Research Article |
Corresponding author: Massimo Meregalli ( massimo.meregalli@unito.it ) Academic editor: Brian Wiegmann
© 2021 Massimo Meregalli, Manfred Kahlen, Riccardo Monguzzi, Valentina Marzia Rossi, Alfredo Santovito.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Dichotrachelus orobicus, a new species from the Bergamasque Prealps, is described. It is closely related to D. grignensis from the Grigna Massif, from which it differs mainly in the COI and ITS2 sequences, and minute morphological characters. Remarks on the possible epoch of speciation between the two taxa, with an analysis of the biogeographic scenario that may have led to the disjunction, are discussed.
Alpine fauna, cryptic species, mitochondrial cytochrome c oxidase 1, internal transcribed spacer 2, molecular clock, new species, phylogeny, speciation
Dichotrachelus grignensis Barajon, 1946, was described based on some specimens found on Mt. Grigna, in western Lombardy (northern Italy). It belongs to a group of species whose host plant is Saxifraga caesia L. and represents the westernmost species of the D. grignensis group, as defined in
Most of the specimens used in this study were sampled during a specific research expedition carried out in July 2018 by two of the authors (M.M. and M.K.), with the addition of several previously collected specimens, mainly conserved in coll. Monguzzi. Specimens were collected usually at night, by searching on the limestone rocks near plants of Saxifraga caesia. Some more specimens were sifted from the same plants, and larvae were found in the roots of the saxifrage. Fresh imagoes and larvae were stored in vials with alcohol 95° immediately after collection and conserved at minus 23°C until they were processed. Specimens from previous collections had been conserved dry on card in entomological drawers and an attempt at DNA extraction from these specimens, not always successful, was made after a short rehydration.
Body length was measured in profile from anterior border of eyes to apex of elytra, excluding rostrum. Length/width ratios were measured from digital photographs, and were always taken at the maximum length and width of the respective parts in dorsal view. Genitalia were cleared with 10% KOH and carefully dissected; female genitalia and male genital sclerite were embedded in resin, male genitalia were mounted dry on the same card as the respective specimen. Photographs were taken using a Nikon P 6000 digital camera mounted on a Leica 6SE stereomicroscope, combining image stacks with Zerene Stacker. All images were cleaned and enhanced as necessary in Adobe Photoshop CS3.
Total DNA was extracted by placing the entire animal body in 400 μl of 5M guanidine-isothiocyanate, after separating the head + prothorax from the rest of the body, or cephalic capsule from the rest of the body for the larvae, to maximize DNA extraction. DNA extraction was destructive for the larvae; adult specimens were conserved as much as possible. Two different regions of the mitochondrial COI gene were amplified and independently analysed. One region was amplified with primers based on
Pairwise distance was calculated for the COI sequences with MEGA-X, implementing Tamura-Nei substitution model (
Bayesian Inference was estimated using MrBayes 3.2 (
An attempt to estimate the epoch of the splitting event between the two D. grignensis clades that resulted from the Bayesian inference, and within the Bergamasque Prealps populations, was carried out by applying a molecular clock to the results of the COI analyses. Criticism of molecular clocks has often been expressed, in particular because variation among taxa and intrinsic theoretical limits of the procedure have been evidenced (
Three different molecular clock models were tested with Bayes factor (B10): no-clock model, strict clock model, and uncorrelated gamma rate relaxed clock model (
There is individual variation among the specimens, so that occasionally one of the characters indicated overlaps between the species. The field of variation increases considering the entirety of the populations from the Bergamasque Prealps and only the shape of the male genital sclerite appears to be constantly different. This structure was demonstrated to be a highly reliable character in the taxonomy of the genus (
Character | Dichotrachelus grignensis | Dichotrachelus orobicus |
Rostrum: ratio width at base/width at antennal insertion | 1.37–1.83 mean 1.61; median 1:59 | 1.07–1.75 mean 1.43; median 1.46 |
Rostrum: ratio length/width at base | 1.28–1.61 mean 1.37; median 1.34 | 1.31–1.93 mean 1.58; median 1.53 |
Pronotum, sides: | usually linearly weakly broadened from base to apical quarter, not strongly narrowed before apex | usually slightly curved outwards, with maximum width at midlength, strongly narrowed before apex |
Protarsus, tarsomere 3: ratio length/width | 0.67–1.06 mean 0.89; median 0.91 | 0.91–1.10 mean 1.04; median 1.00 |
Metatarsus, tarsomere 3: ratio length/width | 0.72–1.16 mean 0.99; median 1.00 | 1.01–1.56 mean 1.20; median 1.17 |
Penis, apical part: ratio length from base of ostium to apex/maximum width of ostium | 2.15–2.32 (n=3) | 2.45–3.31 (n=5) mean 2.84; median 2.65 |
Penis, apical lamella: shape | sides feebly convergent, apex slightly elongated | sides subparallel, apex broadly rounded |
Male genital sclerite, valve: | anterior valve broadly oval, scarcely longer than wide; outer margin of posterior valve scarcely curved, oval | anterior valve oblong, narrow, much longer than wide; outer margin of posterior valve strongly curved, semicircular |
Node ages were calculated with a rate of 0.018 nuclear substitutions per site per My per lineage, according to
The two taxa are extremely similar, and each population shows some morphological peculiarities; moreover, the very limited number of specimens available does not allow evaluation of the stability of the few distinctive characters; due to the paucity of specimens, no statistical analysis of possible biometric variation can be implemented. The following scheme was drawn from the specimens of D. orobicus from the type locality, Pizzo Camino, and surroundings, vs 8 specimens from the Grigna (Figs
Dichotrachelus grignensis, Northern Grigna. Rostrum, dorsal (a). Antenna (c). Pronotum (e). Protarsus (g). Rostrum, profile (i). Apical part of penis (k). Metatarsus (m). Dichotrachelus orobicus, holotype. Rostrum, dorsal (b). Antenna (d). Pronotum (f). Protarsus (h). Rostrum, profile (j). Apical part of penis (l). Metatarsus (n).
The Bayesian inference gave phylogenetic trees with comparable topology for all the primer sets. All populations belonging to the D. grignensis complex clustered in a fully supported clade. Within this clade all the specimens from the Bergamasque Prealps formed a monophyletic group with respect to the specimens from the Grigna Mts with full, or almost full, support for both the regions of COI and the ITS2 (Figs
Bayesian Inference consensus tree based on COI, fragment 825 bp, of the relationships among several species of Dichotrachelus, with emphasis on the Dichotrachelus grignensis complex. Branch post probability support is indicated on the branches, in percentage. Scale bar unit: expected substitutions per site.
Bayesian Inference consensus tree based on COI, fragment 657 bp, of the relationships among a few species of Dichotrachelus, with emphasis on the Dichotrachelus grignensis complex. Branch post probability support is indicated on the branches, in percentage. Scale bar unit: expected substitutions per site.
Several nucleotide variations were detected.
825 bp fragment. 781 sites are constant among all specimens (94.6%); of the 44 sites that vary, 40 are phylogenetically informative, i.e., they are constant in all specimens of each of the two clades and vary between the two clades: they are thus considered as molecular synapomorphies. The majority of the variations are T->C or A->G transitions (39 sites), with only a few A->C or A->T transversions (5 sites). All but 5 of these substitutions are synonymous; the variations in the 275 aa long chain in D. orobicus are the following: a valine replacing a methionine in position 47, a valine replacing an isoleucine in position 75, a leucine replacing a phenylalanine in position 95, an alanine replacing a threonine in position 272 and an histidine replacing an asparagine in position 274.
657 bp fragment. 614 sites are constant among all specimens (93.3%); of the 43 sites that vary, 30 are phylogenetically informative. The C->T and A->G transitions are 22, whereas the A->C, A->T and G->C transversions are 8. All but 3 of these substitutions are synonymous; the variations in the 219 aa long chain in D. orobicus are the following: a glycine replacing a lysine in position 33, a methionine replacing a valine in position 139 and an isoleucine replacing a valine in position 172.
In the sequences of the specimens of the D. grignensis complex the first 228 sites are constant, then limited variation occurs. The main variation regards a series of repeated GAC sequences, starting from position 244, that can be from 4 to 7, depending on the population. The remaining part of the chain is again rather constant, with a few replacements of single nucleotides and a few occasional gaps in one or another of the populations.
According to our analysis, the two clades of the D. grignensis complex separated around 5 million years ago (‘Mya’), with similar results for both the 825 and 657 fragments; the median in-clade divergence among the populations of D. orobicus indicates that they became reciprocally isolated at most around 0.5 Mya, but probably in more recent times (Fig.
The p-distance between the specimens of D. orobicus and those of D. grignensis is, for the 825 fragment, about 0.06 (6%), whereas the interpopulation p-distance for D. orobicus is always below 1%, varying between 0.004 and 0.008; for the 657 fragment the p-distance between D. orobicus and D. grignensis is 0.07 and among the various populations of D. orobicus it is lower than 0.01.
Theoretically, the two clades could be classified as two distinct species or two distinct subspecies in D. grignensis. An analysis of the concept and application of the subspecies is beyond the scope of this paper. It is known that subspecies have no place in the schema of the phylogenetic species concept, so adherents to that concept are predisposed to a finding of “no subspecies” because below the generic level a taxon is either a species or it is nothing (
In our opinion, a comprehensive approach to establish the rank to be assigned to allopatric populations should evaluate equally the entire set of morphological, biogeographical and molecular data.
The two clades of the D. grignensis complex differ by several molecular synapomorphies. Any approach to delimit a threshold in interspecific differentiation in genetic divergence is somewhat arbitrary, since it can vary to a great extent among different taxa. A limit of 5% was roughly considered to be a good indicator (
Moreover, not all the nucleotide differences between the two groups of populations of the D. grignensis complex are synonymous. The two regions of the COI that were amplified, out of a total of 494 amino acids, differ for 8 between the two taxa (variation of 0.016%). The intraspecific amino acid variation usually peaks near zero (i.e., median divergence is 0.009 for Drosophila flies,
The molecular clock places the disjunction between the demes from the Grigna mountains and those from the Bergamasque Prealps at about 5 Mya. Time for speciation cannot be standardized in any way, since it depends on a number of geographical, ecological and genetic factors (
We could not evidence any clear-cut morphological trait to separate the two taxa, excluding a minute variation in the shape of the genital sclerite; nevertheless, these minimal differences give an indication that supports the molecular data.
Based on this multi-data approach, we consider the two taxa to be distinct species, even though they are barely distinguishable based on morphology only.
Two main scenarios can be considered. One, the ancestral species was distributed across the entire area from Mount Camino to the Grigna massifs, and then, around 5 Mya, the Grigna population remained isolated from the others. Alternatively, a second scenario supposes that the ancestral species was distributed only in the Bergamasque area, and then, around 5 Mya, it colonized the Grigna massifs, where it remained isolated from the parent population. Some remarks on orogeny and paleoclimatology of the area may help understanding the distribution pattern of the D. grignensis complex. Bioecology of these weevils must be also taken into account. The Dichotrachelus of the D. grignensis group are strictly associated with calcareous rocks; they are, nowadays, apparently exclusive to heights above 1500 m a.s.l., where they are monophagous on Saxifraga, usually S. caesia. Their vagility is very limited and consequently their dispersion capability is scarce, at least in the short term, and it can occur only in suitable habitats with limestone rocks and the presence of Saxifraga caesia. Their present distribution in the Alps was determined by orogeny of the chain as well as paleoclimatic events.
The southern Alps, separated from the Alpine axial zone by the Insubric line, experienced several deformation phases mainly of Late Eocene-Oligocene age (
The southward propagation of thrusting towards the foreland, as recorded in the Po Plain subsurface, continued to occur throughout the Miocene (5.3–7.2 Mya) (
The present Lago di Como topography originated from a deeply incised Messinian canyon during the Messinian Salinity Crisis (
The divergence between the Grigna and the Bergamasque Prealps populations of the D. grignensis complex can be roughly dated to around 5 Mya, and there does not seem to be any evidence of a sudden possibility of range expansion westbound towards the Grigna in that epoch. The most likely scenario proposes that the ancestral forms were already present in the entire region during the Miocene, before the Messinian, and became isolated following the Messinian incisions of the Valsassina that separated the Grigna from the Resegone and the other calcareous mountains east of the Introbio valley. Consequently, this primary disjunction between the two lineages of D. grignensis-complex appears to be related to tectonic events.
This pattern of disjunction, with sister species present on the two sides of the Valsassina, is not exclusive to the two species of Dichotrachelus but it occurs also in other scarcely vagile steno-endemisms. Examples are Boldoriella manzoniana Monzini, 1995, from the Grigna massif, with its vicariant B. carminatii (Dodero, 1917) from the Bergamasque Prealps (
The populations of D. orobicus present on the limestone outcrops between the Valsassina and the Valcamonica, and probably exclusive to the montane and alpine region, show a limited reciprocal molecular differentiation. Their apparent isolation in the calcareous massifs has undoubtedly a more recent origin, and was likely determined by the Pleistocene climatic oscillations, when glaciers acted as barriers that prevented gene exchange among the populations remained isolated in the nunattaker (Fattorini 2004).
Italy, Lombardy, Prov. Bergamo, Val di Scalve, Pizzo Camino
Named after the Prealpi Orobie, the Bergamasque Prealps, a mountain range in the Italian Alps, located in northern Lombardy.
A cryptic species vicariant of Dichotrachelus grignensis, morphologically extremely similar, only different for the shape of the male genital sclerite (Figs
Body length of the holotype: 6.85 mm. Rostrum narrow, ratio length/width at base 1.65, with sides in dorsal view subparallel, weakly convergent anteriad, with deep interantennal longitudinal groove (rostrum broad, mean ratio length/width at base 1.37; dorsal sides linearly convergent from base to antennal insertion, longitudinal groove very shallow). Pronotum small, slightly constricted near apex with sides moderately curvilinear (pronotum robust, not constricted near apex, sides usually slightly linearly broadened from base to apical third). Elytral shape very similar between the two species. Tarsomere 3 of protarsus as long as wide, lobes slightly developed (tarsomere 3 of protarsus shorter than wide, lobes not developed); tarsomere 3 of metatarsus as long as wide (tarsomere 3 of metatarsus shorter than wide). Sides of body of penis smoothly restricted anteriad, lamella with parallel sides, broadly rounded at apex (sides of body of penis sharply restricted anteriad, distinctly sinuate before lamella, lamella with sides feebly convergent anteriad, slightly elongated at apex). Anterior valve of male genital sclerite (terminology as in
The specimens from Pizzo Camino area are relatively uniform, particularly in the discriminating characters. Those from Pizzo Arera have the rostrum slightly shorter and broader. Those from Presolana have the rostrum similar to those from Pizzo Camino. The specimen from Resegone has the apex of the penis more similar to those of the Grigna massifs, but the genital sclerite has the typical shape of D. orobicus.
(approximate georeference, when not indicated on label, in square parenthesis). Labels reported verbatim; /: different line. Holotype ♂. “Val di Scalve, Schilpario / (BG) Pizzo Camino [45.9867°, 10.1810°] / m 2000 22.VII.2002 / R. Monguzzi leg.” (deposited at Museo Civico di Storia Naturale, Milano, Italy). Paratypes: coll. Monguzzi: same data as the holotype, 1♂, 1♀; “Pizzo Camino / Val di Scalve, Schilpario – BG / m 2000 5.VIII.1979 / Leg. R. Monguzzi” 1♀; “Pizzo Camino (BG) / Schilpario m 2000 / 4.VIII.2000 / R. Monguzzi” 1♀; “Pizzo Camino / (Schilpario) / m 2100 17.VIII.2014 / R. Monguzzi” 1 fragment; “Val di Scalve (BG) / Cimone d. Bagozza [46.0214°, 10.2662°] / m 2100 6.09.2014 / R. Monguzzi leg.” 3♀; “Val di Scalve (BG) / Mass Presolana / M. Ferrante m 2200 [45.9744°, 10.0288°] / 13.9.14 R. Monguzzi” 1♀; “Prealpi Orobie (BG) / M Ferrante m 2300 / vers. Est Gruppo della / Presolana 19.07.2014 / R. Monguzzi leg.” 1♀; “Pizzo Arera (BG) / Mandrone m 2100 [45.9305°, 09.8067°] / 30.vi.09 R. Monguzzi” 1♀; “M. Arera / BG m 2200 / 11.7.81 Rosa” 1♂; “Prealpi Bergamasche / Val Brembana Cima / di Menna [45.9254°, 09.7595°] m 2100 / 20.08.2013 R. Monguzzi”; “Prealpi Bergamasche / Zuccone dei Campelli [45.9580°, 09.5133°] / Vers. Valsassina (LC) / Valle dei Camosci m 2100 / 3.vi.09 R. Monguzzi” 2♀.
Coll. Kahlen: “Prov. Bergamo, Pizzo Arera, Mandrone 2000m 45°56′04″N, 9°48′13″E, 7.7.1990 Saxifraga caesia”, 2♂ 2♀; “Prov. Bergamo, Pizzo Arera, SW-Kar 2050m 45°55′50″N, 9°48′24″E, 20.7.1992, Saxifraga caesia” 2♂ 1♀; “Prov. Brescia, Passo di Baione 2155m 46°01′17″N, 10°15′59″E, 30.7.2018 Saxifraga caesia” 1♀.
Coll. Szallies: “I Alpi Bergam. / Pizzo Arera Ost- / grat Nordkar / 23-/2500 m 3.6.2015 / leg. Szallies” 1♂; “I. Bergam. Alpen / Valle Camonica / Passo di Baione [46.0213°, 10.2666°] / 2150 m 27.7.2016 / leg. Szallies” 3♂; I. Bergam. Alpen / Valle Camonica / Cima dei Ladrinai [46.0165°, 10.2790°] / 2300 m 27.7.2016 / leg. Szallies” 1♀.
Coll. Meregalli: “Val di Scalve, Schilpario / (BG) Pizzo Camino / m 2000 22.VII.2002 / R. Monguzzi leg.” 1♂ 1♀.
Non-type material. “I Bergamask Alpen / Lecco Resegone / 1800 m / 1.7.2016 / leg. Szallies” 1♂ (aedeagus only, body mistakenly destroyed during DNA extraction).
Dichotrachelus orobicus is present in all the calcareous mountains between Valsassina and Valcamonica, where it is usually found above 1800 m asl. (Fig.
The species is monophagous on Saxifraga caesia. The larvae develop among the roots of Saxifraga, often on clumps growing on the soil (Zuccone Campelli, Presolana). The adults feed on the same plant. Their activity is nocturnal, when they can be found in trophic activity on the plants and walking on the surrounding rocks; during the day they shelter below stones and in rock crevices.
Dichotrachelus orobicus does not seem to be endangered at present and it does not fully meet any of the criteria required for inclusion in the categories at risk (
Species | Type categorie | Locality | Georeference |
Dichotrachelus orobicus | holotype, paratypes | Italy, Lombardy, Pizzo Camino | 45.9867′N, 10.1810′E |
Dichotrachelus orobicus | paratypes | Italy, Lombardy, Passo di Baione | 46.0213′N, 10.2666′E |
Dichotrachelus orobicus | paratypes | Italy, Lombardy, Cima dei Ladrinai | 46.0165′N, 10.2790′E |
Dichotrachelus orobicus | paratypes | Italy, Lombardy, Cimone della Bagozza | 46.0214′N, 10.2662′E |
Dichotrachelus orobicus | paratypes | Italy, Lombardy, Massiccio della Presolana | 45.9744′N, 10.0288′E |
Dichotrachelus orobicus | paratypes | Italy, Lombardy, Pizzo Arera | 45.9305′N, 09.8067′E |
Dichotrachelus orobicus | paratypes | Italy, Lombardy, Cima di Menna | 45.9254′N, 09.7595′E |
Dichotrachelus orobicus | paratypes | Italy, Lombardy, Zuccone dei Campelli | 45.9580′N, 09.5133′E |
Dichotrachelus orobicus | non-type specimens | Italy, Lombardy, Monte Resegone | 45.8581′N, 09.4694′E |
Dichotrachelus grignensis | non-type specimens | Italy, Lombardy, Monte Grigna, Rif. Brioschi | 45.9477′N, 09.4012′E |
We wish to thank our colleague A. Szallies for the kind loan of the specimens from his collection and C. Germann for sharing the ITS2 sequences of a few species of Dichotrachelus. C. Lyal (Natural History Museum, London) kindly checked the English text.
M.M., M.K and R.M.: general structuring of the research, field sampling and preparation of the paper; V.R.: remarks on geological aspects; M.M.: phylogenetical analyses; A.S.: laboratory work. The authors declare not to have conflicts of interest.
MW617448 | Dichotrachelus grignensis Grigna, Folmer primers |
MW617382 | Dichotrachelus grignensis Grigna, Rif. Brioschi, Folmer primers |
MZ313365 | Dichotrachelus grignensis Grigna, Jerry/Pat primers |
MZ313369 | Dichotrachelus grignensis Grigna, Rif Brioschi, Jerry/Pat primers |
MZ313362 | Dichotrachelus orobicus Pizzo Arera, Folmer primers |
MZ313363 | Dichotrachelus orobicus Pizzo della Presolana, Folmer primers |
MZ313366 | Dichotrachelus orobicus Colle del Baione, Folmer primers |
MZ313364 | Dichotrachelus orobicus Zuccone dei Campelli, Folmer primers |
MZ313368 | Dichotrachelus orobicus Pizzo Arera, Jerry/Pat primers |
MZ313367 | Dichotrachelus orobicus Pizzo della Presolana, Jerry/Pat primers |
MZ313370 | Dichotrachelus orobicus Colle del Baione, Jerry/Pat primers |
MZ313222 | Dichotrachelus grignensis Grigna, ITS2 primers |
MZ313223 | Dichotrachelus grignensis Grigna, Rif. Brioschi, ITS2 primers |
MZ313210 | Dichotrachelus orobicus Colle del Baione, ITS2 primers |
MZ313224 | Dichotrachelus orobicus Presolana, ITS2 primers |