Appearances can be deceptive: different diversification patterns within a group of Mediterranean earthworms (Oligochaeta, Hormogastridae)

Incomplete lineage sorting and ancient admixture, and speciation without morphological change in ghost-worm cryptic species

Morphologically similar species, that is cryptic species, may be similar or quasi-similar owing to the deceleration of morphological evolution and stasis. While the factors underlying the deceleration of morphological evolution or stasis in cryptic species remain unknown, decades of research in the field of paleontology on punctuated equilibrium have originated clear hypotheses. Species are expected to remain morphologically identical in scenarios of shared genetic variation, such as hybridization and incomplete lineage sorting, or in scenarios where bottlenecks reduce genetic variation and constrain the evolution of morphology. Here, focusing on three morphologically similar Stygocapitella species, we employ a whole-genome amplification method (WGA) coupled with double-digestion restriction-site associated DNA sequencing (ddRAD) to reconstruct the evolutionary history of the species complex. We explore population structure, use population-level statistics to determine the degree of connectivity between populations and species, and determine the most likely demographic scenarios which generally reject for recent hybridization. We find that the combination of WGA and ddRAD allowed us to obtain genomic-level data from microscopic eukaryotes (∼1 millimetre) opening up opportunities for those working with population genomics and phylogenomics in such taxa. The three species share genetic variance, likely from incomplete lineage sorting and ancient admixture. We speculate that the degree of shared variation might underlie morphological similarity in the Atlantic species complex.

Cryptic Clitellata: Molecular Species Delimitation of Clitellate Worms (Annelida): An Overview

Methods for species delimitation using molecular data have developed greatly and have become a staple in systematic studies of clitellate worms. Here we give a historical overview of the data and methods used to delimit clitellates from the mid-1970s to today. We also discuss the taxonomical treatment of the cryptic species, including the recommendation that cryptic species, as far as possible, should be described and named. Finally, we discuss the prospects and further development of the field.

Untangling a mess of worms: Species delimitations reveal morphological crypsis and variability in Southeast Asian semi-aquatic earthworms (Almidae, Glyphidrilus)

Semi-aquatic freshwater earthworms in the genus Glyphidrilus from Southeast Asia are characterized by both an extreme morphological crypsis among divergent phylogenetic lineages and a high morphological variability within the same phylogenetic lineages. The present study provides a new taxonomic framework for this problematic genus in SE Asia by integrating DNA sequence and morphological data. When single-locus and multilocus multispecies coalescent-based (MSC) species delimitation methods were applied to DNA sequence data, they usually yielded highly incongruent results compared to morphology-based species identifications. This suggested the presence of several cryptic species and high levels of intraspecific morphological variation. Applying reciprocal monophyly to the cytochrome c oxidase subunit 1 (COI) gene tree allowed us to propose the existence of 33 monophyletic species. Yet, often substantially more molecular operational taxonomic units (MOTUs) were obtained when species delimitation was based on COI and 16S rRNA sequences. In contrast, the ITS1 and ITS2 sequences suggested fewer MOTUs and did not recover most of the monophyletic species from the Mekong basin. However, several of these latter taxa were better supported when MSC species delimitation methods were applied to the combined mtDNA and ITS datasets. The ITS2 secondary structure retrieved one unnamed Mekong basin species that was not uncovered by the other methods when applied to ITS2 sequences. In conclusion, based on an integrative taxonomic workflow, 26 Glyphidrilus candidate species were retained and two remained to be confirmed. As such, this study provides evidence to suggest nine species new to science and to synonymize 12 nominal morphospecies. It also illustrates that the uncritical use of COI as a universal DNA barcode may overestimate species diversity because COI may be unable to distinguish between divergent conspecific lineages and different candidate species.

Multigene phylogeny reveals a new Iranian earthworm genus (Lumbricidae: Philomontanus) with three new species

Lumbricidae taxonomy is vastly restricted by the morphological simplicity of earthworms and their lack of complex appendices. This has led to confusing results in the Lumbricidae classifications, which in turn, has hindered our ability to identify and assign new and cryptic species to the family. Here we propose the addition of a new Lumbricidae genus from the Zagros and Elburz Mountains of Iran, i.e. Philomontanus gen. nov, including three new species. Our taxonomic inferences were based on the phylogenetic analysis of two nuclear gene regions (28S rDNA and 18S rDNA) and 11 mitochondrial gene regions (16S rDNA, 12S rDNA, NADH dehydrogenase I, cytochrome oxidase subunits I and II and tRNAs Asn, Asp, Val, Leu, Ala and Ser). Philomontanus gen. nov comprises the earthworm species Philomontanus sarii sp. nov., Philomontanus mahmoudi sp. nov. and Philomontanus baloutchi sp. nov. These three species are morphologically similar to each other with only a few characters separating them (e.g. size, pigmentation and position of clitellum). Our findings support the adoption of an integrative approach including molecular information (e.g., DNA sequences) to aid earthworm classification and develop a robust taxonomy.

Integrative systematic revision of a Mediterranean earthworm family: Hormogastridae (Annelida, Oligochaeta)

The problem of reconciling earthworm taxonomy and phylogeny has shown advances with the application of molecular techniques, yet they have proven insufficient. Integrative systematics could solve this by combining multiple sources of evolutionary information. Relatively low diversity, restricted range and low nomenclatural conflict make Hormogastridae Michaelsen, 1900 a desirable target for an integrative systematics approach. The main systematic conflicts within this family are the polyphyly of the species Hormogaster pretiosa Michaelsen, 1899, the widespread presence of cryptic lineages, the lack of resolution of supraspecific relationships and the paraphyly of the genus Hormogaster Rosa, 1877 (found to be composed of four well-supported genus-level clades by molecular phylogenetic inference). This work integrates all the existing information by performing phylogenetic inference based on morphological, molecular and total evidence datasets, comparing their performance with the topology obtained by phylogenomic analyses. It also includes a comparative study of representatives of the main clades based on microcomputed tomography (µCT) reconstructions. The addition of morphological characters improved the resolution of the Hormogastridae tree; ancestral state reconstruction displayed the evolution of character states and provided morphological diagnoses for the genera within a new system, which incorporates information about ecological niches and biogeography.

Pinpointing cryptic borders: Fine-scale phylogeography and genetic landscape analysis of the Hormogaster elisae complex (Oligochaeta, Hormogastridae)

Spatial and temporal aspects of the evolution of cryptic species complexes have received less attention than species delimitation within them. The phylogeography of the cryptic complex Hormogaster elisae (Oligochaeta, Hormogastridae) lacks knowledge on several aspects, including the small-scale distribution of its lineages or the palaeogeographic context of their diversification. To shed light on these topics, a dense specimen collection was performed in the center of the Iberian Peninsula - resulting in 28 new H. elisae collecting points, some of them as close as 760m from each other- for a higher resolution of the distribution of the cryptic lineages and the relationships between the populations. Seven molecular regions were amplified: mitochondrial subunit 1 of cytochrome c oxidase (COI), 16S rRNA and tRNA Leu, Ala, and Ser (16S t-RNAs), one nuclear ribosomal gene (a fragment of 28S rRNA) and one nuclear protein-encoding gene (histone H3) in order to infer their phylogenetic relationships. Different representation methods of the pairwise divergence in the cytochrome oxidase I sequence (heatmap and genetic landscape graphs) were used to visualize the genetic structure of H. elisae. A nested approach sensu Mairal et al. (2015) (connecting the evolutionary rates of two datasets of different taxonomic coverage) was used to obtain one approximation to a time-calibrated phylogenetic tree based on external Clitellata fossils and a wide molecular dataset. Our results indicate that limited active dispersal ability and ecological or biotic barriers could explain the isolation of the different cryptic lineages, which never co-occur. Rare events of long distance dispersal through hydrochory appear as one of the possible causes of range expansion.

Hybridization relics complicate barcode-based identification of species in earthworms

Introgressive hybridization results in mito-nuclear discordance which could obscure the delimitation of closely related taxa. Although such events are increasingly reported, they have been poorly studied in earthworms. Here, we propose a method for investigating the degree of introgressive hybridization between three taxa of the Allolobophora chlorotica aggregate within two field populations (N = 67 and N = 105) using a reference data set including published DNA barcoding and microsatellite data of all known A. chlorotica lineages (N = 85). For this, we used both molecular phylogenetic and population genetic approaches. The test of correspondence between mitochondrial cytochrome c oxidase I (COI) lineages and clusters of nuclear microsatellite genotypes allowed individuals to be sorted in three categories (matching, admixed and nonmatching) and additional markers (mitochondrial NADH dehydrogenase subunit 1, nuclear Histone 3 and Internal transcribed Spacer Region 2) were used for phylogenetic reconstructions in order to check assignments. Although 15 admixed individuals were observed, no early-generation hybrids were detected within the two populations. Interestingly, 14 nonmatching individuals (i.e. with a mtDNA haplotype that did not correspond to their nuclear cluster) were detected, a pattern that would result after multiple generations of unidirectional hybridization of female from one taxon to male of the other taxon. Because earthworms are simultaneous hermaphrodites, these events of unidirectional hybridization suggest sterility of the male function in several crosses and highlight that some individuals can be misidentified if reliance is placed on COI barcodes alone. These findings could improve the use of these barcodes in earthworms for species delineation.

Phylogenomic analyses of a Mediterranean earthworm family (Annelida: Hormogastridae)

Earthworm taxonomy and evolutionary biology remain a challenge because of their scarce distinct morphological characters of taxonomic value, the morphological convergence by adaptation to the uniformity of the soil where they inhabit, and their high plasticity when challenged with stressful or new environmental conditions. Here we present a phylogenomic study of the family Hormogastridae, representing also the first piece of work of this type within earthworms. We included seven transcriptomes of the group representing the main lineages as previously-described, analysed in a final matrix that includes twelve earthworms and eleven outgroups. While there is a high degree of gene conflict in the generated trees that obscure some of the internal relationships, the origin of the family is well resolved: the hormogastrid Hemigastrodrilus appears as the most ancestral group, followed by the ailoscolecid Ailoscolex, therefore rejecting the validity of the family Ailoscolecidae. Our results place the origin of hormogastrids in Southern France, as previously hypothesised.

Fauna Europaea: Annelida - Terrestrial Oligochaeta (Enchytraeidae and Megadrili), Aphanoneura and Polychaeta

Fauna Europaea provides a public web-service with an index of scientific names (including important synonyms) of all living European land and freshwater animals, their geographical distribution at country level (up to the Urals, excluding the Caucasus region), and some additional information. The Fauna Europaea project covers about 230,000 taxonomic names, including 130,000 accepted species and 14,000 accepted subspecies, which is much more than the originally projected number of 100,000 species. This represents a huge effort by more than 400 contributing specialists throughout Europe and is a unique (standard) reference suitable for many users in science, government, industry, nature conservation and education. This paper provides updated information on the taxonomic composition and distribution of the Annelida - terrestrial Oligochaeta (Megadrili and Enchytraeidae), Aphanoneura and Polychaeta, recorded in Europe. Data on 18 families, 11 autochthonous and 7 allochthonous, represented in our continent by a total of 800 species, are reviewed, beginning from their distinctness, phylogenetic status, diversity and global distribution, and following with major recent developments in taxonomic and faunistic research in Europe. A rich list of relevant references is appended. The Fauna Europaea Annelida - terrestrial Oligochaeta data-set, as completed in 2004, will be updated accordingly.

Underground evolution: new roots for the old tree of lumbricid earthworms

Earthworms belonging to the family Lumbricidae are extremely abundant in terrestrial temperate regions. They affect soil properties and nutrient cycling, thus shaping plant community composition and aboveground food webs. Some lumbricids are also model organisms in ecology and toxicology. Despite the intense research efforts dedicated to lumbricids over the last 130years, the evolutionary relationships and taxonomic classification of these organisms are still subject to great debate. Resolution of their systematics is hampered by the structural simplicity of the earthworm body plan and the existence of cryptic species. We sampled 160 earthworm specimens belonging to 84 lumbricid species (28 genera) and 22 Lumbricoidea outgroups, sequenced two nuclear genes, four mitochondrial genes and seven mitochondrial tRNAs and examined 22 morphological characters. We then applied a combination of phylogenetic methods to generate the first robust genus-level phylogeny of the Lumbricidae. Our results show that the current Lumbricidae classification and the underlying hypotheses of character evolution must be revised. Our chronogram suggests that lumbricids emerged in the Lower Cretaceous in the holarctic region and that their diversification has been driven by tectonic processes (e.g. Laurasia split) and geographical isolation. Our chronogram and character reconstruction analysis reveal that spermathecae number does not follow a gradual pattern of reduction and that parthenogenesis arose from sexual relatives multiple times in the group; the same analysis also indicates that both epigeic and anecic earthworms evolved from endogeic ancestors. These findings emphasize the strong and multiple changes to which morphological and ecological characters are subjected, challenging the hypothesis of character stasis in Lumbricidae.

Pheromone evolution, reproductive genes, and comparative transcriptomics in mediterranean earthworms (annelida, oligochaeta, hormogastridae)

Animals inhabiting cryptic environments are often subjected to morphological stasis due to the lack of obvious agents driving selection, and hence chemical cues may be important drivers of sexual selection and individual recognition. Here, we provide a comparative analysis of de novo-assembled transcriptomes in two Mediterranean earthworm species with the objective to detect pheromone proteins and other reproductive genes that could be involved in cryptic speciation processes, as recently characterized in other earthworm species. cDNA libraries of unspecific tissue of Hormogaster samnitica and three different tissues of H. elisae were sequenced in an Illumina Genome Analyzer II or Hi-Seq. Two pheromones, Attractin and Temptin were detected in all tissue samples and both species. Attractin resulted in a reliable marker for phylogenetic inference. Temptin contained multiple paralogs and was slightly overexpressed in the digestive tissue, suggesting that these pheromones could be released with the casts. Genes involved in sexual determination and fertilization were highly expressed in reproductive tissue. This is thus the first detailed analysis of the molecular machinery of sexual reproduction in earthworms.

Compilation of morphological and molecular data, a necessity for taxonomy: The case of Hormogaster abbatissae sp. n. (Annelida, Clitellata, Hormogastridae)

Conflict among data sources can be frequent in evolutionary biology, especially in cases where one character set poses limitations to resolution. Earthworm taxonomy, for example, remains a challenge because of the limited number of morphological characters taxonomically valuable. An explanation to this may be morphological convergence due to adaptation to a homogeneous habitat, resulting in high degrees of homoplasy. This sometimes impedes clear morphological diagnosis of species. Combination of morphology with molecular techniques has recently aided taxonomy in many groups difficult to delimit morphologically. Here we apply an integrative approach by combining morphological and molecular data, including also some ecological features, to describe a new earthworm species in the family Hormogastridae, Hormogaster abbatissaesp. n., collected in Sant Joan de les Abadesses (Girona, Spain). Its anatomical and morphological characters are discussed in relation to the most similar Hormogastridae species, which are not the closest species in a phylogenetic analysis of molecular data. Species delimitation using the GMYC method and genetic divergences with the closest species are also considered. The information supplied by the morphological and molecular sources is contradictory, and thus we discuss issues with species delimitation in other similar situations. Decisions should be based on a profound knowledge of the morphology of the studied group but results from molecular analyses should also be considered.

A time-calibrated molecular phylogeny of the precious corals: reconciling discrepancies in the taxonomic classification and insights into their evolutionary history

Background

Seamount-associated faunas are often considered highly endemic but isolation and diversification processes leading to such endemism have been poorly documented at those depths. Likewise, species delimitation and phylogenetic studies in deep-sea organisms remain scarce, due to the difficulty in obtaining samples, and sometimes controversial. The phylogenetic relationships within the precious coral family Coralliidae remain largely unexplored and the monophyly of its two constituent genera, Corallium Cuvier and Paracorallium Bayer & Cairns, has not been resolved. As traditionally recognized, the diversity of colonial forms among the various species correlates with the diversity in shape of their supporting axis, but the phylogenetic significance of these characters remains to be tested. We thus used mitochondrial sequence data to evaluate the monophyly of Corallium and Paracorallium and the species boundaries for nearly all named taxa in the family. Species from across the coralliid range, including material from Antarctica, Hawaii, Japan, New Zealand, Taiwan, Tasmania, the eastern Pacific and the western Atlantic were examined.

Results

The concatenated analysis of five mitochondrial regions (COI, 16S rRNA, ND2, and ND3-ND6) recovered two major coralliid clades. One clade is composed of two subgroups, the first including Corallium rubrum, the type species of the genus, together with a small group of Paracorallium species (P. japonicum and P. tortuosum) and C. medea (clade I-A); the other subgroup includes a poorly-resolved assemblage of six Corallium species (C. abyssale, C. ducale, C. imperiale, C. laauense, C. niobe, and C. sulcatum; clade I-B). The second major clade is well resolved and includes species of Corallium and Paracorallium (C. elatius, C. kishinouyei, C. konojoi, C. niveum, C. secundum, Corallium sp., Paracorallium nix, Paracorallium thrinax and Paracorallium spp.). A traditional taxonomic study of this clade delineated 11 morphospecies that were congruent with the general mixed Yule-coalescent (GMYC) model. A multilocus species-tree approach also identified the same two well-supported clades, being Clade I-B more recent in the species tree (18.0-15.9 mya) than in the gene tree (35.2-15.9 mya). In contrast, the diversification times for Clade II were more ancient in the species tree (136.4-41.7 mya) than in the gene tree (66.3-16.9 mya).

Conclusions

Our results provide no support for the taxonomic status of the two currently recognized genera in the family Coralliidae. Given that Paracorallium species were all nested within Corallium, we recognize the coralliid genus Corallium, which includes the type species of the family, and thus consider Paracorallium a junior synonym of Corallium. We propose the use of the genus Hemicorallium Gray for clade I-B (species with long rod sclerites, cylindrical autozooids and smooth axis). Species delimitation in clade I-B remains unclear and the molecular resolution for Coralliidae species is inconsistent in the two main clades. Some species have wide distributions, recent diversification times and low mtDNA divergence whereas other species exhibit narrower allopatric distributions, older diversification times and greater levels of mtDNA resolution.

Adding complexity to the complex: new insights into the phylogeny, diversification and origin of parthenogenesis in the Aporrectodea caliginosa species complex (Oligochaeta, Lumbricidae)

The importance of the Aporrectodea caliginosa species complex lies in the great abundance and wide distribution of the species which exist within it. For more than a century, chaos has surrounded this complex; morphological criteria has failed to solve the taxonomic status of these species. This present body of work aims to study the phylogeny of this complex by increasing the number of samples used in previous molecular works and by including morphologically-similar species that were never studied using molecular tools (A. giardi, Nicodrilus monticola, N. carochensis and N. tetramammalis). Two basal clades were obtained: one formed by A. caliginosa and A. tuberculata and the other by the rest of the species. This second clade was divided into two more: one with Eurosiberian and another with Mediterranean forms. A. caliginosa and A. longa were divided into two paraphyletic groups. Both A. giardi and A. nocturna showed characteristics consistent with monophyletic groups. Each of the two recovered lineages of A. trapezoides were phylogenetically related to different sexual species. While lineage I of A. trapezoides was monophyletic, lineage II resulted to be paraphyletic, as well as the three Nicodrilus 'species'. The diversification of the complex occurred during the Late Miocene-Early Pliocene (6.92-11.09 Mya). The parthenogenetic forms within the Mediterranean clade would have diversified before the ones in the Eurosiberian clade (3.13-4.64 Mya and 1.05-3.48 Mya, respectively), thus implying the existence not only of at least two different moments in which parthenogenesis arose within this complex of species, but also of two different and independent evolutionary lines. Neither the 4× rule nor the GMYC method for species delimitation were successful for distinguishing taxonomically-distinct species.