only 22 described, extant species. However, recent mitochondrial and nuclear estimates indicate the presence of up to 49 candidate species based on ecogeographic sampling. Previous studies also suggest a complex history of hybridization between these lineages. Studies in other groups suggest that disregarding admixture may affect both phylogenetic inference and clustering‐based species delimitation. With a dataset comprising 233 Anchored Hybrid Enrichment (AHE) loci sequenced for 896 Desmognathus specimens from all 49 candidate species, we test three hypotheses regarding (i) species‐level diversity, (ii) hybridization and admixture, and (iii) misleading phylogenetic inference. Using phylogenetic and population‐clustering analyses considering gene flow, we find support for at least 47 candidate species in the phylogenomic dataset, some of which are newly characterized here while others represent combinations of previously named lineages that are collapsed in the current dataset. Within these, we observe significant phylogeographic structure, with up to 64 total geographic genetic lineages, many of which hybridize either narrowly at contact zones or extensively across ecological gradients. We find strong support for both recent admixture between terminal lineages and ancient hybridization across internal branches. This signal appears to distort concatenated phylogenetic inference, wherein more heavily admixed terminal specimens occupy apparently artifactual early‐diverging topological positions, occasionally to the extent of forming false clades of intermediate hybrids. Additional geographic and genetic sampling and more robust computational approaches will be needed to clarify taxonomy, and to reconstruct a network topology to display evolutionary relationships in a manner that is consistent with their complex history of reticulation.

Phylogenetic and multivariate analyses of Gekko smithii Gray, 1842 recover a new species from Peninsular Malaysia and support the resurrection of G. albomaculatus (Giebel, 1861) from Sumatra

Abstract

An integrative taxonomic analysis of Sundaic populations of Gekko smithii from the Thai-Malaya Peninsula, Sumatra, and Borneo recovered four deeply divergent mitochondrial lineages that are separated by major geographic barriers (mountains and seaways). Furthermore, they bear a number of concordant statistically significant differences in meristic and morphometric features, morphospatial separation in multivariate space, and discrete differences in color pattern. Gekko smithiisensu stricto is restricted to southern Thailand south of the Isthmus of Kra and Peninsular Malaysia west of the Banjaran (mountain range) Titiwangsa, being that the type locality is on Penang Island, Penang. Gekko hulksp. nov. is a new species from extreme southern Thailand and Peninsular Malaysia east of the Banjaran Titiwangsa and five east coast islands—the type locality being Pulau (island) Tioman, Pahang. Gekko cf. albofasciolatus is tentatively used to include Bornean populations west of the Iran Mountains in Sabah and Sarawak which, in the absence of molecular data, cannot unequivocally be separated morphologically from G. albofasciolatus from the type locality at Banjarmasin, Kalimantan, Indonesia east of the Iran Mountains. In the absence of molecular data, G. albomaculatus is resurrected to include mainland Sumatran, Nias Island, and Banyak Islands populations which, based on their morphology, cannot be separated from descriptions of G. albomaculatus from the type locality of Bangka Island, 15 km off the southeast coast of mainland Sumatra. Further integrative analyses of all Sumatran and Bornean populations are currently underway as well as the enigmatic Wallacean populations from Sulawesi. Data are presented that strongly suggest all references to G. smithii from Java stem from a 151 year-old misidentification of a specimen of G. gecko of unknown provenance. Additionally, there are no vouchered records of G. smithii from Myanmar. The phylogeographic patterns of Sundaic populations of the G. smithii complex are concordant with those of a plethora of other Sundaic lineages.

An integrative taxonomic revision of slug-eating snakes (Squamata: Pareidae: Pareineae) reveals unprecedented diversity in Indochina

Slug-eating snakes of the subfamily Pareinae are an insufficiently studied group of snakes specialized in feeding on terrestrial mollusks. Currently Pareinae encompass three genera with 34 species distributed across the Oriental biogeographic region. Despite the recent significant progress in understanding of Pareinae diversity, the subfamily remains taxonomically challenging. Here we present an updated phylogeny of the subfamily with a comprehensive taxon sampling including 30 currently recognized Pareinae species and several previously unknown candidate species and lineages. Phylogenetic analyses of mtDNA and nuDNA data supported the monophyly of the three genera Asthenodipsas, Aplopeltura, and Pareas. Within both Asthenodipsas and Pareas our analyses recovered deep differentiation with each genus being represented by two morphologically diagnosable clades, which we treat as subgenera. We further apply an integrative taxonomic approach, including analyses of molecular and morphological data, along with examination of available type materials, to address the longstanding taxonomic questions of the subgenus Pareas, and reveal the high level of hidden diversity of these snakes in Indochina. We restrict the distribution of P. carinatus to southern Southeast Asia, and recognize two subspecies within it, including one new subspecies proposed for the populations from Thailand and Myanmar. We further revalidate P. berdmorei, synonymize P. menglaensis with P. berdmorei, and recognize three subspecies within this taxon, including the new subspecies erected for the populations from Laos and Vietnam. Furthermore, we describe two new species of Pareas from Vietnam: one belonging to the P. carinatus group from southern Vietnam, and a new member of the P. nuchalis group from the central Vietnam. We provide new data on P. temporalis, and report on a significant range extension for P. nuchalis. Our phylogeny, along with molecular clock and ancestral area analyses, reveal a complex diversification pattern of Pareinae involving a high degree of sympatry of widespread and endemic species. Our analyses support the "upstream" colonization hypothesis and, thus, the Pareinae appears to have originated in Sundaland during the middle Eocene and then colonized mainland Asia in early Oligocene. Sundaland and Eastern Indochina appear to have played the key roles as the centers of Pareinae diversification. Our results reveal that both vicariance and dispersal are responsible for current distribution patterns of Pareinae, with tectonic movements, orogeny and paleoclimatic shifts being the probable drivers of diversification. Our study brings the total number of Pareidae species to 41 and further highlights the importance of comprehensive taxonomic revisions not only for the better understanding of biodiversity and its evolution, but also for the elaboration of adequate conservation actions.

Global diversification and evolutionary history of onchidiid slugs (Gastropoda, Pulmonata)

Many marine species are specialized to specific parts of a habitat. In a mangrove forest, for instance, species may be restricted to the mud surface, the roots and trunks of mangrove trees, or rotting logs, which can be regarded as distinct microhabitats. Shifts to new microhabitats may be an important driver of sympatric speciation. However, the evolutionary history of these shifts is still poorly understood in most groups of marine organisms, because it requires a well-supported phylogeny with relatively complete taxon sampling. Onchidiid slugs are an ideal case study for the evolutionary history of habitat and microhabitat shifts because onchidiid species are specialized to different tidal zones and microhabitats in mangrove forests and rocky shores, and the taxonomy of the family in the Indo-West Pacific has been recently revised in a series of monographs. Here, DNA sequences for onchidiid species from the North and East Pacific, the Caribbean, and the Atlantic are used to reconstruct phylogenetic relationships among Onchidella species, and are combined with new data for Indo-West Pacific species to reconstruct a global phylogeny of the family. The phylogenetic relationships of onchidiid slugs are reconstructed based on three mitochondrial markers (COI, 12S, 16S) and three nuclear markers (28S, ITS2, H3) and nearly complete taxon sampling (all 13 genera and 62 of the 67 species). The highly-supported phylogeny presented here suggests that ancestral onchidiids most likely lived in the rocky intertidal, and that a lineage restricted to the tropical Indo-West Pacific colonized new habitats, including mudflats, mangrove forests, and high-elevation rainforests. Many onchidiid species in the Indo-West Pacific diverged during the Miocene, around the same time that a high diversity of mangrove plants appears in the fossil record, while divergence among Onchidella species occurred earlier, likely beginning in the Eocene. It is demonstrated that ecological specialization to microhabitats underlies the divergence between onchidiid genera, as well as the diversification through sympatric speciation in the genera Wallaconchis and Platevindex. The geographic distributions of onchidiid species also indicate that allopatric speciation played a key role in the diversification of several genera, especially Onchidella and Peronia. The evolutionary history of several morphological traits (penial gland, rectal gland, dorsal eyes, intestinal loops) is examined in relation to habitat and microhabitat evolutionary transitions and that the rectal gland of onchidiids is an adaptation to high intertidal and terrestrial habitats.

Plotting for change: an analytical framework to aid decisions on which lineages are candidate species in phylogenomic species discovery

A recent study argued that coalescent-based models of species delimitation mostly delineate population structure, not species, and called for the validation of candidate species using biological information additional to the genetic information, such as phenotypic or ecological data. Here, we introduce a framework to interrogate genomic datasets and coalescent-based species trees for the presence of candidate species in situations where additional biological data are unavailable, unobtainable or uninformative. For de novo genomic studies of species boundaries, we propose six steps: (1) visualize genetic affinities among individuals to identify both discrete and admixed genetic groups from first principles and to hold aside individuals involved in contemporary admixture for independent consideration; (2) apply phylogenetic techniques to identify lineages; (3) assess diagnosability of those lineages as potential candidate species; (4) interpret the diagnosable lineages in a geographical context (sympatry, parapatry, allopatry); (5) assess significance of difference or trends in the context of sampling intensity; and (6) adopt a holistic approach to available evidence to inform decisions on species status in the difficult cases of allopatry. We adopt this approach to distinguish candidate species from within-species lineages for a widespread species complex of Australian freshwater fishes (Retropinna spp.). Our framework addresses two cornerstone issues in systematics that are often not discussed explicitly in genomic species discovery: diagnosability and how to determine it, and what criteria should be used to decide whether diagnosable lineages are conspecific or represent different species.

SPART: A versatile and standardized data exchange format for species partition information

A wide range of data types can be used to delimit species and various computer-based tools dedicated to this task are now available. Although these formalized approaches have significantly contributed to increase the objectivity of species delimitation (SD) under different assumptions, they are not routinely used by alpha-taxonomists. One obvious shortcoming is the lack of interoperability among the various independently developed SD programs. Given the frequent incongruences between species partitions inferred by different SD approaches, researchers applying these methods often seek to compare these alternative species partitions to evaluate the robustness of the species boundaries. This procedure is excessively time consuming at present, and the lack of a standard format for species partitions is a major obstacle. Here, we propose a standardized format, SPART, to enable compatibility between different SD tools exporting or importing partitions. This format reports the partitions and describes, for each of them, the assignment of individuals to the "inferred species". The syntax also allows support values to be optionally reported, as well as original trees and the full command lines used in the respective SD analyses. Two variants of this format are proposed, overall using the same terminology but presenting the data either optimized for human readability (matricial SPART) or in a format in which each partition forms a separate block (SPART.XML). ABGD, DELINEATE, GMYC, PTP and TR2 have already been adapted to output SPART files and a new version of LIMES has been developed to import, export, merge and split them.

Phylogenomic Analysis Reveals Dispersal-Driven Speciation and Divergence with Gene Flow in Lesser Sunda Flying Lizards (Genus Draco)

The Lesser Sunda Archipelago offers exceptional potential as a model system for studying the dynamics of dispersal-driven diversification. The geographic proximity of the islands suggests the possibility for successful dispersal, but this is countered by the permanence of the marine barriers and extreme intervening currents that are expected to hinder gene flow. Phylogenetic and species delimitation analyses of flying lizards (genus Draco) using single mitochondrial genes, complete mitochondrial genomes, and exome-capture data sets identified 9-11 deeply divergent lineages including single-island endemics, lineages that span multiple islands, and parapatrically-distributed non-sister lineages on the larger islands. Population clustering and PCA confirmed these genetic boundaries with isolation-by-distance playing a role in some islands or island sets. While gdi estimates place most candidate species comparisons in the ambiguous zone, migration estimates suggest 9 or 10 species exist with nuclear introgression detected across some intra-island contact zones. Initial entry of Draco into the archipelago occurred at 5.5-7.5 Ma, with most inter-island colonization events having occurred between 1-3 Ma. Biogeographical model testing favors scenarios integrating geographic distance and historical island connectivity, including an initial stepping-stone dispersal process from the Greater Sunda Shelf through the Sunda Arc as far eastward as Lembata Island. However, rather than reaching the adjacent island of Pantar by dispersing over the 15-km wide Alor Strait, Draco ultimately reached Pantar (and much of the rest of the archipelago) by way of a circuitous route involving at least five over-water dispersal events. These findings suggest that historical geological and oceanographic conditions heavily influenced dispersal pathways and gene flow, which in turn drove species formation and shaped species boundaries.

Unexpectedly high levels of lineage diversity in Sundaland puddle frogs (Dicroglossidae: Occidozyga Kuhl and van Hasselt, 1822)

One of the most urgent contemporary tasks for taxonomists and evolutionary biologists is to estimate the number of species on earth. Recording alpha diversity is crucial for protecting biodiversity, especially in areas of elevated species richness, which coincide geographically with increased anthropogenic environmental pressures - the world's so-called biodiversity hotspots. Although the distribution of Puddle frogs of the genus Occidozyga in South and Southeast Asia includes five biodiversity hotspots, the available data on phylogeny, species diversity, and biogeography are surprisingly patchy. Samples analyzed in this study were collected throughout Southeast Asia, with a primary focus on Sundaland and the Philippines. A mitochondrial gene region comprising ~ 2000 bp of 12S and 16S rRNA with intervening tRNA Valine and three nuclear loci (BDNF, NTF3, POMC) were analyzed to obtain a robust, time-calibrated phylogenetic hypothesis. We found a surprisingly high level of genetic diversity within Occidozyga, based on uncorrected p-distance values corroborated by species delimitation analyses. This extensive genetic diversity revealed 29 evolutionary lineages, defined by the > 5% uncorrected p-distance criterion for the 16S rRNA gene, suggesting that species diversity in this clade of phenotypically homogeneous forms probably has been underestimated. The comparison with results of other anuran groups leads to the assumption that anuran species diversity could still be substantially underestimated in Southeast Asia in general. Many genetically divergent lineages of frogs are phenotypically similar, indicating a tendency towards extensive morphological conservatism. We present a biogeographic reconstruction of the colonization of Sundaland and nearby islands which, together with our temporal framework, suggests that lineage diversification centered on the landmasses of the northern Sunda Shelf. This remarkably genetically structured group of amphibians could represent an exceptional case for future studies of geographical structure and diversification in a widespread anuran clade spanning some of the most pronounced geographical barriers on the planet (e.g., Wallace's Line). Studies considering gene flow, morphology, ecological and bioacoustic data are needed to answer these questions and to test whether observed diversity of Puddle frog lineages warrants taxonomic recognition.

Gene Flow Increases Phylogenetic Structure and Inflates Cryptic Species Estimations: A Case Study on Widespread Philippine Puddle Frogs (Occidozyga laevis)

In cryptic amphibian complexes, there is a growing trend to equate high levels of genetic structure with hidden cryptic species diversity. Typically, phylogenetic structure and distance-based approaches are used to demonstrate the distinctness of clades and justify the recognition of new cryptic species. However, this approach does not account for gene flow, spatial, and environmental processes that can obfuscate phylogenetic inference and bias species delimitation. As a case study, we sequenced genome-wide exons and introns to evince the processes that underlie the diversification of Philippine Puddle Frogs-a group that is widespread, phenotypically conserved, and exhibits high levels of geographically-based genetic structure. We showed that widely adopted tree- and distance-based approaches inferred up to 20 species, compared to genomic analyses that inferred an optimal number of five distinct genetic groups. Using a suite of clustering, admixture, and phylogenetic network analyses, we demonstrate extensive admixture among the five groups and elucidate two specific ways in which gene flow can cause overestimations of species diversity: (1) admixed populations can be inferred as distinct lineages characterized by long branches in phylograms; and (2) admixed lineages can appear to be genetically divergent, even from their parental populations when simple measures of genetic distance are used. We demonstrate that the relationship between mitochondrial and genome-wide nuclear p-distances is decoupled in admixed clades, leading to erroneous estimates of genetic distances and, consequently, species diversity. Additionally, genetic distance was also biased by spatial and environmental processes. Overall, we showed that high levels of genetic diversity in Philippine Puddle Frogs predominantly comprise metapopulation lineages that arose through complex patterns of admixture, isolation-by-distance, and isolation-by-environment as opposed to species divergence. Our findings suggest that speciation may not be the major process underlying the high levels of hidden diversity observed in many taxonomic groups and that widely-adopted tree- and distance-based methods overestimate species diversity in the presence of gene flow.

PL, Lim KKP, Imbun PY, Anuar MSS. An investigation into the taxonomy of Abavorana luctuosa (Peters, 1871) (Anura, Ranidae) and the resurrection of Rana decorata Mocquard, 1890 from Borneo

The taxonomic status of the ranid frog Abavorana luctuosa (Peters, 1871) was investigated using a combination of molecular and morphological data. The analyses revealed that A. luctuosasensu lato is composed of two species in Borneo. One of these species agrees with the description of Rana decorata Mocquard, 1890 which is resurrected in the combination Abavorana decoratacomb. nov. (Mocquard, 1890). Abavorana decorata is recovered as the sister lineage to the remainder of Abavorana and differs by a 16.0–17.0 % uncorrected pairwise sequence divergence from its congeners A. nazgul and A. luctuosa, respectively. It is distinguishable morphologically from A. luctuosa and A. nazgul by its ventral pattern (bold, black and white reticulations on its venter along with bold banding on the underside of hind limbs vs. generally immaculate and spotted in the latter two species), and a prominent white streak beneath the eye and/or tympanum extending to the corner of the jaw. Abavorana decorata further differs from A. luctuosa by having a significantly wider head and snout, larger interorbital and tympanum diameters, longer femur in both sexes, and various combinations of other mensural characters. Both species are sympatric in Borneo and this discovery adds to a growing number of widespread Sundaic species shown to be species complexes with distinct forms in Borneo.

Phylogenetics of mud snakes (Squamata: Serpentes: Homalopsidae): A paradox of both undescribed diversity and taxonomic inflation

Mud snakes (Serpentes: Homalopsidae) are a family of 55 described, mainly aquatic, species primarily distributed throughout mainland Southeast Asia and the Indo-Australian Archipelago. Although they have been the focus of prior research, the basic relationships amongst genera and species remain poorly known. We used a combined mitochondrial and nuclear gene dataset to infer their phylogenetic relationships, using the highest levels of taxon and geographic sampling for any homalopsid phylogeny to date (62% generic and 62% species coverage; 140 individuals). Our results recover two reciprocally monophyletic groups: the fangless Brachyorrhos and its sister clade comprised of all rear-fanged homalopsids. Most genera and interspecific relationships were monophyletic and strongly supported, but intergeneric relationships and intraspecific population structure lack support. We find evidence of both undescribed diversity as well as cases of taxonomic inflation within several species. Tree-based species delimitation approaches (mPTP) support potential new candidate species as distinct from their conspecifics and also suggest that many named taxa may not be distinct species. Divergence date estimation and lineage-through-time analyses indicate lower levels of speciation in the Eocene, with a subsequent burst in diversification in the Miocene. Homalopsids may have diversified most rapidly during the Pliocene and Pleistocene, possibly in relation to tectonic shifts and sea-level fluctuations that took place in Sundaland and the Sahul Shelf. Our analyses provide new insights on homalopsid taxonomy, a baseline phylogeny for the family, and further biogeographic implications demonstrating how dynamic tectonics and Quaternary sea level changes may have shaped a widespread, diverse family of snakes.