Figures
Abstract
Local endemic species with their unique evolutionary history always stirred the interest of scientists. One such area especially rich in endemics is northern Italy. In case of pill millipedes of the genus Glomeris Latreille, 1803, only a single species is found in northern Europe, while 22 country-endemics alone are known from Italy. Many of these endemics, however, have not been studied in several decades; therefore we aimed to determine whether this diversity is the result of overlooked synonymies or natural processes. A focus was placed on the local endemics that are in some aspects morphologically similar to the widespread and variable G. klugii Brandt, 1833. The local endemics Glomeris larii Verhoeff, 1921, G. primordialis Verhoeff, 1930, G. oblongoguttata Verhoeff, 1894, G. oropensis Verhoeff, 1936, G. transalpina Koch, 1836, G. romana Verhoeff, 1900, G. ligurica Latzel, 1884 and G. apuana Verhoeff, 1911 were included in a molecular analysis incorporating ribosomal nuclear (28S) and mitochondrial (COI) genes. Individuals were sequenced and compared to 31 specimens from 18 localities of G. klugii. The final dataset included 657 base pairs for 56 terminals in the COI, and 14 terminals with 1068 base pairs in the combined 28S and COI analysis. Our analysis shows intraspecific distances of up to 5% in the COI gene in G. klugii that are not strictly correlated to geography or color pattern. G. larii is discovered to be genetically and morphologically identical to G. klugii and is synonymised with the latter. Interspecific distances in our dataset vary between 6.7 to 15.9%, with the lowest (6.7–9.0%) between G. primordialis and G. klugii. Our analysis confirms the species status of the local endemics G. primordialis, G. oblongoguttata, G. oropensis, G. transalpina, G. ligurica and G. apuana. We also confirm the synonymy of G. undulata Koch, 1844 under G. klugii. G. genuensis Latzel, 1886 is indistinguishable from G. ligurica.
Citation: Wesener T, Conrad C (2016) Local Hotspots of Endemism or Artifacts of Incorrect Taxonomy? The Status of Microendemic Pill Millipede Species of the Genus Glomeris in Northern Italy (Diplopoda, Glomerida). PLoS ONE 11(9): e0162284. https://doi.org/10.1371/journal.pone.0162284
Editor: Bi-Song Yue, Sichuan University, CHINA
Received: July 14, 2016; Accepted: August 20, 2016; Published: September 15, 2016
Copyright: © 2016 Wesener, Conrad. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All sequence files are available from Genbank database (accession numbers are in the manuscript, in the Table). All other relevant data are within the paper and its Supporting Information files.
Funding: The author(s) received no specific funding for this work.
Competing interests: The authors have declared that no competing interests exist.
Introduction
This study analyses the mitochondrial and nuclear genes of six Italian pill millipede species to determine whether the species are indeed local endemics or simply the result of incorrect taxonomy. Pill millipedes belong to the order Glomerida, a relatively species-poor (>280 species [1]), basal taxon [2] of the Diplopoda with a holarctic distribution [3]. The highest genus-level diversity of the Glomerida is in Europe [1,4]. The most species-rich European genus, Glomeris Latreille, 1803 [4,5], includes more than 70 species described from Europe and North Africa [6,7]. Thirty-five of these species are found in Italy and indeed the highest species-level diversity in Europe is reached in Italy just south of the Alps [6,8].
Endemic species are often categorized as microendemics, local endemics, regional endemics, or country endemics. Italy is home to more endemic pill millipede species than in all other European countries combined. Of Glomeris species endemic to Italy, 18 occur exclusively in northern Italy [6] and are often only known from a localized area, sometimes just the type locality. Very little is known about these northern Italian endemics, despite their importance for biogeographical studies and nature conservation; fourteen of them have no records since more than 50 years. The species included in our study are six of the 22 endemic Glomeris species occurring in Italy [9–11].
The Italian Glomeris species studied here all belong historically to the G. klugii Brandt, 1833 species-group [12,13]. The G. klugii species-group is a good representative of the potential of molecular studies in helping to clarify taxonomic problems in the genus. Taxonomic progress in Glomeris is currently hindered by the unclear status of many of its more than 70 named species, with more than 300 subspecies and varieties being described in the past (see [14]). First molecular studies based on allozyme data settled long suspected synonymies and wrongly applied names in several of the widespread species of the genus [15–18], including G. klugii [16].
G. klugii is widespread and is known to show great phenotypic plasticity including many color variations; more than 40 varieties and subspecies have been described [16]. The basic color of G. klugii is light brown to red, with different amounts of black freckles fusing towards larger spots, and further on towards fully black individuals, creating distinct color patterns [16,19]. Some of the specimens look rather striking, with certain color patterns restricted to special geographic areas, one of the reasons for the rich application of taxonomic names and long list of synonyms in G. klugii. Two color morphs of G. klugii were viewed as separate species (G. undulata Koch, 1844 and G. conspersa Koch, 1847) for more than 150 years until genetic studies showed their conspecificity [16]. The name G. klugii Brandt, 1833 was long forgotten and was only recently revised [20]. Its type specimen clearly represents a specimen with the characteristic coloration (red with black spots) of the subspecies G. klugii porphyrea Koch, 1847 from the Balkan Peninsula. If the subspecies G. k. porphyrea would be a distinct species, the name G. undulata could be re-validated for the central European populations of G. klugii.
Modern genetic studies involving single genes, especially the barcoding gene [21,22] cytochrome c oxidase subunit I (COI), can help to further clarify taxonomic problems in pill millipedes. Previous studies show that large genetic p-distances exist between pill millipede genera [23,24]. Large interspecific genetic distances also occur inside the Glomeridae genera, such as in Trachysphaera [25] and in Glomeris [26], at least between the few species where sequence data is available. First studies based on COI in Glomeris revealed two things, (1) local endemic species that were synonyms of more widespread taxa [27]; and (2) 'forgotten' local endemics that could be revised with the help of barcode data [11].
With the help of mitochondrial and nuclear genes, this study evaluates the taxonomic status of six more or less localized endemic Italian Glomeris species that historically belong to the G. klugii species-group: G. larii Verhoeff, 1921, G. primordialis Verhoeff, 1930, G. oblongoguttata Verhoeff, 1894, G. oropensis Verhoeff, 1936, G. transalpina Koch, 1836 and G. romana Verhoeff, 1900. The abovementioned species morphologically resemble the variable and widespread species G. klugii in the most important morphological characters (coloration and absence of a main stria at the thoracic shield). To clarify the relationships of the local endemics with G. klugii, specimens of the latter covering the geographical range of the species, as well as representing different color morphs, were included. Our molecular genetic analysis characterizes the different genetic lineages of G. klugii, as well as shows whether the different local endemic Italian taxa represent valid species or additional synonyms of G. klugii.
Materials and Methods
Specimen selection
To study the intraspecific variability of G. klugii and the taxonomic status of related species we selected 31 specimens of G. klugii from 18 localities (Fig 1) and downloaded four additional COI sequences of G. klugii from GenBank (coming from [26]). Glomeris species receive no special protection status anywhere in Europe. For specimens collected from natural protection areas, a detailed list of the authority and locations is provided in the acknowledgments. Given the budgetary constraints, we often decided to study only two specimens per locality so that we could include samples from many different populations that represent the whole range and morphological variation of the species. More than two specimens were studied only in especially morphologically diverse populations (Euganean Hills, Hagen, Malgrate).
Known area of distribution of G. klugii highlighted in green (after Hoess 2000). Letters a-i refer to specific analyzed species (letters marked by a red diamond refer to microendemics), numbers in rectangles to populations (1–16) of G. klugii (as listed in Table 1).
ZFMK = Zoological Research Museum A. Koenig, Bonn, Germany; ZSM = Zoologische Staatssammlung Munich, Germany. Asterisk marks sequences downloaded from Genbank.
For the study of the northern Italian local endemic species, we selected specimens of G. larii, G. primordialis, G. oblongoguttata, G. oropensis, G. transalpina, G. romana, G. ligurica Latzel, 1884 and G. apuana Verhoeff, 1911; the sequences of G. ligurica and G. apuana came from earlier studies [11]. Most of the local endemic species are rare, meaning that only a few specimens, often from only a single locality, could be included.
We also selected specimens of G. marginata and G. connexa to serve as outgroups for the study of the 28S rDNA gene as no pill millipede sequences were available on Genbank (06.2015).
All specimens were directly conserved in 95% ethanol and then, within days or weeks, stored at -20°C until DNA extraction.
DNA extraction, PCR, and sequencing
For DNA extraction, we removed inter-segmental muscle tissue from the specimens and then, following the manufacturer’s extraction protocol, extracted genomic DNA from muscle tissue using the DNAeasy Blood & Tissue kit from Qiagen.
Voucher specimens of each sequenced animal are stored at the ZFMK (Table 1). PCR and sequencing success for the COI gene was low; several iterations of PCRs and sequencing reactions were necessary.
We conducted the PCR of the COI gene with the LCO1490 and Nancy primer pair [28], as well as the Folmer primers [29]. Each reaction involved 2.5 μl DNA, 2.3 μl water, 2 μl Q-solution, 10 μl Qiagen multiplex-mix, and 1.6 μl of each primer (10 pmol/μl) for a total reaction volume of 20 μl. Each PCR run included a negative control without any DNA. The touchdown PCR temperature profile was 35 seconds of denaturation at 94°C, 90 seconds annealing at 55°C (40°C), 90 seconds of elongation at 72°C at 15 (25) cycles. Of 112 samples, only 41 were successfully amplified. Several samples of G. klugii did not yield any PCR bands. This was similar to the results of a previous study (Spelda et al. 2011). Studies using a more degenerated primer pair had a much higher success rate in Glomeris species (see [11,27,30].
For the 28S analysis, we studied the D3-D5 fragment, using primers successfully utilized in previous studies of millipedes [31]. Each reaction involved 2 μl DNA, 2.8 μl water, 2 μl Q-solution, 10 μl Qiagen multiplex-mix, and 1.6 μl of each primer (10 pmol/μl) for a total reaction volume of 20 μl. Each PCR run included a negative control without any DNA. The PCR temperature profile was 60 seconds of denaturation at 94°C, 60 seconds annealing at 50.5°C, 60 seconds of elongation at 72°C at 35 cycles. The PCR was successful for all 14 samples.
Subsequent Sanger sequencing of purified PCR-products was outsourced and conducted with the same primers at Macrogen, Netherlands. We confirmed sequence identities with BLAST searches [32], checked our data for multi-peaks, translated sequences into amino-acids, and checked whether nucleotide substitutions were primarily found at the third codon position to rule-out pseudogenes [33]. All 41 new COI sequences (2x G. romana, 2x G. transalpina, 1x G. oropensis, 5x G. oblongoguttata, 3x G. primordialis, 1x G. larii, 27x G. klugii), as well as all 14 new 28S sequences (1x G. larii, 4x G. klugii, 1x G. oropensis, 1x G. oblongoguttata, 2x G. primordialis, 1x G. romana, 1x G. marginata, 1x G. connexa, 2x G. ligurica) were deposited in GenBank (see Table 1 for access numbers).
Phylogenetic and distance analyses
The COI analysis included all species and specimens that could be sequenced (see above). Double strand sequences were concatenated in SEQMAN and aligned by hand in Bioedit [34]. Evolutionary analyses were conducted in MEGA6 [35]. For the analysis of the COI gene, the evolutionary history was inferred by using the Maximum Likelihood method based on the Hasegawa-Kishino-Yano model [36]. The tree with the highest log likelihood (-3621.8103) is shown. The percentage of trees in which the associated taxa clustered together is shown next to the branches. Initial tree(s) for the heuristic search were obtained automatically by applying Neighbor-Join and BioNJ algorithms to a matrix of pairwise distances estimated using the Maximum Composite Likelihood (MCL) approach, and then selecting the topology with superior log likelihood value. A discrete Gamma distribution was used to model evolutionary rate differences among sites (5 categories (+G, parameter = 0.7657)). The rate variation model allowed for some sites to be evolutionarily invariable ([+I], 60.6677% sites). The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. The analysis involved 56 nucleotide sequences. Codon positions included were 1st+2nd+3rd+Noncoding. There were a total of 657 positions in the final dataset.
Aside from G. klugii, the 28S analysis included specimens of the local endemic species G. larii, G. primordialis, G. oblongoguttata, G. oropensis, G. ligurica and G. romana, as well as the most widespread Glomeris taxa (after [6]), G. marginata and G. connexa as outgroups. Budgetary constraints did not allow for a broader analysis. Successfully amplified and sequenced were G. klugii specimens from the Cinque Terre (W Italy), the Euganean Hills (E Italy), Hagen (NW Germany) and the Siebengebirge (W Germany), which cover the geographical range of the species. Double strand sequences were concatenated with Seqman (DNASTAR Inc.). Noisy ends were filled-in with 'N's. The dataset was aligned with MUSCLE [37]. The uncut 28S data set had 425 positions. Because of an unclear homology, positions 170–183 were cut before further analyses were conducted, resulting in a dataset with 411 positions.
Because the 28S dataset showed extremely little resolution on the here analyzed level (maximum distances 2.3%), COI sequences from the same specimens were added to concatenate the dataset.
For the combined dataset, the evolutionary history was inferred by using the Maximum Likelihood method the same way as in the COI analysis (see above). A discrete Gamma distribution was used to model evolutionary rate differences among sites (5 categories (+G, parameter = 0.1627)). The rate variation model allowed for some sites to be evolutionarily invariable ([+I], 38.1750% sites). The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. The analysis involved 14 nucleotide sequences. There were a total of 1068 positions in the final dataset (S1 Alignment).
For both the analysis of the COI gene and the analysis of the combined dataset, a bootstrap consensus tree inferred from 1000 replicates [38] is taken to represent the evolutionary history of the analyzed taxa.
For the distance analysis, only the COI dataset was used. Uncorrected p-distances were calculated using MEGA6 [35]. The number of base differences per site from between sequences are shown (S1 Table). The analysis involved 56 nucleotide sequences. Codon positions included were 1st+2nd+3rd+Noncoding. All ambiguous positions were removed for each sequenced pair. There were a total of 657 positions in the final dataset.
Results
The genetic distance analysis
Aside from G. larii, interspecific distances in our COI dataset vary between 6.7–16.1%, while intraspecific distances are between 0–5% (Fig 2). A clear barcoding gap is present between 5% and 6.7% (Fig 2), which rises well above 8.5% if one species (G. primordialis) is removed.
All red points in the blue area refer to Glomeris larii. Basic table see S1 Table.
G. larii, described as an endemic of the Lake Como area, shows little to no genetic distance in the COI barcoding fragment to G. klugii specimens from the same locality (0.2%, or a single basepair) and a small distance of 0.3–4.4% to the other populations of G. klugii, falling clearly inside the observed intraspecific variation of Glomeris species (<5%, see Fig 2).
G. primordialis differs 6.7–9% from G. klugii. The three sequenced specimens of G. primordialis show an intraspecific variation of 0.8–2.6%.
G. oblongoguttata differs from its closest relatives, G. klugii and G. transalpina, by 9.1–10.4% and 10.8–11.4% respectively. Intraspecific variation of the five analyzed G. oblongoguttata specimens, all from the same population, is low, 0–0.3%.
G. oropensis clearly differs from the two nearby populations of G. transalpina by 9.9%, from G. primordialis, which occurs almost in sympatry, by 9.4–10.1%, and by 9.3–11.7% from the widespread G. klugii.
G. transalpina shows its lowest genetic distances to G. klugii, with genetic distances of 8.4–9.4%. Both sequenced G. transalpina populations show identical haplotypes (0% distance).
G. romana shows large genetic distances to all other analyzed species, varying between 13.1–15.4%. The two analyzed subspecies, G. romana romana Verhoeff, 1900 and G. romana marinensis Verhoeff, 1928 differ at a single basepair.
Distances between the species G. marginata, G. connexa, G. apuana and G. ligurica are high, always >10%, while their intraspecific distances are between 0–2.0%
Phylogeny of the Glomeris species
The phylogeny of the analyzed Glomeris species was resolved congruently in both the COI (Fig 3) and the combined dataset (Fig 4). Because the COI dataset included more taxa, it is here used for further discussions. The G. klugii species-group excluding G. romana is recovered as monophyletic with strong statistical support (98%). Inside the G. klugii species-group, no species grouping receives statistical support. A basal trichotomy includes a grouping uniting the high-alpine endemics G. transalpina and G. oropensis, the Bergamasque endemic (Fig 1) G. oblongoguttata, and a grouping uniting G. primordialis and G. klugii, with G. larii nested deeply inside G. klugii. The monophyly of each species (except G. larii) is statistically strongly supported (100%). The grouping of G. larii inside G. klugii is statistically strongly supported (100%).
Numbers represent nodal support values from the maximum likelihood (1000 bootstrap replicates) analysis. Values >65% not shown. Sequences from GenBank marked with single asterisk after name. Species and subspecies surrounded by boxes. G. klugii species-group highlighted by brown background.
Numbers represent nodal support values from the maximum likelihood (1000 bootstrap replicates) analysis. Values <65% not shown. Sequences from GenBank marked with single asterisk after name. Species and subspecies surrounded by boxes. G. klugii species-group highlighted by brown background.
The monophyly of the other analyzed Glomeris species, G. romana, G. marginata, G. ligurica, G. apuana and G. connexa, is strongly supported (all 100%). The only supported grouping (Fig 3) is the G. connexa species-group (88%), including G. ligurica, a western Italian endemic (Fig 1: c), in a basal position opposed to the weakly-supported (69%) sister-taxa G. connexa and G. apuana.
Color morphs and geographical variation in G. klugii
The characteristically colored subspecies G. klugii porphyrea from the Balkan Peninsula is nested within G. klugii (Fig 3) and shows genetic distances to the other populations of the species in the COI barcoding fragment of 1.9–5.0%. These Balkan specimens do not form a monophylum with other similarly colored specimens from Italy (Fig 2).
Inside G. klugii, the resolution of the COI (Fig 3) and combined (Fig 4) analyses is limited. In both analyses the specimens from Liguria, collected around the Cinque Terre (Fig 1: 3), are the most-basal specimens, opposed, but statistically not supported, to the remaining specimens. The unusually colored population from Cremona in the Po Valley (Fig 1: 16) is in a basal trichotomy with the specimens from the Cinque Terre and all remaining G. klugii. The next split /includes two specimens (100% bootstrap support) from disjunct populations, one a red-black specimen from the west coast of the Lago di Garda (Fig 1: 14), the second a normal colored specimen from Chieti, located in the southern margin of the species range (Fig 1: 15). The following split (99% statistical support) includes the specimens of the subspecies G. klugii porphyrea from two sites in Croatia at the eastern edge of the range of the species. The next split opposes two specimens from Southern Tyrol, Italy (Fig 1: 11) and the German and remaining Italian G. klugii (Fig 4). A further split then separates all German specimens from the remaining specimens from Italy (Fig 3). All German specimens group together with moderate statistical support (86%). All seven sampled German populations, spanning the northern and NE margin of the species range, show genetic distances of 0–0.6% from one another, with up to 0.3% observed in the same population (Hagen, four specimens sampled), with the same haplotype present in geographically close populations.
For the remaining Italian populations, the resolution is limited (Fig 4). The genetic distances in the group are moderate, 0–2.5%. Two specimens from the Euganean Hills (Fig 1: 8) are basally located, while a third specimen from the same population groups with two specimens from Riva, which is on the northern coast of the Lago di Garda (Fig 1: 9). The specimens from Malgrate and Bocca, two sites near Lecco on the Lago di Como (Fig 1: 10), group together with the specimen of G. larii from the same area (Fig 4).
Discussion
Intraspecific differences in G. klugii
The observed 5% intraspecific distances within G. klugii are high in comparison to other Glomeris species, which usually fall below 2.5% [26] and 3.5% [27]. However, all previous studies included less than 10 specimens per species and never covered a large geographic area. For example, barcoding studies of German ground beetles (Carabidae) showed intraspecific distances of up to 3.8% in the COI gene, weakly overlapping with interspecific distances which showed a minimum 3% p-distance [39]. Other Europe-wide scale studies focusing on rose beetles found intraspecific distances of up to 9% [40].
The present study explicitly included G. klugii specimens from the extremes of the species’ northern, eastern and southern range. If we included only German populations in our analysis, intraspecific variation would be at up to 0.5% only. The highest intraspecific distances were observed in the Italian G. klugii. In Italy, no geographic pattern of the distribution of related haplotypes could be detected. For example, a specimen (CC023) from Salo is more closely related to a specimen from Chieti (Fig 3) than to a nearby population from Riva (Fig 1). Also, different, unrelated haplotypes exist in the same Italian populations of G. klugii, as is evident by the three analyzed specimens from the Euganean Hills (Fig 1). Two of them are close to one another, while a third specimen groups more closely to specimens from Riva (Fig 3).
No phylogenetic pattern could be discerned from the individual coloration pattern of the specimens (Fig 5A–5F). Even the same population often shows incredible individual variation in the color patterns (Fig 5A), but shows identical or almost identical haplotypes. The basic color of G. klugii is yellow with varying amounts of black freckles (e.g. [19]), while the Balkan subspecies G. klugii porphyrea is characterized by a red basic color. However, both yellow and red colored specimens of G. klugii porphyrea were collected from the same population in the Balkans (Fig 5E and 5F). The sequenced typically red colored specimens (Fig 5E) of G. klugii porphyrea, which closely resemble the holotype of G.klugii [20], clearly group within a clade of yellow colored Italian and German G. klugii (Fig 3). Characteristically red-black colored specimens also occur in Italy, such as in the population from Salo (Fig 6A), or even a red 'conspersa'-like from Cremona (Fig 6B), but do not group together (Fig 3). These observations confirm an earlier allozyme study [16] that found that specimens of the 'conspersa' color morph (Fig 5A right, 5B) are more closely related to specimens of the distinct 'undulata' color morph from the same locality (Fig 5A left) than to other 'conspersa' specimens from a different locality. Breeding experiments might reveal the hereditary basis of the development of different color patterns of G. klugii, or perhaps even show that the color patterns vary during different developmental stages of the same individual.
(A) Living, rolled-up specimens from Tübingen, SW Germany, left the 'undulata' morph, right the 'conspersa' morph, photograph courtesy of JP Oeyen; (B) Living, walking specimen from Tübingen, SW Germany, the 'conspersa' morph, photograph courtesy of JP Oeyen; (C) Specimen from Malgrate at the Lago di Como (ZFMK MYR4559); (D) Specimen from Malgrate at the Lago di Como (ZFMK MYR4767); (E) Specimen of G. klugii porphyrea from Croatia (ZFMK MYR734); (F) Population of G. klugii porphyrea from Croatia, photograph courtesy of P. Kautt.
(A) Living, walking specimen from Austria, showing the red 'undulata' morph, photograph courtesy of JP Oeyen; (B) Living, walking specimen from Cremona, showing the rare red 'conspersa' morph, photograph courtesy of JP Oeyen.
More Italian specimens of G. klugii, especially from currently unsampled areas such as the Piedmont or the Sea Alps, should be included in further studies to gain further insights in the intriguing haplotype diversity of G. klugii in Italy.
Genetic distances of the COI barcoding fragment between the different Glomeris taxa
Aside from the distances observed between G. larii (Fig 7A) and G. klugii (0.2–4.4%), which fall clearly inside the intraspecific variation observed in G. klugii (0–5%, Fig 2), all other Glomeris species differ from one another by 6.9–15.9%. This degree of interspecific p-distances in the COI gene falls well within those observed in German ground beetles (3–20% [39]), but significantly lower than the interspecific distances observed in central European centipedes, which are between 16–19% in Geophilus [30] and 13.9–22% in Cryptops [41].
(A) Glomeris larii Verhoeff, 1921 (ZFM MYR4562); (B) Glomeris primordialis Verhoeff, 1930 (ZFMK MYR4741); (C) Glomeris oblongoguttata Verhoeff, 1894 (ZFMK MYR4570) from Pisogne; (D) G. oblongoguttata, living specimen from Oltre il Colle, photograph courtesy of JP Oeyen; (E) Glomeris oropensis Verhoeff, 1936, adult specimen (ZFMK MYR4534); (F) G. oropensis, juvenile.
Closest to G. klugii, and falling inside the barcode gap of 5–8.5% otherwise observed between Glomeris species, is G. primordialis. G. primordialis is only known from two sites in the Piedmont area north of Biella [42,43] and has had no specimen records for 75 years. The sequenced specimens, while showing different haplotypes, all come from the same area (Table 1) and can be considered topotypes. Given the unique color pattern of G. primordialis (Fig 7B) and the genetic distance to G. klugii, a strong argument can be made to maintain the current classification as separate species. That said, more genetic studies involving Piedmont populations of G. klugii, which were not successfully included in the current study (Fig 1), should be conducted in the future.
The identity of G. oblongoguttata, a species endemic to the Bergamasque area with a color pattern comparable to the red morphs of G. klugii (Fig 7C and 7D), is confirmed as a separate species. The separation of G. transalpina and G. klugii was already supported by allozyme data [16] and is confirmed by our analysis.
Morphologically, G. oropensis (Fig 7E), a high-altitude endemic currently only known from the mountain above Oropa [43], closely resembles G. transalpina (Fig 8A) as the species only differ in their juvenile coloration (Fig 7F). G. oropensis clearly differs genetically from the two nearby populations of G. transalpina, as well as from populations of G. primordialis and G. klugii. The observed large genetic distances confirm that G. oropensis is currently best viewed as a separate species, microendemic to the Biellese Alps.
(A) Glomeris transalpina Koch, 1836, living specimen from the Riederalm, photograph courtesy of JP Oeyen (ZFMK MYR2609); (B) Glomeris romana marinensis Verhoeff, 1928 (ZFMK MYR797); (C) Glomeris ligurica Latzel, 1884, living specimens from a population close to Melogno, photograph courtesy of P. Kautt.
Our analysis also confirms earlier allozyme studies [17] that G. transalpina (Fig 8A) is a species clearly separate from, but close to, G. klugii. Interestingly, the sequenced specimens of G. transalpina originating from two separate high-altitude populations (Riederfurka and Simplon, Fig 1) show identical haplotypes, hinting to a recent dispersal (from an ice age refugium?) of the species to both localities.
G. romana and G. klugii are clearly separate genetically despite their morphological similarity (both species show a unique color pattern produced by varying densities of black freckles) and geographical exclusiveness (G. romana occurs further south than G. klugii, both species apparently never occur in direct sympatry [6,14]). This confirms again results of earlier allozyme studies [6,8]. The present study provides no molecular phylogenetic support for the melanistic subspecies G. romana marinensis Verhoeff (Fig 8B) described from San Marino (which shares the absence of any main stria on the thoracic shield with G. klugii).
All other analyzed species, G. marginata, G. connexa, G. apuana and G. ligurica differ clearly from one another and from the species of the G. klugii species group. Intraspecific distances of all species aside from G. klugii are much lower than in G. klugii, 0–2.5% vs. 0–5%. One possible explanation for this difference might lie in the fact that this study only included a few samples for the other species and that these samples do not cover the geographical extremes of their specific range [6]. This is a contrast to G. klugii, a focus of this study, for which the specimens were collected from 18 different localities representing the entire geographic range of the species (Fig 1).
G. larii, a new synonym of G. klugii
G. larii was described by Verhoeff as a species similar to, but separate from G. klugii [44]. The main argument for the species status of G. larii, despite its great similarity to G. klugii in all morphological aspects, was the color pattern of the anal shield (Fig 7A). Furthermore, G. klugii and G. larii were supposed to be geographically exclusive, with only G. larii occurring on the eastern coast of the Lago di Como around Lecco. G. larii was only collected twice; the type series and a second batch of specimens a few years later [45]. Aside from these two occasions, there are no further records of G. larii and the species is only mentioned in faunal lists [6,9,10].
Here, we document the color variability of G. klugii (Fig 5A–5F), which encompasses the color pattern observed in G. larii (Fig 7A). G. klugii also regularly occurs around Lecco (own observations, collections of the Bergamo Museum). Our own sample from a locality very close to Lecco includes specimens with the color pattern of G. larii (Fig 7A) and specimens with the color pattern more associated with G. klugii (Fig 5C and 5D). These specimens all group together, well within G. klugii, in the COI and combined analyses (Figs 3 and 4).
The genetic distances between G. larii and G. klugii fall well within the intraspecific variation observed in G. klugii and other Glomeris species (Fig 2). Based on the combination of (1) no geographical separation (direct sympatry, (Fig 1)), (2) no reliable differences in their morphology and coloration (Figs 5C, 5D and 7A), and (3) genetic distances of both mitochondrial and nuclear genes well within the intraspecific distances observed in other Glomeris species (Fig 2), the name G. larii is best viewed as being synonymous with G. klugii.
The status of G. genuensis
Our attempts to separate genetically specimens of G. genuensis from G. ligurica failed; all sequenced specimens grouped with previously sequenced G. ligurica. Even specimens collected at the type locality of G. genuensis, both showing the 'freckled' look associated with G. genuensis and the connexa-look associated with G. ligurica (similar to Fig 8C) show identical haplotypes. Authors who recollected G. genuensis always describe key morphological characters, the striae and the anal shield coloration, as identical to those known from G. ligurica. One population of G. genuensis was even described as a variety aptly named 'pseudoligurica' [42] because of the great similarity to specimens of G. ligurica.
All recorded specimens of G. genuensis and G. ligurica [12,13,44,46,47] show identical striated patterns on their thoracic shield, an important taxonomic character. A direct comparison of their colors indicates that both species also share similar unique characteristics on their anal shield. In fact, the main difference between G. genuensis and G. ligurica seems to be in the more freckled color pattern of the former, This difference is similar to the characters used to justify the distinctiveness of G. conspersa and G. undulata, now both known as color variants of G. klugii [14,16]. Currently, G. genuensis and G. ligurica appear to be morphologically and genetically inseparable. However, further studies involving more sample localities and specimens of Ligurian Glomeris are necessary to evaluate the status of G. genuensis and formally synonymise the name under G. ligurica.
Phylogeny of the Glomeris species
Our choice of molecular markers is not the best to resolve the phylogeny of the different species of Glomeris. The chosen 28S rDNA fragment [31] was too conservative for our dataset, providing little to no resolution. A different 28S rDNA fragment might work better for genus-level studies of the Glomerida, but such a gene has not been explored so far in the order, or even in Diplopoda in general. The mitochondrial COI gene provided surprisingly good resolution in our phylogeny on the species-level (Fig 3), as well as for the relatively closely related G. klugii and the G. connexa species-groups. However, the relationships between the different Glomeris species is still little resolved. As a mitochondrial marker, which is only inherited from the maternal line, interpretation of the COI results has to be taken with care. Mitochondrial markers are prone to show high variability because of the presence of inherited endosymbionts [48]. However, Wolbachia infections have not yet been recorded in the Diplopoda. Additional risks of interpreting the results of studies depending on mitochondrial molecular markers are gene transfers (e.g. [49–51]) as well as sex-dependent dispersal strategies, in which males are much more mobile then females [52]. While both events have not yet been recorded in the Diplopoda, the still small number of genetic studies in millipedes [2], in combination with our generally limited knowledge of the ecology and life history of pill millipedes [14], demands a careful interpretation of the obtained results.
Conclusion
G. klugii exhibits large intraspecific distances of up to 5% across its area of distribution, especially among Italian populations. Individuals of G. klugii belonging to the same color morphs do not form phylogenetically related groups. A clear geographic distribution pattern of the different Italian haplotypes cannot be discerned in our dataset. Only distinctly related populations of G. klugii occur in close proximity to one another.
G. larii is synonymous to G. klugii. The other studied local Italian endemics, G. primordialis and G. oropensis both microendemics in the Biellese Alps, as well as G. oblongoguttata endemic to the Bergamasque Alps, are, despite their morphological similarity to G. klugii, supported as separate species. The barcoding COI fragment is a useful tool to identify difficult to determine pill millipede species.
The results of our study show how important integrative taxonomic studies are to distinguish between 'endemic' species that exist only as names in the literature ([16,27] G. larii this study) and 'real' local endemic species ([11,17] this study) before any biogeographical interpretations are undertaken. Our analyses confirm that northern Italy, especially the Biellese and Bergamasque Alps, is indeed unusually rich in local endemic pill millipede species. The evolutionary processes that led to such local endemism remain unknown. A correct aging of the splits between the different Glomeris taxa might elucidate whether these local endemic species evolved relatively recently, i.e. during the last ice ages, or whether far older, geological processes might have shaped the current center of endemism patterns in northern Italy. Future research on these evolutionary processes that will include local endemics that are not pill millipedes is planned.
Supporting Information
S1 Alignment. Fasta alignment combined COI & 28S dataset.
https://doi.org/10.1371/journal.pone.0162284.s001
(FAS)
Acknowledgments
Marco Valle (Museo Civico di Scienze Naturali "E. Caffi", Bergamo), Henrik Enghoff (Copenhagen), and Peter Kautt (Tübingen) generously provided us with numerous specimens. Many thanks to Axel Schönhofer (Mainz), Hans Reip (Jena), Jörg Spelda (Munich), Peter Kautt (Tübingen) and Fernando Scarlassara (Italy) for not only providing us with specimens, but also for their advice and for reading a previous version of the manuscript. Collection permits for natural protection areas were kindly provided by the following people: H. Mölleken (Ressort Umweltschutz, Stadt Wuppertal), S. Müller (Umweltamt, Stadt Hagen) W. Wasch (Personal- und Organisationsamt, Bundesstadt Bonn), H. Zimmermann & Fr. Makiolczyk (Amt für Natur- und Landschaftsschutz, Rhein-Sieg Kreis), M. Ehling (Struktur- und Genehmigungsdirektion Nord, Rhineland-Palatine), and U. Schmitz (Amt für Umweltschutz, Rhein Kreis Neuss). Our appreciation also goes to Jan Philip Oeyen (Bonn) for sharing his photographs and producing the diagram shown in Fig 2 and to Holly Wesener for her indispensible editorial assistance. We thank the two reviewers Ivan H. Tuf and George Sangster as well as the academic editor Bi-Song Yue for their comments which improved the quality of our article.
The here presented work includes results of a Bachelor of Science (BSc) thesis conducted by CC at the Rheinische Friedrich-Wilhelm University of Bonn in the Biology program under the supervision of TW and Prof. Dr. J.W. Wägele. The lab work at the ZFMK would not have been possible without the help of the lab manager, Claudia Etzbauer.
Author Contributions
- Conceptualization: TW.
- Data curation: TW CC.
- Formal analysis: TW CC.
- Funding acquisition: TW.
- Investigation: TW CC.
- Methodology: TW.
- Project administration: TW.
- Resources: TW CC.
- Software: TW.
- Supervision: TW.
- Validation: CC.
- Visualization: TW CC.
- Writing – original draft: TW.
- Writing – review & editing: TW CC.
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