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Novel integrative data for two Milnesium Doyère, 1840 (Tardigrada: Apochela) species from Central Asia
expand article infoWitold Morek, Bartłomiej Surmacz, Łukasz Michalczyk
‡ Institute of Zoology and Biomedical Research, Jagiellonian University, Kraków, Poland
Open Access

Abstract

Tardigrada are a phylum of microscopic animals inhabiting a variety of ecosystems, both aquatic and terrestrial, being recognised for their remarkable abilities to withstand tough environmental conditions. The order Apochela groups exclusively carnivorous species, with the vast majority representing the genus Milnesium Doyère, 1840. Representatives of this genus are characterised by simplified morphology, therefore possessing an extremely limited set of taxonomically meaningful morphological traits. Nevertheless, the taxonomy of Milnesium is mostly based on classical data: observations and measurements in light microscopy with the majority of descriptions lacking integrative data, most importantly DNA barcodes, but also scanning electron microscopy photographs and developmental variability analysis. Hence, re-descriptions that include novel integrative data are urgently needed. In this contribution, we provide new taxonomic data for two species described from Central Asia, Milnesium almatyense (a single population) and Milnesium reductum Tumanov, 2006 (five populations): morphometrics, DNA barcodes, SEM observations and description of developmental variability. As a result, we amend the description of both species and reveal phylogenetic relationships of those species and other sequenced congeners. The integrative data confirm the validity of the two species and include them in the growing set of Milnesium species associated with DNA sequences.

Key Words

developmental variability, DNA barcoding, integrative description, M. almatyense, M. reductum, phylogeny

Introduction

Tardigrades, commonly named as water bears, are a phylum of cosmopolitan invertebrates, inhabiting almost all environments across the world (Nelson et al. 2018). Currently around 1300 tardigrade species are recognised (Degma et al. 2009–2019) with 44 belonging to the order Apochela Schuster et al. 1980 which is characterised by a set of unique traits, i.e. a separation of the primary and secondary claw branches or the presence of peribuccal lamellae. Additionally, in contrast to parachelans, apochelans lack important morphological characters commonly utilised in their taxonomy, for example, placoids and septulum in the muscle pharynx or ornamented egg shells. The great majority of extant species (41 of 44) within the family Milnesiidae Ramazzotti, 1962, the only one in order Apochela, are classified within genus Milnesium Doyère, 1840 (37 listed in Degma et al. 2009–2019, excluding two nomina dubia: M. dujiangensis and M. tardigradum trispinosa, but with a further four taxa described later on by Kaczmarek et al. 2019, Moreno-Talamantes et al. 2019, and Surmacz et al. 2019). Due to the limited number of taxonomically meaningful traits the classification of this genus is challenging and, so far, has been based mostly on classical morphological data that were shown to be at least partially insufficient in resolving the taxonomy of these tardigrades, as pseudocryptic species have recently started to be recognised (e.g. Morek et al. 2016a; Morek et al. 2019a; Surmacz et al. 2020). Moreover, it has been demonstrated that some Milnesium species exhibit developmental variability, in which the immature life stages, i.e. hatchlings and juveniles (1st and 2nd instar, respectively; Morek et al. 2019b) may exhibit different morphologies than the adults (3rd instar onwards), for example, in the number of points on secondary branches of claws (so called claw configuration, CC; Michalczyk et al. 2012a, b), dorsal cuticle sculpturing or buccal tube morphology (Morek et al. 2016a; Morek et al. 2019b; Morek et al. in press).

So far, the tardigrade fauna of Kazakhstan and Kyrgyz Republic have not been thoroughly investigated, with only a few contributions published in the current century, focusing mostly on descriptions of new species (e.g.: Tumanov 2003, 2005, 2006, 2007; Kaczmarek et al. 2018; Zawierucha et al. 2018 or Coughlan et al. 2019). In this region, three Milnesium species were described and two of them, M. almatyense Tumanov, 2006 and M. reductum Tumanov, 2006, are characterised by rare morphological traits: the [2-3]-[2-2] CC (M. almatyense) and the lack of accessory points (M. reductum). Although the descriptions are accurate and detailed, they lack the integrative data, such as scanning electron microscope imaging, information on ontogenetic variability and, most importantly, DNA sequences. Additional phenotypic data and genetic data for these species are needed, not only to aid their identification and to test against potential synonymies, but also to pinpoint the phylogenetic positions of both taxa. The phyletic relationships of M. almatyense and M. reductum are particularly interesting because of the rare phenotypic traits mentioned above. Specifically, in the first extensive phylogeny of Milnesium in Morek and Michalczyk (2020) only two species with a [2-3]-[2-2] CC and no species without accessory points were included.

Thus, the aim of this study was to collect integrative data for two species, M. almatyense and M. reductum, based on the material collected in the proximity of their loci typici in Kazakhstan and Kyrgyz Republic, respectively. The integrative analysis of six populations of the two species provided novel morphological and developmental traits and DNA barcodes, amending the descriptions of these taxa and allowed us to identify their kin.

Materials and methods

Sampling and specimens

We analysed six samples containing M. almatyense (one sample) and M. reductum (five samples) originating from Kazakhstan and Kyrgyz Republic (detailed collection data are listed in Table 1). The samples were examined according to the protocol described by Stec et al. (2015). The extracted specimens were afterwards split into four analyses: (i) imaging and morphometry in phase-contrast light microscopy (PCM), (ii) DNA extraction and sequencing, (iii) imaging in scanning electron microscopy (SEM) and (iv) developmental variability analysis (Morek et al. 2016a). The exact numbers of specimens per population utilised for each analysis are provided in Table 1.

Table 1.

The collection details of populations analysed in this study. Analysis types: LCM – morphometry and general morphology in PCM; DNA – DNA extraction and sequencing; SEM – imaging in SEM; DEV – developmental analysis (hatching). The “?” indicates the lacking sequence.

Sample code Locality Coordinates Altitude Sample type Specimens analysed GenBank accession numbers
LCM DNA SEM DEV
KZ.003 Kazakhstan, Ile-Alatau 43°1'54.6"N, 76°36'54.36"E, 1866 m asl lichen 3 4 10 2 18S rRNA: MT509118; 28S rRNA: MT509119; ITS-2: MT509111; COI: MT511064
KG.012 Kyrgyz Republic, Tashkömür 41°22'22.2"N, 72°14'43.02"E, 726 m asl moss +lichen 5 4 5 2 18S rRNA: MT509115; 28S rRNA: MT509120; ITS-2: MT509112; COI: MT511060, MT511061
KG.013 Kyrgyz Republic, Tashkömür 41°22'22.5"N, 72°14'46.62"E, 762 m asl moss +lichen 24 4 5 0 18S rRNA: MT509115; 28S rRNA: ?; ITS2: MT509112; COI: MT511060
KG.014 Kyrgyz Republic, Tashkömür 41°22'23.22"N, 72°14'48.3"E, 784 m asl moss +lichen 4 4 0 0 18S rRNA: MT509116; 28S rRNA: MT509121; ITS-2: MT509113; COI: MT511062
KG.142 Kyrgyz Republic, Toluk 41°55'7.86"N, 73°37'49.32"E, 1520 m asl lichen 18 4 10 0 18S rRNA: MT509117; 28S rRNA: MT509122; ITS-2: MT509114; COI: MT511063
KG.147 Kyrgyz Republic, Toluk 41°55'6.42"N, 73°37'52.74"E, 1522 m asl moss +lichen 4 1 0 0 18S rRNA: MT509115; 28S rRNA: MT509123; ITS-2: MT509114; COI: MT511063

In addition to the new populations, paratypes of both species were examined: a single specimen of M. almatyense (slide No. 199) and five specimens of M. reductum (slide No. 192). The slides were loaned from the Zoological Institute of the Russian Academy of Sciences, St. Petersburg, Russia.

Microscopy, imaging and morphometry

The specimens were mounted on permanent microscope slides according to the method by Morek et al. (2016b). The measurements of buccal tube follow Michalczyk et al. (2012a, b), whereas the remaining traits were measured following Tumanov (2006). The branch height ratio is a ratio of the secondary to the primary branch height and it is expressed as a percentage. The pt is a ratio of a given structure to the length of the buccal tube (Pilato 1981), expressed as a percentage and, in the text and tables, is given in italics. The number of measured specimens follows the recommendations by Stec et al. (2016). The morphometric data were handled using the Apochela spreadsheet ver. 1.3., available from Tardigrada Register, www.tardigrada.net (Michalczyk and Kaczmarek 2013). The measurements and photographs were taken with an Olympus BX53 PCM associated with an Olympus DP74 digital camera. For deep focus structures that could not be focused in a single photo, a series of up to 24 photos were taken and then merged into one focused image using Corel Photo-Paint X8. Specimens were processed for SEM imaging according to the protocol by Stec et al. (2015) and examined under high vacuum with Versa 3D DualBeam SEM at the ATOMIN facility of the Jagiellonian University.

Developmental variability analysis

Cultures were established from live specimens and viable eggs deposited in exuviae, which were incubated at standard rearing conditions described by Kosztyła et al. (2016) with rotifers Lecane inermis Bryce, 1892, as a food source. In order to test for the potential presence of developmental variability, hatching was applied because developmental tracking (Morek et al. 2016a) failed (hatchlings died before reaching the juvenile stage). As ontogenetic variability in CC in both analysed species was detected, we further applied the analytical method described by Surmacz et al. (2020) to assess whether our morphometric dataset encompassed juveniles in addition to hatchlings and adults and to determine whether the species exhibit early or late CC change.

Moreover, the analytical method by Surmacz et al. (2020) was also applied to data available for M. berladnicorum Ciobanu et al., 2014 from the Tardigrada Register and an additional, topotypic hatchling (exhibiting the [2-2]-[2-2] CC), to test whether the species exhibits the early or late change. The data unequivocally indicated the early positive shift to [2-3]-[2-2] CC (see also Suppl. material 1).

Genotyping

Genomic DNA was extracted from four individuals from each of the six analysed populations (except for a single population of M. reductum where only one specimen was sequenced). The extraction method follows Chelex® 100 resin (Bio-Rad) protocol by Casquet et al. (2012), with modifications by Stec et al. (2015). Prior to extraction, the specimens were mounted on temporary water slides to determine their CC. Whenever possible, voucher specimens were obtained from the extraction vial (in total 13 vouchers out of 21 extractions). Four genetic markers were sequenced, three nuclear: the small ribosomal subunit (18S rRNA), large ribosomal subunit (28S rRNA), Internal Transcribed Spacer 2 (ITS-2); and one mitochondrial, Cytochrome Oxidase C subunit I (COI). The PCR protocols follow Stec et al. (2015), primers and PCR programmes with relevant references are listed in Table 2. The chromatograms were manually checked using BioEdit ver. 7.2.5 (Hall 1999). As the COI is a protein coding gene, the obtained fragments were translated into amino acids using MEGA 7 (Kumar et al. 2016) to test against pseudogenes. All sequences are deposited in GenBank (the accession numbers are listed in Table 1).

Table 2.

PCR protocols and primers references for specific protocols for amplification of the four DNA fragments sequenced in the study.

DNA fragment Primer name Primer direction Primer sequence (5’-3’) Primer source PCR programme
18S rRNA 18S_Tar_Ff1 forward AGGCGAAACCGCGAATGGCTC Stec et al. (2017) Zeller (2010)
18S_Tar_Rr1 reverse GCCGCAGGCTCCACTCCTGG
28S rRNA 28S_Eutar_F forward ACCCGCTGAACTTAAGCATAT Gąsiorek et al. (2018) Mironov et al. (2012)
28SR0990 reverse CCTTGGTCCGTGTTTCAAGAC Mironov et al. (2012)
ITS-2 ITS2_Eutar_Ff forward GCATCGATGAAGAACGCAGC Stec et al. (2018b) Stec et al. (2018b)
ITS2_Eutar_Rr reverse TCCTCCGCTTATTGATATGC
COI COI_Mil.tar_Ff forward TATTTTATTTTTGGTATTTGATGTGC Morek et al. (2019b) Morek et al. (2019b)
COI_Mil.tar_Rr reverse CCTCCCCCTGCAGGATC
jjLCO1490 forward TITCIACIAAYCAYAARGAYATTGG Astrin and Stüben (2008) Michalczyk et al. (2012ab)
jjHCO2198 reverse TAIACYTCIGGRTGICCRAARAAYCA

Phylogenetic analysis

To uncover the phylogenetic relationships of the two analysed species, the recent dataset by Morek and Michalczyk (2020) comprising 34 Milnesium populations was utilised. To this dataset, we added the 23 new sequences representing the six populations and four DNA fragments (with only a single 28S rRNA sequence for M. reductum, population KG.013, lacking). First, the nucleotide sequence of each marker was aligned using MAFFT version 7 (Katoh et al. 2002; Katoh and Toh 2008), with the default settings used for ITS-2 and COI, whereas for 18S rRNA and 28S rRNA, the Q-INS-I strategy was applied. As the outgroup, the following taxa were utilised: Mesobiotus philippinicus Mapalo et al., 2016 (18S rRNA: KX129793, 28S rRNA: KX129794, ITS-2: KX129795 and COI: KX129796; Mapalo et al. 2016), Macrobiotus hannae Nowak & Stec, 2018 (18S rRNA: MH063922, 28S rRNA: MH063924, ITS-2: MH063923 and COI: MH057764; Nowak and Stec 2018) and Paramacrobiotus lachowskae Stec et al., 2018 (18S rRNA: MF568532, 28S rRNA: MF568533, ITS-2: MF568535 and COI: MF568534; Stec et al. 2018a). In next step, the obtained alignments were visually checked in BioEdit and trimmed to 1047 (18S rRNA), 865 (28S rRNA), 648 bp (ITS-2) and 580 bp (COI). Afterwards, the four obtained alignments were concatenated in SequenceMatrix (Vaidya et al. 2011). We used the Bayesian Information Criterion in PartitionFinder version 2.1.1 (Lanfear et al. 2016) in order to find the most suitable substitution model for posterior phylogenetic analysis. As the COI is a protein coding fragment, before partitioning, the alignment was always divided into three data blocks constituting three separated codon positions. The analysis was run to test for all possible models implemented in the programme (for further, Bayesian Inference, BI). The best fit-models for six partitions in BI were: GTR+I+G for 18S rRNA, 28S rRNA, the second and the third codon positions in COI, whereas the GTR+G was indicated for ITS-2 and the first codon position in COI.

BI marginal posterior probabilities were calculated using MrBayes v3.2 (Ronquist and Huelsenbeck 2003). Random starting trees were utilised and the analysis was run for ten million generations, sampling the Markov chain every 1000 generations. An average standard deviation of split frequencies of < 0.01 was used as an indicator that the two independent analyses had converged. We used Tracer v1.3 (Rambaut et al. 2014) to ensure Markov chains had reached stationarity and to determine the correct “burn-in” for the analysis (the first 10% of generations). A consensus tree was obtained after summarising the resulting topologies and discarding the “burn-in”. In the BI consensus tree, clades recovered with posterior probability (PP) between 0.95 and 1.00 were considered well supported, those with PP between 0.90 and 0.94 were considered moderately supported and those with lower PP were considered unsupported. The consensus tree was visualised in FigTree v.1.4.3, available from http://tree.bio.ed.ac.uk/software/figtree.

As the dataset analysed herein is only slightly larger than the dataset by Morek and Michalczyk (2020), we did not perform the Maximum Likelihood analysis, as previously it resulted in weakly-supported nodes.

Results

Genetic diversity of analysed populations

Milnesium almatyense was represented by single haplotype in each gene, whereas M. reductum exhibited three 18S rRNA haplotypes, four 28S rRNA haplotypes, three ITS-2 haplotypes and four haplotypes of COI. The maximum genetic distance between the analysed populations of M. reductum was as follows: 0.2% in 18S rRNA, 0.5% in 28S rRNA, 1.3% in ITS-2 and 2.9% in COI, thus there was very little structuring within the M. reductum clade. The summary of haplotypes and the matrices of genetic distances between the populations are available in Suppl. materials 2.

Novel morphological traits

Thanks to the observations in high quality PCM and SEM, as well as morphometric measurements of multiple specimens, including sexually-immature instars (the analysis with the algorithm developed by Surmacz et al. 2020, confirmed the presence of the first three life stages), we were able to identify novel traits and update differential diagnoses for both species.

Specifically, for M. almatyense (Fig. 2A):

  1. Morphometric data missing in the original description, such as the dimensions of peribuccal and lateral papillae, anterior and posterior widths of buccal tube, II internal, I–III external and anterior claws lengths, are provided in Table 3. Morphometric ranges have expanded in the majority of traits due to the increased sample size (six in the original description and 13 herein), but mostly because of the inclusion of immature life stages, which were not present in the original description. The only discrepancy in the measurements was the standard buccal tube width both in absolute and relative dimensions (14.1–16.3 µm and 36.5–44.0 in the original description vs . 6.5–13.1 µm and 26.2–33.1 herein). The gap in absolute dimensions can be explained by the corresponding gap in body lengths between the type series and the new population analysed herein: 672–798 µm vs . 311–665 µm, respectively (Tumanov 2006 measured only larger specimens, hence the skewed body size in the original description). The gap in relative values is harder to explain, but (i) the pt index does not remove the allometric effects entirely (Bartels et al. 2011) and (ii) buccal tube width is susceptible to deformation under cover-glass pressure and even minor changes in pressure may considerably affect pt values (Morek et al. 2016b). Importantly, the susceptibility to deformation increases with the size of the buccal tube (Morek et al. 2016b), thus, given that larger specimens were present in the type series than in the new population, any differences in cover-slip pressure would magnify the effect, resulting in a discrepancy in buccal tube width. Thus, we consider the gaps in buccal tube width between the type series and the new population analysed herein as an artefact caused by differences in body size, possibly magnified by differences in cover-slip pressure.
  2. The analysis of ontogenetic variability in CC showed that hatchlings (Fig. 1A) exhibit a [2-2]-[2-2] CC (Fig. 1B), whereas juveniles and adults have a [2-3]-[2-2] CC (Fig. 2D), thus the species undergoes the early positive anterior CC change (see also Suppl. material 3).
  3. As there are recognised species characterised exclusively by a [2-2]-[2-2] CC, they need to be compared with hatchlings of M. almatyense to ensure they, indeed, represent different species. Specifically, M. katarzynaeKaczmarek et al., 2004 differs from M. almatyense by more posteriorly-inserted stylet supports (73.3–78.3 in M. katarzynae vs. 70.2–70.7 in hatchlings of M. almatyense), whereas M. koguiLondoño et al., 2015 differs from M. almatyense by relatively shorter primary branches IV (37.9–40.4 in M. kogui vs . 52.0–65.9 in hatchlings of M. almatyense).
  4. Observation of the mouth opening in SEM confirmed that the species has six peribuccal lamellae, but it also indicated that the lamellae are of unequal size, i.e . the pair of dorsal and ventral lamellae are larger than the two lateral lamellae, i.e. the configuration is 4+2 (Fig. 2C).
  5. Observations under a high quality PCM and SEM indicated that M. almatyense has a sculptured cuticle, which is visible both, in the population analysed herein (Fig. 3A, C, E, F), as well as in the analysed paratype (Fig. 3B, D). The sculpturing has a form of delicate, irregular wrinkles, similarly to M. berladnicorum and M. variefidum (Morek et al. 2016a). Pseudopores and dorsal pseudoplates are also clearly visible in the analysed paratype (Fig. 3B) and the new population (the scheme of the pseudoplate arrangement is depicted in Fig. 2B, based on the new population). Row I is not visible, whereas for rows II and III, the determination of exact shape was impossible, due to the obscured outline of the pseudoplates (numbering according to Moreno-Talamantes et al. 2019). The cuticle in hatchlings is similar, but the dorsal sculpture is more evident and regular, forming a delicate reticulation (Fig. 1C).

Similarly, for M. reductum (Fig. 4A):

  1. Morphometric data missing in the original description, such as the dimensions of peribuccal and lateral papillae, anterior and posterior widths of buccal tube, II internal, I–III external and anterior claws lengths, are provided in Table 4. Morphometric ranges have slightly expanded in the majority of traits due to the increased sample size (six in the original description and 34 herein) and mostly because of the inclusion of immature life stages, which were not recognised in the original description.
  2. The analysis of ontogenetic variability in CC showed that hatchlings exhibit a [2-2]-[2-2] CC (Fig. 5A, B), whereas juveniles and adults have a [2-3]-[3-2] CC (Fig. 5C), thus the species undergoes the early positive CC change (see also Suppl. material 4). SEM observations confirmed that the accessory points on primary branches are, indeed, lacking (Fig. 5C, D).
  3. As there are described species characterised exclusively by a [2-2]-[2-2] CC, they need to be compared with hatchlings of M. reductum. Both M. katarzynae and M. kogui differ from M. reductum by having accessory points on primary branches (clearly visible in PCM).
  4. SEM observations confirmed that the species has six peribuccal lamellae, but it also indicated that the lamellae are of unequal size, i.e . the pair of dorsal and ventral lamellae are larger than the two lateral lamellae, i.e. the configuration is 4+2 (Fig. 4B).
  5. Observations under a high quality PCM of all new populations, as well as of the analysed paratype, revealed that this species possesses pseudopores visible in the dorsal cuticle (Fig. 4D, E). A weak outline of a single pseudoplate is also visible (Fig. 4C, E). The SEM imaging further confirmed the species has s smooth dorsal cuticle, with a barely visible pseudoplate in row VIII (Fig. 4C). As only a single pseudoplate is observable both in PCM and SEM, a scheme of pseudoplate arrangement is not provided.
Figure 1. 

The morphology of Milnesium almatyense Tumanov, 2006 hatchlings, PCM. A Habitus; B Claws II, with the [2-2] CC; C Dorsal cuticle with visible sculpture in form of reticulation. All the scale bars are given in µm.

Table 3.

Measurements (in μm) and the pt values of selected morphological structures of 13 adult females of Milnesium almatyense Tumanov, 2006 from Kazakhstan, KZ.003, mounted in Hoyer’s medium. All available specimens were measured (N – number of specimens/structures measured, RANGE refers to the smallest and the largest structure amongst all measured specimens; SD – standard deviation).

Character N Range Mean SD
µm pt µm pt µm pt
Body length 13 311–665 1254–1679 502 1512 107 122
Peribuccal papillae length 9 4.9–8.2 14.6–21.1 6.4 19.3 1.0 2.1
Lateral papillae length 12 3.6–6.0 12.2–15.8 4.8 14.7 0.9 1.0
Buccal tube
Length 13 24.6–40.2 32.9 5.2
Stylet support insertion point 12 17.4–28.4 67.0–71.9 22.8 69.6 3.7 1.6
Anterior width 10 7.8–16.0 31.7–41.3 12.1 36.2 2.9 3.2
Standard width 10 6.5–13.1 26.2–33.1 10.0 29.9 2.2 2.2
Posterior width 10 6.6–13.7 26.6–35.9 10.0 30.0 2.4 2.9
Standard width/length ratio 10 26%–33% 30% 2%
Posterior/anterior width ratio 10 73%–92% 83% 7%
Claw 1 heights
External primary branch 11 10.4–16.6 39.3–49.2 13.8 42.3 2.1 2.6
External base + secondary branch 7 9.0–16.2 35.3–42.4 13.0 38.5 2.6 2.4
External branches length ratio 6 87%–98% 93% 4%
Internal primary branch 12 9.7–15.6 35.6–48.8 13.1 40.1 1.8 3.2
Internal base + secondary branch 8 8.6–15.4 34.2–39.8 12.2 37.1 2.2 2.0
Internal spur 6 3.4–5.3 11.7–13.6 4.4 12.6 0.7 0.8
Internal branches length ratio 8 89%–100% 94% 4%
Claw 2 heights
External primary branch 13 11.3–19.3 42.5–56.1 15.4 47.0 2.4 3.2
External base + secondary branch 11 9.0–16.3 36.3–44.8 13.3 40.5 2.3 2.2
External branches length ratio 11 74%–96% 86% 6%
Internal primary branch 12 10.6–17.5 41.5–50.4 14.8 44.3 2.1 2.4
Internal base + secondary branch 9 8.7–15.8 35.1–41.4 13.1 38.3 2.0 1.9
Internal spur 7 4.3–5.4 12.3–15.6 4.7 13.7 0.5 1.4
Internal branches length ratio 9 82%–92% 88% 3%
Claw 3 heights
External primary branch 12 10.9–19.6 44.0–56.1 15.8 48.8 2.4 3.3
External base + secondary branch 10 8.6–17.2 34.7–44.8 13.5 41.6 2.9 3.0
External branches length ratio 9 71%–91% 85% 7%
Internal primary branch 9 10.8–19.3 43.5–48.7 15.4 46.3 2.8 1.8
Internal base + secondary branch 8 9.6–16.6 39.0–47.3 13.9 41.3 2.5 2.6
Internal spur 9 3.8–7.2 10.8–18.2 4.7 13.7 1.0 1.9
Internal branches length ratio 6 83%–91% 87% 3%
Claw 4 heights
Anterior primary branch 7 12.9–23.6 52.0–63.4 18.5 58.5 3.7 3.4
Anterior base + secondary branch 9 9.8–19.7 39.5–50.2 15.1 45.9 3.5 3.4
Anterior branches length ratio 6 68%–85% 78% 6%
Posterior primary branch 9 13.0–23.6 52.0–65.9 19.0 59.6 3.4 4.8
Posterior base + secondary branch 8 10.1–19.7 40.7–52.3 15.4 47.5 3.1 3.6
Posterior branches length ratio 7 74%–84% 80% 3%
Figure 2. 

The morphology of Milnesium almatyense Tumanov, 2006 adults. A Habitus, PCM; B The pseudoplate arrangement based on adult specimens from population KZ.003, drawing; C SEM photograph of mouth opening; with six, unequal in size peribuccal lamellae, so-called 4+2 configuration; D SEM photograph of claws II with a [2-3] CC and visible accessory points. All the scale bars are given in µm.

Figure 3. 

The morphology of Milnesium almatyense Tumanov, 2006 adults cuticle. A Sculptured dorsal cuticle, with visible pseudoplates, specimen from KZ.003 population, PCM; B Sculptured dorsal cuticle, with visible pseudoplates, paratype, PCM; C Dorsal cuticle with visible pseudopores, the same specimens on A from KZ.003 population, PCM; D Dorsal cuticle with visible pseudopores, paratype, PCM. E SEM photograph of dorsal cuticle with visible sculpture and caudal, complex pseudoplate. F SEM photograph of details of sculpture of the dorsal cuticle. The photographs A and C, as well as B and D depict the same specimen and paratype, respectively. All the scale bars are given in µm.

Table 4.

Joined measurements (in μm) and the pt values of selected morphological structures of 34 adult females of Milnesium reductum Tumanov, 2006 from five populations from Kyrgyz Republic, KG.012; KG.013; KG.014, KG.142 and KG.147, mounted in Hoyer’s medium. Individuals were chosen to represent the entire body length range, with as equal representation of all available life stages as possible (N – number of specimens/structures measured, RANGE refers to the smallest and the largest structure amongst all measured specimens; SD – standard deviation).

Character N Range Mean SD
µm pt µm pt µm pt
Body length 31 363–730 1217–1693 555 1535 143 110
Peribuccal papillae length 30 3.7–8.8 13.3–22.1 6.4 17.8 1.3 2.1
Lateral papillae length 31 4.6–10.0 14.6–23.7 7.1 19.1 1.6 2.2
Buccal tube
Length 34 25.8–43.9 36.4 5.7
Stylet support insertion point 34 18.3–29.3 62.9–72.9 24.8 68.4 3.3 2.5
Anterior width 33 7.7–16.8 25.8–41.3 11.9 32.2 2.4 3.6
Standard width 33 5.6–16.2 20.0–39.8 9.8 26.4 2.6 4.6
Posterior width 33 6.6–17.8 22.2–43.7 11.3 30.3 2.9 4.7
Standard width/length ratio 33 20%–40% 26% 5%
Posterior/anterior width ratio 33 74%–120% 94% 11%
Claw 1 heights
External primary branch 32 14.9–24.0 48.1–60.4 19.5 54.0 2.5 3.5
External base + secondary branch 30 9.2–15.1 28.9–38.8 12.7 34.6 1.7 2.4
External branches length ratio 28 57%–72% 64% 4%
Internal primary branch 33 14.2–23.4 45.3–57.9 18.6 51.3 2.6 3.7
Internal base + secondary branch 33 8.8–14.7 24.4–37.4 12.2 33.6 1.7 3.2
Internal spur 26 4.6–8.1 13.1–20.7 6.2 16.4 0.8 2.0
Internal branches length ratio 32 53%–75% 66% 5%
Claw 2 heights
External primary branch 33 16.8–26.8 49.5–65.8 21.7 59.2 2.9 3.7
External base + secondary branch 30 9.8–16.0 33.0–40.4 13.1 36.7 2.0 1.8
External branches length ratio 29 56%–70% 62% 3%
Internal primary branch 33 15.5–26.4 48.8–64.9 20.7 57.2 3.1 3.8
Internal base + secondary branch 30 8.3–17.9 31.7–50.7 13.0 35.8 2.1 3.5
Internal spur 25 5.0–8.8 14.8–21.3 7.0 18.3 0.9 2.1
Internal branches length ratio 29 50%–87% 63% 6%
Claw 3 heights
External primary branch 33 16.0–26.9 49.8–70.1 22.0 60.5 3.1 4.7
External base + secondary branch 31 9.3–16.2 32.8–42.0 13.5 36.9 2.0 2.3
External branches length ratio 30 53%–67% 61% 4%
Internal primary branch 31 14.2–25.9 48.6–64.5 20.9 58.3 3.2 4.4
Internal base + secondary branch 28 9.3–16.0 31.7–41.2 13.1 35.8 2.1 2.3
Internal spur 26 5.3–9.5 15.0–23.3 7.1 18.6 1.1 2.1
Internal branches length ratio 26 53%–71% 62% 5%
Claw 4 heights
Anterior primary branch 34 17.4–31.3 59.6–80.9 25.7 70.8 3.9 4.7
Anterior base + secondary branch 32 9.9–20.5 31.3–50.4 14.4 39.6 2.6 3.3
Anterior spur 26 5.1–8.5 14.6–25.3 6.8 18.0 1.1 2.6
Anterior branches length ratio 32 45%–65% _ 56% 5%
Posterior primary branch 33 18.4–31.5 61.2–82.7 26.2 72.2 3.8 5.1
Posterior base + secondary branch 33 9.4–19.2 36.4–48.8 15.2 41.7 2.8 3.2
Posterior branches length ratio 32 50%–65% 58% 4%
Figure 4. 

The morphology of Milnesium reductum Tumanov, 2006 A Habitus of adult female, PCM; B SEM photograph of mouth opening; with six, unequal in size peribuccal lamellae, so-called 4+2 configuration; C SEM photograph of smooth dorsal cuticle with visible single, complex pseudoplate; D smooth dorsal cuticle, with visible single pseudoplate and faint pseudopores, specimen from KG.013 population, PCM; E smooth dorsal cuticle, with visible single pseudoplate and faint pseudopores, paratype, PCM. All the scale bars are given in µm.

Figure 5. 

The morphology of claws of Milnesium reductum Tumanov, 2006 A Photograph of hatchlings claws III with a [2-2] CC, PCM; B Photograph of hatchlings claws IV with a [2-2] CC, PCM; C SEM photograph of claws II with a [2-3] CC lacking accessory points; D SEM photograph of tip of primary branch of claws lacking accessory points. All the scale bars are given in µm.

Phylogenetic position of M. almatyense and M. reductum

Adding the two species to the dataset published by Morek and Michalczyk (2020) did not alter the overall topology of Milnesium phylogenetic tree (see figure 3 therein). Both species are embedded in “clade A” (sensu Morek and Michalczyk 2020), which is shown in Fig. 6. M. almatyense is recovered as a sister species to M. berladnicorum, both being in a polytomy with M. variefidum Morek et al. 2016a and a well-supported subclade comprising M. tardigradum Doyère, 1840 and M. pseudotardigradum Surmacz et al. 2019. On the other hand, M. reductum is represented by five closely-related populations forming a well-supported subclade that is in a sister relationship with all remaining taxa of “clade A” in Morek and Michalczyk (2020).

In the currently available dataset, the species with the closest affinity to M. almatyense is M. berladnicorum, thus we compared the genetic distances between the two species, which are as follows: 0.2% in 18S rRNA, 0.9% in 28S rRNA, 4.0% in ITS-2 and 6.9% in COI. M. reductum is indicated as the sister species for the entire clade (genetic distances between M. reductum and the remaining species of clade A are as follows: 0.5–2.4% in 18S rRNA, 3.4–5.7% in 28S rRNA, 10.2–16.3% in ITS-2 and 13.8–16.3% in COI).

Figure 6. 

Fragment of the Bayesian phylogenetic tree (“clade A”), based on the analysis of concatenated 18S rRNA + 28S rRNA + ITS-2 + COI nucleotide sequences, obtained by Morek and Michalczyk (2020) with an addition of six new populations, showing the positions of the two Milnesium species analysed in this contribution (shaded in grey): M. almatyense Tumanov, 2006 and M. reductum Tumanov, 2006. The numbers at nodes represent Posterior Probability (PP) supports, with the values in grey font indicating poor support (conventionally recognised as polytomy). The scale bar shows the number of substitutions per site. For the remaining parts of the tree, please see Morek and Michalczyk (2020).

Considering the similarity and problematic taxonomy of M. tardigradum and M. pseudotardigradum, as well as of M. almatyense, M. berladnicorum and M. variefidum, we introduce names for these two species clusters: the tardigradum complex and the almatyense complex, respectively (Fig. 6). The names are derived from the first species described within respective complexes.

Discussion

New observations under a high class PCM and SEM, as well as ontogenetic variability analysis, allowed for a substantial amendment of the original descriptions of M. almatyense and M. reductum. Furthermore, sequencing of the four standard DNA barcodes delivered a new line of evidence supporting the erection of the two species and allowed an inference of their phylogenetic positions. DNA barcodes also provide an important and useful tool for identification of both species. Currently, integrative data are available only for 14 of 41 formally-described Milnesium species (34%; Michalczyk et al. 2012a, b; Morek et al. 2016a; Jackson and Meyer 2019; Kaczmarek et al. 2019; Morek et al. 2019a; Morek et al. 2019b; Surmacz et al. 2019; Morek et al. in press; present contribution), which is a worryingly low percentage, considering the challenging taxonomy of this genus reflected by the low number of useful phenotypic traits for species differentiation and the widespread developmental variability. Moreover, within the 14 species for which integrative data are available, only in 10 species has the ontogenetic variability been analysed. This shows how much there is still to be done to stabilise Milnesium taxonomy, both in terms of re-descriptions and encouraging taxonomists to publish descriptions supported by ontogenetic variability and genetic analyses. Especially, that recent studies revealed a considerable undescribed species diversity within the genus (Morek and Michalczyk 2020) and demonstrated evidence for pseudocryptic Milnesium species (Morek et al. 2019b; Surmacz et al. 2019), the identification of which would not have been possible with the sole use of classical tools.

Although the number of species that were tested against developmental variability in CC is not too high (20 species, including the undescribed species that were analysed by Morek and Michalczyk 2020), some patterns seem to emerge. Specifically, amongst the 10 tested species with adult [3-3]-[3-3] CC, ontogenetic variability was observed only in a single species (an undescribed Milnesium sp. nov. 10 PH.014 in Morek and Michalczyk 2020). In contrast, all 10 tested species with adult CC different from [3-3]-[3-3], exhibited developmental variability in CC. Moreover, in the majority of cases (7 of 10 species; 70%) an increase in the number of secondary claw branches is observed (i.e. positive CC change). Importantly, in the vast majority of cases (9 of 10 species; 90%), the shift takes place between hatchlings and juveniles (i.e. early CC change). Given that hatchlings are most likely to be missing in the type series, taxonomists should be especially cautious when describing and analysing taxa in which adults exhibit the CC other than [3-3]-[3-3].

In contrast to the original description of M. almatyense, our analysis of the type and new material showed that the dorsal cuticle is not smooth, but covered with fine sculpturing and pseudoplates. These characteristics were missed due to the insufficient quality of the light microscope used by Tumanov (2006) and because such traits were not recognised at the time. This amendment, along with the exact same pattern of ontogenetic CC change (i.e. early positive) makes M. almatyense extremely similar to M. berladnicorum. Specifically, in the original differential diagnosis of M. berladnicorum (Ciobanu et al. 2014), the species was discriminated from M. almatyense by the sculptured cuticle and ontogenetic variability in CC was not known to occur in either species. Moreover, measurements of all morphometric traits presented herein (Table 3) and in the original description (Table 3 in Tumanov 2006) overlap with ranges for M. berladnicorum. Another trait used by Ciobanu et al. (2014) to differentiate M. berladnicorum from M. almatyense was the lack of eyes in M. almatyense. However, the absence of eyes was observed by Tumanov (2006) in specimens fixed on microscope slides, thus it is not known whether the eyes were absent or they were present in living animals, but they dissolved in acetic acid and Faure medium (D. Tumanov, pers. inf.). Importantly, the eyes were present in all living individuals in the M. almatyense population analysed herein and dissolved in Hoyer’s medium in hatchlings. Thus, it could be assumed that M. almatyense exhibits eyes as M. berladnicorum does, making this trait invalid in the differentiation of these two species. We have, however, found a single morphological trait that could potentially differentiate M. almatyense and M. berladnicorum: pseudoplate arrangement, specifically in M. almatyense pseudoplate IV is singular (Fig. 2B), but it is double in M. berladnicorum (fig. 6A in Morek et al. 2016a). However, the taxonomic value of dorsal pseudoplates in Milnesium has been recently questioned by Moreno-Talamantes et al. (2019). Thus, further studies on this trait are needed to validate its value in species differentiation. Moreover, the new genetic data, presented herein, show that M. almatyense and M. berladnicorum are a pair of a very closely-related species (Fig. 6), with genetic distances in the analysed variable markers between the two species being moderate in both ITS-2 (4.0%) and COI (6.9%). Considering that there are Milnesium species with high intraspecific genetic distances (e.g. M. tardigradum with p-distances up to 4.0% in ITS-2 and 11.4% in COI, Morek et al. 2019b or M. eurystomum Maucci, 1991 with respective maximum distances of 1.7% and 8.9%, Morek et al. in press), it is not clear whether M. almatyense and M. berladnicorum are a single or distinct species. Thus, a more detailed analysis, for example, with the use of new genetic markers, is needed to verify whether M. berladnicorum is a good species or a junior synonym of M. almatyense. If the taxonomic status of M. berladnicorum is supported, then this pair of species should be considered pseudocryptic, as the phenotypic differences are difficult to spot and are not straightforward. In other words, a confident identification of these species may require DNA barcoding.

The recent phylogenetic analysis of the genus Milnesium suggested that species generally cluster by geography (Morek and Michalczyk 2020). Thus, as both M. almatyense and M. reductum originate from the Palearctic realm (sensu Holt et al., 2013), we expected them to be placed either in clade A or clade B shown by Morek and Michalczyk (2020) and the current analysis confirmed this prediction by demonstrating that both species represent clade A (Fig. 6). Therefore, the correlation between molecular phylogeny and geographic origin of populations is maintained and strengthened. M. reductum is so far the only species characterised by the lack of accessory points, for which its phylogenetic position is known. Therefore, it is difficult to speculate, whether this trait was lost only once in Milnesium or this is yet another example of convergent evolution (as has been shown for many other traits, such as cuticular sculpturing, claw or lamellae configuration; Morek and Michalczyk 2020). However, given that generally Milnesium species seem to exhibit limited geographic ranges and since the other four known species that lack the accessory points originate from different zoogeographic realms, i.e. India (M. longiungue Tumanov, 2006) and USA (M. alabamae Wallendorf & Miller, 2009; M. swansoni Young, Chappell, Miller & Lowman, 2016 and M. zsalakoae Meyer & Hinton, 2010), it would not be surprising if this trait was lost independently in different lineages.

Acknowledgements

We are deeply grateful to Denis Tumanov (Saint-Petersburg State University, Russia) for the loan of the type material and valuable comments on the manuscript. We also thank the reviewers, Reinhardt M. Kristensen (University of Copenhagen, Denmark) and Peter Degma (Comenius University, Slovakia), for their valuable comments that improved our manuscript. The study was supported by the Polish Ministry of Science and Higher Education via the Diamond Grant programme (grant no. DI2015 016845 to WM, supervised by ŁM) and by the Jagiellonian University (grant no. DS/MND/WB/IZ/16/2018 to WM). Some of the analyses were carried out with the equipment purchased from the Sonata Bis programme of the Polish National Science Centre (grant no. 2016/22/E/NZ8/00417 to ŁM).

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