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The Hyrcanian Forests present a unique Tertiary relict ecosystem, covering the northern Elburz and Talysh Ranges (Iran, Azerbaijan), a poorly investigated, unique biodiversity hotspot with many cryptic species. Since the 1970s, two nominal species of Urodela, Hynobiidae, Batrachuperus (later: Paradactylodon) have been described: Paradactylodon persicus from northwestern and P. gorganensis from northeastern Iran. Although P. gorganensis has been involved in studies on phylogeny and development, there is little data on the phylogeography, systematics, and development of the genus throughout the Hyrcanian Forests; genome-wide resources have been entirely missing. Given the huge genome size of hynobiids, making whole genome sequencing hardly affordable, we aimed to publish the first transcriptomic resources for Paradactylodon from an embryo and a larva (9.17 Gb RNA sequences; assembled to 78,918 unigenes). We also listed 32 genes involved in vertebrate sexual development and sex determination. Photographic documentation of the development from egg sacs across several embryonal and larval stages until metamorphosis enabled, for the first time, comparison of the ontogeny with that of other hynobiids and new histological and transcriptomic insights into early gonads and timing of their differentiation. Transcriptomes from central Elburz, next-generation sequencing (NGS) libraries of archival DNA of topotypic P. persicus, and GenBank-sequences of eastern P. gorganensis allowed phylogenetic analysis with three mitochondrial genomes, supplemented by PCR-amplified mtDNA-fragments from 17 museum specimens, documenting <2% uncorrected intraspecific genetic distance. Our data suggest that these rare salamanders belong to a single species P. persicus s.l. Humankind has a great responsibility to protect this species and the unique biodiversity of the Hyrcanian Forest ecosystems.
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genes
G C A T
T A C G
G C A T
Article
Shedding Light on a Secretive Tertiary Urodelean
Relict: Hynobiid Salamanders (Paradactylodon
persicus s.l.) from Iran, Illuminated by
Phylogeographic, Developmental, and
Transcriptomic Data
Matthias Stöck 1, * , Fatemeh Fakharzadeh 2, Heiner Kuhl 1, Beata Rozenblut-Ko´scisty 3,
Sophie Leinweber 1, Riddhi Patel 4, Mehregan Ebrahimi 5,6, Sebastian Voitel 7,
Josef Friedrich Schmidtler 8, Haji Gholi Kami 9, Maria Ogielska 3and Daniel W. Förster 4
1Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Müggelseedamm 301, D-12587 Berlin,
Germany; kuhl@igb-berlin.de (H.K.); solein@pivko.de (S.L.)
2
Department of Biology, Faculty of Sciences, Shahid Chamran University of Ahvaz, Ahvaz 61357-43135, Iran;
akharzadeh2015@gmail.com
3
Department of Evolutionary Biology and Conservation of Vertebrates, Wroclaw University, Sienkiewicza 21,
50-335 Wroclaw, Poland; beata.rozenblut-koscisty@uwr.edu.pl (B.R.-K.); maria.ogielska@uwr.edu.pl (M.O.)
4
Evolutionary Genetics Department, Leibniz-Institute for Zoo and Wildlife Research, Alfred-Kowalke-Str. 17,
10315 Berlin, Germany; riddhibio27@gmail.com (R.P.); foerster@izw-berlin.de (D.W.F.)
5Department of Biology, College of Sciences, Shiraz University, Shiraz 71467-13565, Iran;
m.ebrahimi@shirazu.ac.ir
6School of Biological Sciences, Flinders University, Adelaide, SA 5001, Australia
7Independent Researcher, Spangenbergstraße 81, D-06295 Eisleben, Germany; sebastian.voitel@t-online.de
8Zoologische Staatssammlung, Münchhausenstraße 21, 81247 München, Germany; josef@schmidtler.eu
9Department of Biology, Faculty of Sciences, Golestan University, Gorgan 49136-15759, Iran;
hgkami2000@yahoo.com
*Correspondence: matthias.stoeck@igb-berlin.de; Tel.: +49-30-64-181-629
Received: 17 March 2019; Accepted: 11 April 2019; Published: 18 April 2019


Abstract:
The Hyrcanian Forests present a unique Tertiary relict ecosystem, covering the northern
Elburz and Talysh Ranges (Iran, Azerbaijan), a poorly investigated, unique biodiversity hotspot with
many cryptic species. Since the 1970s, two nominal species of Urodela, Hynobiidae, Batrachuperus (later:
Paradactylodon) have been described: Paradactylodon persicus from northwestern and P. gorganensis
from northeastern Iran. Although P. gorganensis has been involved in studies on phylogeny and
development, there is little data on the phylogeography, systematics, and development of the genus
throughout the Hyrcanian Forests; genome-wide resources have been entirely missing. Given the
huge genome size of hynobiids, making whole genome sequencing hardly aordable, we aimed to
publish the first transcriptomic resources for Paradactylodon from an embryo and a larva (9.17 Gb
RNA sequences; assembled to 78,918 unigenes). We also listed 32 genes involved in vertebrate sexual
development and sex determination. Photographic documentation of the development from egg sacs
across several embryonal and larval stages until metamorphosis enabled, for the first time, comparison
of the ontogeny with that of other hynobiids and new histological and transcriptomic insights into
early gonads and timing of their dierentiation. Transcriptomes from central Elburz, next-generation
sequencing (NGS) libraries of archival DNA of topotypic P. persicus, and GenBank-sequences of
eastern P. gorganensis allowed phylogenetic analysis with three mitochondrial genomes, supplemented
by PCR-amplified mtDNA-fragments from 17 museum specimens, documenting <2% uncorrected
intraspecific genetic distance. Our data suggest that these rare salamanders belong to a single species
P. persicus s.l. Humankind has a great responsibility to protect this species and the unique biodiversity
of the Hyrcanian Forest ecosystems.
Genes 2019,10, 306; doi:10.3390/genes10040306 www.mdpi.com/journal/genes
Genes 2019,10, 306 2 of 16
Keywords:
Urodela; Hynobiidae; phylogeography; RNAseq; genomics; gene expression; gonadal
development; histology; systematics
1. Introduction
The Caspian or Hyrcanian Forests present a unique Tertiary relict ecosystem that mostly covers
the northern and few interior chains of the Elburz Range and the Talysh Mountains (Iran, Azerbaijan)
as well as their western adjacent ranges [
1
,
2
]. Together with the Colchic broadleaf forests of Georgia,
the Hyrcanian Forests that in part even cover the southern Caspian coastal plain, constitute the
most important refugia and the last relics of primary temperate deciduous broad-leaved forests
worldwide [
3
]. They have been almost poetically called “the crib” of the Central European woods [
1
],
and while this is true regarding their function as a Pleistocene refugium for some temperate species,
its prominence
for global biodiversity and conservation may be better highlighted by their importance
and uniqueness as a Tertiary relict ecosystem. Despite biogeographic connections to the Colchis,
both regions
became geographically isolated following the uplift and folding of the Lesser Caucasus
during the Palaeocene–Miocene and subsequent volcanic uplift during the Pliocene–Quaternary [
4
,
5
].
Thus, it is not very surprising that the Hyrcanian Forests as an ancient ecosystem harbor many endemic
species, with numerous plants [
1
,
3
,
5
] and animals [
2
], and also comprise a great cryptic biodiversity [
6
]
that still remains to be discovered.
With respect to vertebrates, the Hyrcanian Forests are inhabited by some of the most western living
representatives of the ancient urodelan family Hynobiidae (only Salamandrella keyserlingii reaches more
western latitudes in northern Eurasia). These salamanders belong to the genus Paradactylodon Risch,
1984 [
7
] (previously Batrachuperus). To date, they comprise in Iran two nominal taxa, Paradactylodon
persicus and P. gorganensis (details below). Their original descriptions were not comparable as they
were based on the morphology of two dierent life stages (larvae vs. adults). Specifically, only merely
50 years ago, the Persian mountain salamander has been originally described as Batrachuperus persicus
Eiselt & Steiner, 1970 [
8
]. Due to the secretive lifestyle of the species, its description was based on five
larvae, collected near Assalem in the Talysh Mountains of the Gilan province of
northwestern Iran
.
In 1971
, J.J. and J.F. Schmidtler [
9
] collected some larvae from the type locality (“topotypic larvae”).
After their metamorphosis in captivity, a brief description of the juvenile salamanders was presented [
9
].
The second nominal Iranian hynobiid taxon is based on a large, 23 cm long adult male type
specimen, deposited in the Mus
é
um National d’Histoire Naturelle in Paris (MNHN). This salamander
was discovered in a cave at the eastern edge of the Hyrcanian corridor [
10
] and only later described as
Batrachuperus gorganensis Clergue-Gazeau and Thorn, 1979 [
11
]. Stöck ([
12
]; therein “Fig. 8”) depicted
the type and provided a flow-cytometric DNA measurement (34.77 pg), although based on GC-biased
DAPI-staining, see also [
13
], and a Giemsa-stained karyotype (2n =62), obtained from fin clips of
topotypic Paradactylodon (as Batrachuperus)gorganensis larvae (i.e., the eastern taxon).
This shows
the
genome size in the upper part of the range of large amphibian and urodelean genomes [
14
]. Therefore,
whole genome sequencing still remains a major challenge (cf. [
15
]). Stöck [
12
] also described external
larval morphological changes during the development from a total length of 40 mm until metamorphosis
(100 mm). These authors [
12
] reviewed the specific literature and provided a map with geographic
coordinates of all records published until that time ([
12
]; therein “Fig. 1”).
Ebrahimi et al. [16]
depicted,
measured and compared for the first time the egg sacs of P. gorganensis with those of other hynobiid
species, showing them to be among the largest of extant hynobiids (surpassed only by eggs sacs of
Ranodon sibiricus [
17
,
18
]). Without providing further taxonomic reasoning, several authors [
19
23
],
published additional data on the biology and distribution of the Iranian hynobiid salamanders from
Ardabil and Gilan provinces, all nomenclaturally assigned to P. persicus, although several of the
specimens were from eastern Iran (i.e., nominal gorganensis). Using skeletochronology, a recent paper
Genes 2019,10, 306 3 of 16
examined the age structure in topotypic P. gorganensis [
24
] and suggested a lifespan of 13 years for
females and 11 years for males.
Based on a complete mitochondrial genome, Zhang et al. [
25
] showed topotypic P. gorganensis to be
a ca. 40 My diverged sister taxon of P. mustersi from Afghanistan and to form a phylogenetic clade with
Ranodon sibiricus (see also [
26
]). While multiple nuclear genes in general supported the age of the clade
involving P. mustersi and R. sibiricus (40 My; [
27
]), this phylogeny did not include Iranian hynobiids
and thus could not further contribute to elucidate their intrageneric or intraspecific relationships.
In the present study, our aims were (i) the clarification of the phylogenetic relationships of the
Iranian Paradactylodon, (ii) to produce the first genome-wide resource for this rare species (beyond
few existing PCR-based sequences); and (iii) to report the first larval and sexual developmental data.
To achieve these three goals, we generated phylogenetic data from a geographically comprehensive
collection of available DNA-samples from the entire range of Iranian hynobiids, including in part
archival museum samples from scientific collections, and study their phylogeny based on three complete
mitochondrial genomes. Given the huge genome size of hynobiids that make whole genome analyses
still almost unaffordable, as the first genomic resources of Paradactylodon, we publish two transcriptomes,
based on RNAseq of a whole embryo and a larva. We use these transcriptomes to derive a list of genes
involved in sexual development and sex determination in other vertebrates.
We also
contribute to the
sparse knowledge about the ontogenetic development of the species.
All of
this will facilitate future
studies on the genomic level and provides new biological data for these poorly known salamanders.
2. Materials and Methods
2.1. Animals and Samples
For the present study, we used three (i–iii) sources of DNA-bearing materials (Table S1, Figure 1):
(i) tissue samples derived from adults and larvae deposited in museum collections, namely the Natural
History Museum of Tehran, Iran (MMTT, Muze-ye Melli-ye Tarikh-i Tabi’i, Tehran), the Bavarian State
Collection of Zoology, Munich, Germany (ZSM), and the Senckenberg Collections of Dresden, Germany
(MTKD); (ii) mixed tissues of an entire embryo in developmental stage 46 (see below),
as staged
in
Onychodactylus japonicus by Iwasawa & Kera [
28
] or 36–37, as staged in Hynobius nigrescens by Iwasawa
& Yamashita [
29
], collected on 24 April 2015 from egg sacs, found at locality 4 (Figure 1) and stored
in the field in RNAlater; (iii) multiple tissues from a sibling larva kept in an aquarium and prepared,
when reaching a total length of 53 mm at 38 days after hatching, anesthetized by immersion in tricaine
methanesulfonate (MS 222; Sigma-Aldrich), transferred into RNAlater and stored at 80 C (Table 1).
This larva presented stage 60 according to Reference [
28
], stage 57 according to Reference [
29
], or stage
XII of Vassilieva & Smirnov [30].
Figure 1.
Map with sampling localities.
1
–Gilan Province, Talysh-mountains, 12 km S Assalem,
700 m a.s.l.;
2
–Mazandaran Prov., SE of Chalous city, Lashkenar village, valley of Zereshkdarreh;
3
–Yeilagh-e-Sarasi, ca. 45 km SE Khalkhal, Delmadeh (Daylamdeh) village;
4
–Mazandaran Province,
near Veysar village;
5
–Iran, Mazandaran Province, Veysar village, Noshahr City, Zaresk-Dareh;
6
–Shirabad Cave, 5 km SE (by air) of Shirabad, 60 km E (by air) of Gorgan. Locality-IDs as in Table S1.
Genes 2019,10, 306 4 of 16
2.2. Gonadal Gross Morphology and Histology
Three selected larvae (Table 1) were anesthetized by immersion in tricaine methanesulfonate
(
MS 222
; Sigma–Aldrich). Whole larvae, with opened peritoneum, were fixed in Bouin’s solution
(24 h) and subsequently rinsed several rounds in 70% ethanol. Gonadal anatomy was inspected
after removing the digestive tract. Histological sections were prepared and analyses were performed
according to established protocols [
31
,
32
]. Using Stemi SV11 (Zeiss, Germany) microscope and camera,
separated gonads were photographed, embedded in paraplast, sectioned into 7
µ
m longitudinal slices,
stained with Mallory’s trichrome, and examined using a Zeiss Axioskop 20 microscope. Images were
acquired by a cooled Carl Zeiss AxioCam HRc CCD camera.
2.3. DNA Extraction of Topotypic Samples
We included topotypic tissue from larvae collected by one of us (JFS) on June, 1, 1970 (see
Reference [
9
]), from the type locality of Paradactylodon persicus [
8
] and stored in ethanol, first in a
private collection that was meanwhile transferred to ZSM. After unsuccessful experiments in 2004,
we extracted
DNA from these archival samples using the Qiagen DNeasy Blood & Tissue kit (Qiagen,
Hilden, Germany) with modifications, namely including an overnight lysis and 15 min incubation at
37
C during the elution, in dedicated archival DNA facilities (cf. [
33
]). For all other tissue samples,
extraction followed the manufacturer’s protocols.
2.4. Primers Developed for PCR-Amplification of mtDNA-Fragments from Archival Samples
For the majority of the individuals, almost the entire mitochondrial cytochrome bwas amplified
with the newly developed primer pair PgorgCytb_F1/PgorgCytb_R2 (Supplementary Materials
Table S2
), yielding a ~940 bp product, using the PCR protocol 96
C, 2 min, initial denaturation, a cycle
of 38
×
including (94
C, 2 min, denaturation; 53.5
C, 1 min, annealing; 72
C, 1.5 min, extension)
and final extension at 72
C for 5 min. For two shorter fragments from archival samples, two primer
pairs (PgorgCytb_F1/PgorgCytb_R4 and PgorgCytb_F4/PgorgCytb_R2; Table S2), amplifying two
overlapping pieces, together also spanning the entire cytochrome bsequence, were amplified with
a slightly varying protocol: 96
C, 2 min, initial denaturation, 50
×
cycle including (94
C, 2 min,
denaturation; 53.5
C, 1 min, annealing; 72
C, 1 min, extension) and final extension at 72
C for 5 min.
Due to DNA damage, in four topotypic archival DNA samples (ZSM1-ZSM) even shorter fragments
of cytochrome bhad to be amplified, using primers PgorgCytbF2_short/PgorgCytb_R1 under the
same conditions.
Samples from good-quality DNA were amplified in a total reaction volume of 25 µL comprising
15.4
µ
L ddH
2
O, 2.5
µ
L of 10
×
Top Taq PCR buer (Qiagen), 0.47
µ
L of dNTPs (10 mM each nucleotide),
1.25 µL of each primer, 0.13 µL of Top Taq Polymerase (Qiagen) and 3 µL of DNA (10–30 ng/µL).
For archival samples, the total reaction volume of 20
µ
L comprised 11.4
µ
L ddH
2
O, 4
µ
L of 5
×
Phusion PCR buer, 0.4
µ
L of dNTPs (10 mM each nucleotide), 1
µ
L of each primer, 0.2
µ
L (
2 U/rxn.
)
of Phusion Hot Start II High-Fidelity DNA Polymerase (Thermo Fisher Scientific) and
2µL
of DNA
(
10–30 ng/µL
). In all cases, PCR success was tested on 1.5% agarose gels using 4
µ
L of product;
the remaining
volume was used for Sanger sequencing in one or both directions, depending on
product size.
2.5. Sequencing and Reconstruction of a Complete mtDNA-Genome of Topotypic Paradactylodon persicus
Of one topotypic DNA archival sample (ZSM 1, loc. 1), of which small fragments (ca. 140 bp) of
mtDNA had been successfully PCR-amplified as described above, we constructed two libraries from two
independent DNA extracts, following [
34
], incorporating 8-nt barcodes into both adapters. Libraries
were then sequenced on the Illumina NextSeq 500 (Illumina Inc.)., using a mid-output 150-cycle kit at the
Berlin Center for Genomics in Biodiversity Research (BeGenDiv). Paired end-reads were demultiplexed
with bcl2fastq v2.17.1.14 (Illumina Inc.). CUTADAPT v1.3 [
35
] was used to remove Illumina adapters,
Genes 2019,10, 306 5 of 16
Trimmomatic [
36
] for quality-trimming in a sliding window approach, and sequences were merged
(Flash v1.2.; [
37
]). Sequences were aligned to the reference mitogenome (P. gorganensis, GenBank
accession no. NC_008091.1 [
25
]) using BWA v0.7.10 [
38
], and duplicate sequences were removed using
MarkDupsByStartEnd.jar (https://github.com/dariober/Java-cafe/tree/master/MarkDupsByStartEnd).
The consensus sequence was then generated based on a coverage threshold of
3
×
. A second, almost
complete mitochondrial genome (16,057 bp) was assembled using the same bioinformatics pipeline
from the larval transcriptome, obtained at locality 4.
2.6. Phylogenetic Analyses
Mitochondrial DNAs of P. persicus from the type locality near Assalem (loc. 1) and from central
Elburz (loc. 4) were aligned with that of topotypic P. gorganensis (GenBank NC_008091.1) from eastern
Elburz (loc. 6); P. mustersi (GenBank DQ333821.1) and Ranodon sibiricus (GenBank NC_004021.1) were
used as outgroups. To infer the best fitting model of sequence evolution (GTR+G, gamma-shaped: 0.197;
Akaike criterion), we used SMS [
39
]. To illuminate the intraspecific phylogeny of Iranian Paradactylodon,
we ran PhyML [
40
] with this inferred substitution model and 100 bootstrap pseudoreplicates (Figure 2a).
In addition, to better evaluate and depict the relationships and total dierences between the three
mitochondrial genomes from Iran, we used PopART (Population Analysis with Reticulate Trees;
http://popart.otago.ac.nz/index.shtml), an open source population genetics software, developed by the
Allan Wilson Centre Imaging Evolution Initiative. We removed sites with missing data that could not
be obtained from archival specimens and created ancestral parsimony-based networks. To analyze
the mtDNA-phylogeography throughout the range (locs. 1–6), we used an alignment of up to 865 bp
of the mitochondrial cytochrome b, that was supplemented with all available fragments obtained by
PCRs as described from the archival specimens and analyzed them with the same software and under
the maximum likelihood parameters as above and added a parsimony-based network based on only
119 bp, available from all samples.
Figure 2.
Phylogeny of Central Asian and Iranian hynobiid salamanders and network of Iranian
Paradactylodon. (
a
) Maximum-likelihood phylogeny withbranch support from 100 bootstrap pseudoreplicates
Genes 2019,10, 306 6 of 16
using PhyML in % based on almost complete mitochondrial genomes (
16,300 bp)
, (small arrows in
a
refer to the corresponding sample names in
b
; (
b
) Parsimony-based network of the three mtDNA
genomes (16,140 bp). (
c
) Maximum likelihood phylogeny based on 865 bp of the cytochrome b
fragments, parameters as in
a
; (
d
) Parsimony-based network of the 119 bp of short fragments obtained
by PCR of archival samples. For sample numbers and localities: Figure 1and Table S1. Note that
samples labeled ZSM1-ZSM4 all belong to larvae, jointly archived under collection label ZSM 821/2006
(Table S1); bootstrap support values are only shown when >65. According to the present paper, all
ingroup-sequences belong to a single species, Paradactylodon persicus sensu lato (=s.l.,
see Discussion
).
2.7. Sequencing of Two Transcriptomes, Functional Annotation, and Classification
RNA was extracted using a standard Trizol protocol from mixed tissues from the whole embryo
(after removing the yolk sac) and from a larva (Table 1), using single organs (liver, eye, brain, heart,
muscle, and gonad). Larval organs’ RNAs were adjusted to equal concentrations and pooled before
RNAseq. Complementary DNA (cDNA) was synthesized and sequenced by BGI (BGI-Hongkong Co.,
Ltd., China), using the Hiseq4000 sequencing system (Illumina, San Diego, CA, USA). Reads were
assembled using Trinity [
41
]; Tgicl [
42
] was used to assemble transcripts to unigenes. For annotation,
BLASTx (v2.2.23; [
43
]) was used to align unigenes to five protein databases (NT, NR, COG, KEGG,
and SwissProt), and Blast2GO [44] to obtain GO annotations.
2.8. Genes Involved in Sex Determination or Sex Dierentiation
We aimed at retrieving 37 genes potentially involved in male or female sex determination and sexual
dierentiation (list by M. Schartl, pers. comm., modified for Urodela). Template protein sequences from
the anuran model species Xenopus tropicalis or X. laevis were obtained from Xenbase [
45
] and aligned with
BLAT [
46
] (
t=dnax
q=prot) against P. persicus transcripts, assembled from
RNAseq data
. Matching
transcripts were extracted, aligned against the NR database using NCBI blastx [
43
],
and individually
inspected to identify and remove paralogs or unspecific matches. Transcripts were considered
homologous, if the vast majority of the top scoring BLASTx hits were related to the gene of interest.
Genes from the list that were not found in the transcriptome assemblies, were aligned with
the raw RNAseq read data, using a local BLAST approach (45,774,124 sequences; 4,577,412,400 total
nucleotides). Here, available template RNA or protein sequences were used as queries for tblastn
or tblastx approaches, preferentially of Urodela (if available) or Anura. Reads with matches were
extracted and assembled by the IDBA transcriptome assembler [
47
]. Assembled transcripts were
checked by NCBI BLASTx as above. If no transcripts for the gene of interest could be successfully
assembled, the extracted reads were first aligned with the P. persicus transcriptome assembly (BLAT
default parameters) to sort out paralogous gene family matches. Subsequently, reads not already
matching other Paradactylodon paralogs were submitted to NCBI BLASTx and manual inspection to
reveal low-expression-level transcripts.
3. Results
3.1. Transcriptomes
In total, we have generated approximately 9.17 Gb bases of RNA sequences by Illumina Hiseq.
Assembly of all samples yielded 78,918 unigenes, with a total length of 77,071,006 bp, an average
length of 976 bp, an N50 value of 2019 bp (Figure S1), and a GC content of 46.49%. Raw reads from
both transcriptomes were deposited in the NCBI Sequence Read Archives (SRA, http://www.ncbi.nlm.
nih.gov/Traces/sra/; SUB3150164, SUB3166015) under the BioSample accession number PRJNA415277.
Annotation of unigenes in seven functional databases yielded 35,780 (NR: 45.34%), 29,271 (NT: 37.09%),
30,414 (Swissprot: 38.54%), 10,920 (COG: 13.84%), 28,732 (KEGG: 36.41%), 7388 (GO: 9.36%),
and 25,944
(Interpro: 32.87%) of annotated unigenes (File S1). Using functional annotation results (Figure S2),
we detected 35,888 CDS (coding sequences), and after gene prediction using ESTScan [
48
] on the
Genes 2019,10, 306 7 of 16
remaining unigenes, we obtained 3346 additional CDS. We have also detected 11,337 SSRs (simple
sequence repeats; micro- and mini-satellites), distributed on 8802 unigenes.
3.2. Genes Involved in Sex Determination or Sex Dierentiation
Of a list 37 genes (Table S3), ca. 86% (26) had significant blast hits in the P. persicus transcriptomes
in Xenopus, providing the first sequence data for these genes for this rare urodelean species or even
the family Hynobiidae. For six other important vertebrate genes involved in sex determination or
sex dierentiation (DMRT1, DMRT3, AMHR2, FOXL2, SF1, and WNT1), at least a few reads yielded
Blast hits when urodelean (or other amphibian) queries were used (Table S3, File S2). However,
no matches
were obtained for the five remaining genes (ALDH1A2, DMRT6/DMRTB1, FGF16, NR0B1,
and SRD5A3).
3.3. Mitochondrial Genomes
The two sequencing libraries of sample ZSM 1 from topotypic P. persicus yielded combined
7,704,874 PE-reads of archival DNA. In total, only 2511 sequences mapped to the reference mitogenome
(i.e., only 0.03% of the shotgun data), covering 16,140 bp of assembled mtDNA. In addition,
from one
transcriptome (loc. 4), we generated a total of 16,057 bp of mitochondrial DNA. Sequences are deposited
in GenBank (accession numbers MK737945-MK737946).
3.4. Phylogenetic Analyses
In the maximum likelihood phylogeny, the three Iranian Paradactylodon mtDNA-genomes
(Figure 2a) formed a highly supported clade, with the mtDNAs from the Western (loc. 1) and
central Elburz (loc. 4) falling into a very weakly supported subclade, as compared to the topotypic,
nominal P. gorganensis (loc. 6). Accordingly, the network (based on 16,140 bp, equally available from
all three Iranian Paradactylodon mtDNAs; Figure 2b) shows 175 changes between Paradactylodon from
localities 1 and 4 (ca. 1.08%) but 203 (ca. 1.25%) between those from localities 1 and 6. Additional
phylogenetic analyses of fragments of up to 865 bp of the mitochondrial cytochrome b, obtained with
overlapping PCRs from many more, in part archival samples (Figure 2c), show a slightly diering
topology (most probably due to missing data from some samples) but clearly confirm close relationships
between all P. persicus sensu lato (i.e., comprising nominal P. persicus as well as P. gorganensis), as does
a network, constructed from only 119 bp of this gene, equally available from each of these samples
(Figure 2d).
3.5. Larval Ontogeny with First Observations of Gonadal Development
We present a photographic sequence of the ontogenetic development from the first day of
spawning, first cleavage, formation of the neural crest, advanced embryos, and then from hatching
until metamorphosis (Figure 3), which allowed us to compare stages with other hynobiids (Table 1)
and to assign these stages to the early gonadal development of larval P. persicus for the first time.
The gonads
are paired elongated organs, situated longitudinally in the medial line of the body, parallel
to the proximal portions of the mesonephroi (Figure 4a,d,g; see also Figure S3a,d,g). Until day 29 after
hatching, the gonads remained sexually undierentiated and contain big primordial germ cells (PGCs),
loaded with yolk (Figure 4c,f). At day 41 after hatching, the gonads started sexual dierentiation
(Figure 4g–i). Fat bodies, which dierentiated from the most proximal portions of the gonadal anlage
(Figure 4b), further developed towards the caudal part of the body (Figure 4), in parallel to the gonads
and up to their distal tips (Figure 4h). At the time of sexual dierentiation, their cells were filled with
fat droplets (Figure 4i).
Genes 2019,10, 306 8 of 16
Table 1.
Developmental stages and samples taken for transcriptomics and gonadal histology of Paradactylodon in comparison to stages in three genera of hynobiid
salamanders (Onychodactylus,Hynobius, and Ranodon), according to dierent authors.
Developmental Stages in
Paradactylodon persicus Onychodactylus
japonicus (Iwasawa &
Kera 1980) [28]
Hynobius nigrescens
(Iwasawa &
Yamashita 1991) [29]
Ranodon sibiricus
(Vassilieva &
Smirnov 2001 [30]
Photographs
Figure 3 Days of Development Samples Examined
a1 day after spawning embryos in the
sac, cleavage 3–4 3 –
b12 days after spawning, embryos in
the sac, gastrulation 22 12B –
c18 days after spawning, embryos in
the sac, neural plates 29 18 –
esingle embryo, egg sac removed Transcriptome 1 46 36–37
flarvae before hatching, external gills 48 39
g, h larva at day 4 after hatching Gonadal histology
(Figure 4a–c) 49 42–43 II
i24 days 55 53 VI
k29 days Gonadal
histology(Figure 4d–f)
58 55 VII–IX
l38 days Transcriptome 2 60 57 XII
m41 days Gonadal
histology(Figure 4g–i)
66 63 XV
n58 days 69 65
o58 days 69 65
p60 days 69 66
Genes 2019,10, 306 9 of 16
Figure 3.
Development of Paradactylodon persicus (for staging see Table 1). (
a
c
) Egg sacs of descendants
of Paradactylodon (topotypic P. gorganensis), obtained at loc. 6 that reproduced in captivity at 6.5
C water
Genes 2019,10, 306 10 of 16
temperature. (
a
) day 1 after spawning, visible cleavage (white arrow head); (
b
) detail of the sac with
gastrulation visible (red arrow), 12 days after spawning;
c:
neurulation at day 18 after spawning; (
dp
)
developmental series of P. persicus in captivity over 58 days, during which water temperature raised
from 12–20
C; at each stage, the largest larva is shown (
d
) one of two egg sacs (photographed in a
cuvette) found at locality 4 in 1550 m a.s.l. at 8–9
C water temperature; (
e
) isolated single embryo
(about 2 cm) from the sac shown in (
d
), of which one transcriptome was sequenced, after the yolk was
removed; (
f
) detail of dwith external gills and yolk sacs of embryos visible; (
g
) and (
h
) larvae at day
4 after hatching (lasting 14 days from the same egg sac at 12
C; gonadal histology in Figure 4a–c);
(i) 24 days
(gonadal histology in Figure 4d–f);
k
: 29 days; (
l
) 38 days: larval stage at which RNA of
six organs was obtained for the second transcriptome.; (
m
) 41 days (gonadal histology in Figure 4g–i);
(
np
) three dierent larvae at 58 days; salamanders left water at ca. 60 days at 60 mm total length.
Photographs: (ac) Michael Fahrbach, (dp) Sebastian Voitel.
4. Discussion
4.1. Transcriptomes of Embryo and Larva of Paradactylodon Among the First in Hynobiidae
Mainly due to enormous genome size [
14
], amphibian genomics, and particularly that in
urodela, remains extremely challenging, with only one model urodelan species’ genome fully
sequenced [
15
], but even there is awaiting improved annotation. The genome size of Paradactylodon
(
ca. 34.7 pg/nucleus; [12]
) probably keeps whole genome sequencing unaordable in the near future.
Therefore, beyond a limited number of existing PCR-based sequences, the two transcriptomes provide a
first genome-wide resource for the genus and to our knowledge the second and third in Hynobiidae [
49
].
The first transcriptome in Hynobiidae was studied Hynobius chinensis [
49
], a taxon, which is about
55 My
diverged from Paradactylodon (www.timetree.org; average of eight molecular studies), and thus
expected to show major evolutionary dierences.
Our research revealed the stage of sexual dierentiation of gonads in P. persicus (histologically
documented at day 41), and accordingly, in the second larval transcriptome, prepared at day 38 after
hatching (Table 1), we detected low levels of expression of candidate genes that may be especially
relevant for sex determination (Table S3, such as DMRT1, AMH, AMHR2, and FOXL2. This might
facilitate future studies in the genus Paradactylodon and other Hynobiidae. Sex chromosomes of
most salamanders are homomorphic [
50
,
51
], and in most species, mainly the observation of balanced
sex ratios from clutches is interpreted as indication for genetic sex determination but has remained
essentially without genetic evidence [
52
]. In Hynobiidae, gene expression in context to sexual
development has been studied using histology and qPCR of a single gene (P450) in Hynobius retardus
by Sakata et al. [
53
]. They have shown that P450 aromatase was expressed predominantly in the adult
ovary and brain, weakly in testis, but not in other somatic organs. A typical sexual dimorphism in
P450 aromatase expression was detected in normally developing larvae by a quantitative competitive
RT-PCR; strong expression in the female gonads but very weak in male gonads [53].
4.2. Gonadal Dierentiation in Paradactylodon and Other Hynobiid Salamanders
Morphological and histological data of adult Hynobiidae have been reported to asses the
reproductive cycles in both sexes of Salamandrella schrenckii [
54
] and S. keyserlingii [
54
58
]. The larval
development of gonads from 0–60 days was described in Hynobius retardatus [
50
] at 16–20
C. In this
species, genital ridges with primordial germ cells (PGCs) were formed within 10 days after hatching
and sexual dierentiation occurred within 20–30 days after hatching (stage 53 of [29]; Table 1).
Genes 2019,10, 306 11 of 16
Figure 4.
Developing gonads of Paradactylodon persicus (for staging: Figure 3and Table 1). (
a
c
) Larva
at day 4 after hatching; (
d
f
) at day 24 after hatching; (
g
i
) at day 41 after hatching; (
a
), (
d
) and (
g
)situs
viscerum (digestive tract removed). Note the position of gonads (outlined in black: Figure S3) in relation
to the mesonephroi; (
b
), (
e
) and (
h
) gonads dissected with fat bodies; (
c
) and (
f
) longitudinal section
of undierentiated gonads containing primordial germ cells loaded with yolk platelets (stained red);
(i) fat
body and gonocytes in early testis; inset: enlarged portion of the testis core invaded by a group
of gonocytes.
Abbreviations: fb
: fat body
; fl
: forelimb bud;
g
: gills;
m
: mesonephros;
myo
: trunk
musculature (myomeres separated by myoseptae); white arrowheads: melanophores in the dorsal
peritoneum; black arrowheads: germ cells; white or black arrows: gonads.
Scale bars
: a, b, d, e, g, h:
1 mm; c, f, i: 100 µm.
Here, we document the earliest stages of gonad dierentiation in Paradactylodon, just after PGCs
had invaded the gonadal anlage in stages 42–43 and 55 (at day 24 and 29 after hatching, Table 1).
PGCs were clearly distinguishable by their big size and heavy yolk load, as in other amphibians (for
Genes 2019,10, 306 12 of 16
review: [
59
]). Sexual dierentiation of gonads–in this case testes–took place before metamorphosis,
at stage 63
, at day 41 of (hatched) larval life. The dierences in time (20–30 days) required to achieve
sexual dierentiation reported for H. retardatus [
53
] and P. persicus (41 days, this study) seemed to
be not only species-specific, but also resulted from rearing temperature (16–20
C and 12–20
C,
respectively). Development of gonads in H. retardatus during 1–3 years of its life was described by
Kanki and Wakahara [
60
]. Unfortunately, similar later stages of gonad development were not available
in our study.
4.3. Phylogeography, Taxonomy, and Conservation
Using three almost complete mitochondrial genomes from the three geographic extremes, involving
both type localities (locs. 1, 6) and a sample from central Elburz (loc. 4), we were able to perform a
comparative phylogenetic analysis of Iranian Paradactylodon, supplemented with additional archival
samples and available sequences from GenBank (Figure 2a–d). The largest genetic distance between
these three mitochondrial genomes is about 2%, suggesting that these secretive salamanders may
belong to a single species, however, clearly exhibiting some intraspecific variation. While our
data suggest that both taxa belong to a single species, we also can see some variation along the
Hyrcanian Forests of Iran, suggesting there is “clinal variation” along their range corridor as previously
proposed [
12
].
The distances
reported among the Paradactylodon mitochondrial DNAs (1–2%, Figure 2)
roughly correspond to those between “subspecies” or within species in some other Urodela (e.g.,
Plethodontidae [
61
,
62
] and Salamandridae [
63
,
64
]), including Hynobiidae. Using COI and 16S rDNA,
Xia et al. [
65
] found 2-parameter genetic distances (K2P) of the mean intraspecific variation for COI
and 16S rDNA to be 1.4% and 0.3%, respectively. Uncorrected pairwise distances of cytochrome
bin multiple Batrachuperus species from East Tibet were found to show even larger intraspecific
variation [66].
So far, according to the International Union for Conservation of Nature (IUCN) red list, these two
nominal species of Iranian hynobiids are considered “critically endangered” (P. gorganensis) or “near
threatened” (P. persicus). Although our data on mitochondrial DNA suggest that they presumably
represent a single species, further studies on nuclear genes across the entire distributional area are
required. Hence, we plead for strict conservation of endangered populations of “P. persicus sensu lato”
throughout their range.
The nomenclature of the genus Paradactylodon has been debated for many years [
12
,
67
,
68
].
The genus
has recently also been called Iranodon, however, Paradactylodon Risch 1984 was in “error
considered a nomen nudum by Dubois and Raaëlli, 2012” [69], according to Frost [70].
These salamanders can be considered as bioindicators for “intact and healthy” stream ecosystems
of the Hyrcanian Forests. However, in some cases, P. persicus might persist for decades even after
anthropogenic deforestation has long destroyed the native vegetation [
21
], as long as intact streams or
springs allow relic populations to endure. These hynobiids have survived in the Hyrcanian Forests
since the Tertiary; therefore, humanity and namely Iran has a great responsibility to protect these species
within their ecosystems in the long term, along with the unique biodiversity of their habitats [71].
Supplementary Materials:
Supplementary materials can be found at http://www.mdpi.com/2073-4425/10/4/306/s1.
Table S1. List of samples of Iranian Paradactylodon; sample IDs according to Figure 1 (PDF). Table S2. Primers used
to amplify mtDNA fragments of cytochrome bof Paradactylodon (PDF). Table S3. Male and female sex-determining
and sex dierentiation gene inventory and homologous P. persicus transcripts (red entries refer to low levels of
expression) (PDF). Figure S1. Frequency length distribution of unigenes in the Paradactylodon persicus transcriptome.
(JPG). Figure S2. Functional GO annotations of Paradactylodon persicus transcriptome unigenes. (JPG).
Figure S3
.
Annotated version of Figure 4with the gonads outlined (JPG). File S1. Annotation results of unigenes from
five databases (txt). File S2. Contigs of RNA sequences of extracted male and female sex-determining and sex
dierentiation genes, obtained from the P. persicus transcriptomes (txt).
Author Contributions:
M.S., F.F., and M.O. designed the research; F.F., M.E., H.G.K., J.F.S., S.V. and M.S. collected
samples; S.V. raised animals; S.V., and B.R.-K. performed dissections; B.R.-K., M.O., S.V. and M.S. performed
larval staging; B.R.-K. and M.O. made histological analyses; R.P. and S.L. extracted DNA and did PCRs of
mtDNA-fragments; M.S. prepared RNA and arranged for transcriptomics; H.K. and M.S. analyzed transcriptomes,
Genes 2019,10, 306 13 of 16
H.K. and D.F. assembled the complete mtDNAs, D.F. performed N.G.S. and bioinformatics of archival topotypic P.
persicus; MS and FF did phylogenetic analyzes; M.S., B.R.-K., M.O. and F.F., drafted the manuscript; B.R.-K., M.O.,
S.V. and M.S. prepared the figures; all authors have contributed to the final manuscript.
Funding:
This work was funded by the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB) by
a grant to MS. The publication of this article was funded by the Open Access Fund of the Leibniz-Institute of
Freshwater Ecology and Inland Fisheries (IGB) and the Open Access Fund of the Leibniz Association.
Acknowledgments:
We are grateful to Jamshid Darvish, who passed away, for help with the collection permit
by the University of Mashad (95/3/20/81984) in 2015 and export permit M/503/33174. We thank Frank Glaw and
Michael Franzen for help to access topotypic specimens of JFS, deposited in ZSM; Wibke Kleiner, Eva Kreuz,
and Nadine Possnien for help in the laboratory; Werner Kloas for laboratory access; Michael Fahrbach for
images from egg sacs in captivity; Sylvia Hofmann for drafting the map; Johanna L. Paijmans for help with
mtDNA annotation; Tatajana Dujsebayeva for discussion, references and translations from Russian on Ranodon
sibiricus; and the editor, Sebastian Steinfartz, for the invitation to publish in this Special Issue. Sequencing of
the NGS-libraries for the mtDNA was performed at the Berlin Center for Genomics in Biodiversity Research
(BeGenDiv).
Conflicts of Interest: The authors declare no conflict of interest.
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... These results are consistent with phylogenetic relationships reconstructed in previous analyses, although B. mustersi and B. gorganensis have been classified under the genus Paradactylodon in earlier studies (Peng et al. 2010;Tang et al. 2015). Batrachuperus mustersi and B. gorganensis have been referred to as Paradactylodon in some other studies as well, and the nomenclature of the genus Paradactylodon has been debated for many years (Stock et al. 2019); in one earlier study, Paradactylodon was considered a junior synonym for Batrachuperus (Reilly 1987). Another study highlighted that Batrachuperus is diphyletic, comprising a Central-Western Asia group and a West China group (Thorn and Raffaëlli 2001). ...
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