Hybridization

Hybridization

Interspecies hybridization in European Clausiliidae

Hartmut Nordsieck (X. 2020)

I. Introduction and general remarks

Since 1963, in the course of morphological work on different groups of Alopiinae, I recognized hybridization above subspecies level. At first I found hybridization between taxa which by morphological characters and distribution revealed to be more than subspecies (Nordsieck 1963 Charpentieria: so-called stenzioid subspecies of Citala; 1966 ClausiliaC. whateliana / exoptata). At this time, because of that hybridization, I treated these semispecies as subspecies. In the following decades I stated hybridization between species in several genera of Alopiinae from S. E. Europe, where the species ranges are in contact or overlap (Nordsieck 1969 Delima, 1970 Medora, 1971a, 1974 Herilla, 1974 Isabellaria, 1984 enantiomorph Alopia species; survey 1984). Szekeres (1976) was the first who studied hybridization of enantiomorph Alopia species (though he treated these species as subspecies). Wiese (1989) and Welter-Schultes (1992) communicated on hybrids between Albinaria species from the island of Dia, Crete. Kemperman (1992), in his work on the Albinaria species of the Ionian Islands, traced hybrids between all four species from Kefalonia, for the first time proved also by allozyme studies. Schilthuizen et al. (1993) noted hybridization between two very different species of Crete. Giusti et al. (1995) communicated on hybrids between species of Lampedusa and Muticaria from Malta. Neubert (1998) stated hybrids between two Albinaria species from northern Peloponnese. Welter-Schultes (1998, 2000) united much different Albinaria species from Crete, because he found hybrid zones between them. In the following years I published on further examples of more or less probable interspecies hybridization in genera of S. E. Europe (Albinaria Nordsieck 1997, 2004, 2007, 2017, Medora 2009, enantiomorphic Alopia species 2007, 2016).
Since 1995 the phylogeny of Clausiliidae, at first that of Albinaria and related genera, was studied by DNA analyses. Was interspecies hybridization still discussed in the work of Van Moorsel (2001) on Albinaria phylogenies, this phenomenon has not or nearly not been considered in the papers of Greek authors on phylogenies of Albinaria subgroups and in the thesis of Uit de Weerd (2004) on the phylogeny of the whole Albinaria group, though there was no reason to assume that interspecies hybridization in the Albinaria groups treated by these authors occurs to a minor extent than, e. g., in the Albinaria species of Kefalonia (Kemperman 1992). The same non-consideration of interspecies hybridization can be registrated in the work of Fehér et al. (2013) on the phylogeny of Alopia, though since 1976 hybridization of enantiomorphic Alopia species was known. The conclusions of these authors, high frequency of transformation of closing apparatus = CA from N-type to G-type in the Albinaria group (Van Moorsel, Uit de Weerd) and high frequency of change of coiling direction in Alopia (Fehér et al.) and an Albinaria group from Peloponnese (Kornilios et al. 2015) should therefore be regarded with reserve, hence also their phylogenetic hypotheses and proposals of new systems.
In the following, all possible cases of interspecies hybridization in European Clausiliidae, which I have observed myself or found in the literature, are listed. The species rank of the assumed parental species is based on morphology and distributional relationships (Nordsieck 2007: chapter VI). In general, these species occur in more or less close neighbourhood of the assumed hybrid taxa.

II. Genera with observed interspecies hybridization

Abbreviations of collections: SMF = Forschungsinstitut Senckenberg Frankfurt am Main, SMNS = Staatliches Museum für Naturkunde Stuttgart.

Alopiinae, Medora group:

Carinigera:

In Nordsieck (1984: 194) a form intermediate between Isabellaria = Carinigera chelidromia and Cpraestans from Piperi island, northern Sporades, an assumed hybrid subspecies, has been described as Ccpiperica.

Isabellaria:

In Nordsieck (1974: 132) a form intermediate between Isabellaria thermopylarum and Ialmae from Amfissa, Phocis, an assumed hybrid subspecies, has been described as Itfaueri.

Albinaria:

Continental Greece:
In Nordsieck (1974: 137) I communicated on a copulation of Albinaria scopulosa and Asenilis from Plakoti, Epirus.

Ionian Islands:
Kemperman & Degenaars in Kemperman (1992) stated that the subspecies of Albinaria jonica and Aadrianae from Kefalonia were in allozyme frequencies less similar to the other subspecies than to neighbouring subspecies of Acontaminata and Asenilis, respectively (: 142-143, dendrograms figs. 7-8). This might be caused by introgression from hybridization. Introgression was also assumed because of the occurrence of rare alleles in neighbouring populations of different species on the island (: 151-152).
Kemperman & Gittenberger in Kemperman (1992) found that in contact zones of all species taxa from Kefalonia intermediates which have originated by interspecies hybridization occur:
Ac.contaminata and As. senilis (: 185, fig. 9);
Accontaminata and Amaculosa samiensis = mliebetruti (: 191);
Accontaminata and Ajonica assicola (: 207, fig. 17);
Acperiporon and Aaadrianae (: 190, 201, 203, figs. 15A-C);
Acperiporon and Aadrianae dubia (: 190, 204, figs. 15D-F);
Assenilis and Aj.jonica (196, 205);
Assenilis and Aadrianae dubia (: 197, fig. 14).

Peloponnese:
Gittenberger (1987: 84) described Isabellaria Albinaria vrondamasa from Vrontamas, Laconia, as having a closing apparatus = CA intermediate between N- and G-type (with weak spiral lamella and very weak parallel lamella). In the collection of Fauer (Nordsieck 1997: 59) I found a sample from Vrontamas, in which most specimens had a G-type CA, but about a fourth of the specimens a N-type CA or an intermediate CA like the types of Avrondamasa. Obviously this population is the product of a hybridization of I. = Abutoti (G-type CA) and Agrisea (N-type CA).
Gittenberger (1994: 58) described a form intermediate between Adiscolor (N-type CA) and Ahaessleini (G-type CA) from Molai and Elea, Laconia, as Adeureka. In the coursa of a collecting trip in the eastern Peloponnese I could confirm the occurrence of this and further taxa of Ahaessleini with CAs intermediate between N-type and G-type (Nordsieck 2007: 114, 166).
Neubert (1998: 153) communicated on interspecies hybrids between Akrueperi and Agrisea in the northwestern Peloponnese.
In the course of the already mentioned collection trip (Nordsieck 2007: chapter VII, new species taxa 163-168), I discovered in northeasternmost Arcadia three assumed hybrid subspecies as follows:
Apetrosa tanocola, intermediate between Apetrosa and Amixta (: 164);
Alitoraria profugella, intermediate between Alitoraria and Aprofuga (: 164);
Amixta interposita, intermediate between Amixta and Apetrosa (: 165).
Later on (Nordsieck 2015: 6), I described an assumed hybrid subspecies fom Ithome, Messinia: Aarcadica occulta, intermediate between Aarcadica and Amaculosa (= former Aschuchii).

Dia Island:
Wiese (1989: 41-42) communicated on hybrids Albinaria retusa x Atorticollis on Dia island near Crete.
Schultes & Wiese (1990: 32-33) gave the same information, additionally on hybrids Ajaeckeli Atorticollis, observed in a terrarium.
Welter-Schultes (1992) gave more detailed informations on hybrids between A. species from Dia island. He found nearly no hybrids (< 0.1%) of Ateres and Atorticollis. As concerns Aretusa and Atorticollis, 4 of 10 specimens between Ankinara peninsula and mainland Dia were hybrids; other syntopic occurrences were without hybrids.

Crete:
Schilthuizen et al. (1993), in their revision of Albinaria hippolyti, mentioned hybrids Ahhippolyti x Aspratti (: 143) and Ahholtzi x Aspratti as well as Ahholtzi x Aidaea (: 146). Near Milatos cave a broad area with extensive hybridization between Aharthuriana and Amaltzani was said to exist (: 153).
Welter-Schultes (1998) discovered hybrid zones at the boundary of the ranges of Aleonisorum and Acorrugata (: 276, 278) and of Afulvula and Acorrugata (: 278). He found intermediates between Amoreletiana and Acorrugata (: 278). Also at the contact of the ranges of Asturanyi and Ateres a hybrid zone was stated (: 278). The same information was given in Welter-Schultes (2000), and once more that on the hybrid forms Aarthuriana x Amaltzani and Aretusa x Atorticollis.
Gittenberger et al. (2001) found assumed hybrids Acretensis cf. vexans = Atroglodytes niproensis x Asphakiota in the Imbros gorge, Sfakia (: 79).
Nordsieck (2004) mentioned intermediate forms between Atroglodytes and Avirginea in the region west of Rethimno (: 61), between Atenuicostata and Avirginea in westernmost Crete (: 64), and between Asublamellosa and Avirginea in Agia Roumeli, Sfakia (: 61).
Nordsieck (2017) described the hybrid form Aarthuriana x Amaltzani from Milatos cave, which was already mentioned by Schilthuizen et al. (1993), as Aaxenogena (: 14). Furthermore, a hybrid population Atenuicostata x Aloosjesi from Tigani-Balos, Gramvousa peninsula, was communicated (: 13). The sample of Atroglodytes from Agios Nektarios, Sfakia (: 15), was assumed to contain hybrids of Atroglodytes and Asublamellosa. The subspecies described as A.maltzani ecristata (:19-20) is intermediate between Amaltzani and Acorrugata.
Most recently I found hybrids of Arebeli and Ateres from Kavousi, in a sample together with pure Arebeli, designated by A. J. Wagner on the label as syntypes of Arebeli (SMF).
Bamberger et al. (2021), by analysis of genome-wide ddRAD sequences, could prove extensive hybridization between species of the Albinaria cretensis group (: tree fig. 2). The most interesting case is A. vexans from Samaria gorge, which exhibits a genome of A. sublamellosa with shares from three or four other species.

Cristataria:

A. Schmidt (1868: 102-103) was the first who described hybrids between Cristataria fauciata (= Cdelesserti) and Cstrangulata (Clausilia fauciatae similis, C. strangulatae similis; Nordsieck 1971b: 241). Another hybrid form of both species is Csancta Bourguignat (: 243).
Calbersi from Nahr el-Kelb near Beyrouth is in several characters intermediate between Cboissieri and Cstrangulata, which occur syntopically in this valley. It could be a species which has originated by interspecies hybridization (Nordsieck: 243).

Medora:

In the course of the revision of the genus (Nordsieck 1970) I could trace intermediates in shell morphology between several species from central Dalmatia:
Mdalmatina from Živogošće, with characters of Mcontracta (: 32);
Mdalmatina josephinae from Staza pass above Podgora, with characters of Mmacascarensis (: 34);
Mstenostoma klemmi fromTurija cave, transitional to Mdalmatina (: 41);
Malmissana mariae from Dubci, transitional to Mmacascarensis (: 60).
In the supplement of the revision (Nordsieck 2009) a form intermediate between Mmacascarensis and Mstenostoma from Biokovo, between Vošac and Sveti Jure (: 6, 10), has been described.

Agathylla:

Fehér et al. (2013) described and figured interspecies hybrids of Abiloba and Aneutra from the Drin valley in northern Albania (Shkodër and Tropojë districts) (: 8, figs. 4 E-J). In the mtDNA tree one of them (from Koman) is basal within the Abiloba clade. Specimens from the localities concerned could be confirmed by me as intermediates.

LampedusaMuticaria (with figures 1-6):

Giusti et al. (1995: 370-374) communicated on hybrids Lampedusa imitatrix x Muticaria macrostoma from Malta.
Giusti et al. (: 364-365) informed on the examination of the relationships of the known Muticaria species from Malta and Sicily by an allozyme study. As result some Msyracusana were indistinguishable from Mneuteboomi, while others were different. Therefore, they united Msyracusana and Mneuteboomi to one species. The lunellar, which was figured as belonging to Msyracusana (from Fontane Bianche near Siracusa, fig. 404), exhibits an elongation of principalis rudiment like in Mneuteboomi (fig. 407), unlike the lunellar of Msyracusana from Siracusa (type of syracusana Rossmässler, Fig. 1). Both results make probable that the syracusana specimens, which were indistinguishable from Mneuteboomi in the allozyme study, and the syracusana specimen from Fontane Bianche were hybrids Msyracusana x Mneuteboomi.
Colomba et al. (2010) found that Msyracusana and Mneuteboomi are separated as two clades in their COI tree; therefore, they regarded them as different species. On the other hand, they figured forms of Mneuteboomi from Pantalica with palatal plicae more like those of Msyracusana (Colomba et al.: figs. 13-14). In contrast to them, the specimens of Mneuteboomi from Pantalica examined by me (Fig. 6) are more coarsely ribbed and have a lunellar like the type form of the species (Fig. 5).
Colomba et al. (2012) showed that a form of Msyracusana from Cugno Lungo, Mbrancatoi, comes out in their COI tree outside of the clade Msyracusana + Mneuteboomi; therefore, it was treated as a different species. The paratype at my disposal (Fig. 4) differs from Msyracusana only by the sculpture, not in the lunellar.
Another form from Epipoli, which is also not much different from the type form of MsyracusanaMcyclopica, shall be a different species, too (Liberto et al. 2016), because in COI analysis it has a position outside of the clade Msyracusana + Mneuteboomi like Mbrancatoi (Colomba et al. 2019). Two samples at my disposal (Fig. 2) are intermediate between Msyracusana and Mneuteboomi, because the principalis rudiment is not connected with the anterior upper palatal plica. In a form of Mcyclopica from Magnisi (Fig. 3), unknown until now, the principalis rudiment is even somewhat elongated in front.
These results can only be explained by the fact that the Msyracusana specimens genetically close to Mneuteboomi have suffered introgression from that species (like a part of the syracusana specimens examined by Giusti et al., see above), while Mbrancatoi and Mcyclopica are subspecies of Msyracusana, which are genetically more distant from Mneuteboomi.

Figs. 1-6. Muticaria species from Sicily.
Frontal and body whorl dorsal. Shell height = H (mm).

1. Mssyracusana, I, Sicily, Syrakus = Siracusa, holotype of syracusana Rossmässler, SMF 67317; H 14.4.
2. Mscyclopica, I, Sicily, Castello di Eurialo near Siracusa, ex N 1831 = SMNS ZI0119827; H 14.5.
3. Ms. cyclopica, I, Sicily, Magnisi, SMF 349576; H 13.4.
4. Msbrancatoi, I, Sicily, Cugno Lungo near Siracusa, paratype, ex SMF 341353; H 11.7.
5. Mneuteboomi, I, Sicily, Cava d’Ispica, paratype, ex SMF 329825; H 13.4.
6. Mneuteboomi, I, Sicily, Pantalica, ex N 1828 = SMNS ZI0119828; H 15.45.

Alopiinae, other Alopiini:

Montenegrina:

By a mtDNA study of the genus Montenegrina Mason et al. (2020) could find two cases of interspecies hybridization of Albanian species:
Mrugicollis desaretica x Mdofleini prespaensis in Sveti Marena at Lake Prespa, and
Mnnana x Mperstriata callistoma near Lunik.
For other taxa interspecies hybridization can be assumed because of their aberrant position in the mtDNA tree: Specimens of MdrimmeriMt.tomorosi and Mperstriata callistoma each in another clade with another species than the rest of the samples.
Some species, much differing in shell characters but occurring in close neighbourhood, are combined within the mtDNA tree. This leads to the suspicion that the genetical similarity is caused by interspecies hybridization. The following taxa are concerned:
Mdofleini pinteri in the same subclade as Mstankovici (shore of Lake Ohrid);
Mdofleini plenostoma and Mdoccidentalis in the same subclade as Mperstriata drimica (region of Struga);
Mffuchsi in the same subclade as Mrugilabris golikutensis (region of Tepelenë);
Mf. pallida in the same subclade as Mr.lambdaformis (northern Epirus near Albanian frontier);
Mfmuranyii in the same clade as Mtomorosi (Tomorr Mountains).

Herilla:

In the course of the revision of the genus Herilla (Nordsieck 1971a) I found intermediates in shell morphology between some species from Serbia and Montenegro:
Hbosniensis from Javorovo near Rožaj, with a clausilium plate like Hziegleri (: 69-70), later on (1974: 124) described as Hzedlaueri;
Hbosniensis from upper Tara valley and adjacent Durmitor (: 71), with lunellar and clausilium plate like Hziegleri, later on (1974: 125) described as Hbdux;
Hbosniensis from lower Piva valley (: 72-73), with a white surface layer like Hillyrica, described as Hbhannae.

Alopia:

Interspecies hybridization of enantiomorphic or closely related Alopia species has been described in two of my publications (Nordsieck 2007: chapter VI, 102-106 and 2016).
Hybridization between left- and right-coiled Alopia species can be demonstrated phenotypically (by shell morphology), if the CA of one species concerned is more strongly developed than in the other. This is the case in the following species pairs which I have examined (Nordsieck 2016):
Astraminicollis subspecies (left-coiled, CA fully developed) and Alivida subspecies (right-coiled, CA strongly reduced), with syntopic occurrences in several places in the Bucegi Mountains (well-examined: Valea Velicanului, Cheile Tătarului, Valea Suchelniţei with adjacent Bătrâna);
Aglorifica galbina (left-coiled, CA fully developed) and Alivida deaniana (right-coiled, CA strongly reduced), with syntopic occurrences in a part of the Piatra Craiului Mountains (Măgura Mică, Măgura Mare);
Anixa (left-coiled, CA strongly reduced) and Alivida bipalatalis (right-coiled, CA less strongly reduced), with syntopic occurrences in two places in the Bucegi Mountains (Valea Doamnelor with adjacent Bătrâna , Valea Obârşia Ialomiţei).
The hybridization between Anixa and Albipalatalis in Valea Obârşia Ialomiţei could also be proved by examination of the genitalia of the latter (end genitalia tending to those of Anixa, own investigations).
In a former paper (Nordsieck 1984: 191-192) a hybrid form Amauritii (left-coiled, CA strongly reduced) x Ah. helenae (right-coiled, CA fully developed) from Ciucaş Mountains (Gropşoare) has been described as Ahinterjecta. In a recent paper (Nordsieck 2019) I communicated on hybrids of Ahhelenae and Ahzagani from the valley separating Gropşoare and Zăganu, demonstrated by their reduced CA, which they could have got by hybridization with Amauritii or Anefasta.
Hybridization between left- and right-coiled Alopia species cannot be demonstrated phenotypically, if the CAs of both species are equally developed, though it is certainly present. This is, e. g., the case in the following species pairs:
Aglorifica (left-coiled) and Alischkeana (right-coiled), both with fully developed CA, with syntopic occurrences in the main part of Piatra Craiului Mountains (e. g., Agboettgeri and Alobesa in Valea Râului, Agglorifica and Allischkeana in Valea Brusturetului);
Anixa (left-coiled) and Afussi (right-coiled), both with strongly reduced CA, with syntopic occurrences in the higher parts of the Bucegi Mountains (e. g., Obârşia-S).
The hybridization of Aglorifica and Alischkeana is made probable by the subspecies which cluster in COI analysis (Fehér et al. 2013) with the other enantiomorphic species (Agboettgeri from Valea Râului with Alischkeana subspecies, Allischkeana (deceptans) from Valea Brusturetului with Aglorifica subspecies).
The hybridization of Anixa and Afussi is made probable by their clustering in one clade of COI analysis (Fehér et al.), though A.fussi is in shell morphology nearly indistinguishable from Alivida.
In my paper on interspecies hybridization of Alopia (Nordsieck 2016: 7) I communicated on a probable case of interspecies hybridization of Apomatias and Afussi.
One case of interspecies hybridization of Alopia, the best known one (Astraminicollis x Alivida), has been analysed more exactly by a DNA study of Koch et al. (2017). In a further paper (Koch et al. 2020) the authors stated hybridization between all species taxa of the Alivida group from Bucegi Mountains (AstraminicollisAlividaAnixaAfussi) and united them to the (mega)species Alivida, in spite of morphological differences, especially of Anixa and Afussi to the two other species (own investigations).

Alopiinae, Delimini:

Charpentieria:

Some forms of Charpentieria from the southern limestone Alps between Como Lake and Garda Lake are similar to Cstenzii by some shell characters and their habitat, but agree with Citala in essential characters of shell and genitalia (Nordsieck 1963). Therefore, in that paper, they were treated as so-called stenzioid subspecies of CitalaCiclavataCivariscoiCibalsamoi and Cilorinae (Citrepida included).
Their relationships to Citala are different from normal subspecies of that species, but also from Cstenzii (1963: 171, 173).
The normal subspecies of Citala are connected by intermediate forms and do not occur syntopically with other subspecies.
Forms transitional between Cstenzii and Citala have never been found; C.stenzii occurs syntopically with Citala in several localities.
Cilorinae does not occur syntopically with Citala, but is connected with it by transitional forms (allatollae, 1963: 186-187), distributed next to the range of Ci. lorinae.
Ciclavata occurs syntopically with Citala at some localities; a form transitional to Citala is known from Lecco (leccoensis, 1963: 182-183).
The same is true for Ci. variscoi; at one locality (mouth of Val Taleggio) Ci. variscoi has been found together with Citala and transitional specimens.
Cibalsamoi occurs syntopically with Citala; transitional forms have not been found.
In Nordsieck (1984: 171) I proposed to regard the stenzioids as separate species, Cclavata. The transitional forms were interpreted as interspecies hybrids.
Scheel & Hausdorf (2012), in their DNA study, treated the stenzioids as subspecies of Citala. In their AFLP network the stenzioids form five clusters, because C. i. trepida forms a cluster of its own (though this taxon does not differ from Ci. lorinae by shell characters), and the transitional forms (Ciallatollae) an additional cluster.
Xu & Hausdorf (2020), in their genome-wide DNA study, separated the stenzioids in two species. The western stenzioids, because free of genomic shares with C. itala, are treated as independent species, while the eastern stenzioids, which exhibit genomic shares with C. itala, are regarded as subspecies of that species (C. i. lorinae).

Siciliaria:

The assumed interspecies hybrid forms of Siciliaria from Sicily are the following (material in SMF):
Sgrohmanniana from Priola, with characters of Sseptemplicata;
Sseptemplicata from Parco, with characters of Scalcarae;
Sseptemplicata near Cinisi, with characters of Stiberii, described as Ssalcamoensis and Sshemmeni;.
Scalcarae from Monte Belliemi, with the ribbing of Stiberii, described as S. cbelliemii;
Stiberii from Monte della Scala, with characters of Scalcarae;
Sleucophryna from Sferracavallo, with characters of Scalcarae;
Scrassicostata from Scurati near Custonaci, transitional to Seminens.

Delima:

As result of the taxonomic revision of the genus Delima (Nordsieck 1969: 274-275) I found several intermediate forms, especially in central Dalmatia, which have characters of two neighbouring species, but none of their own. These assumed interspecies hybrid forms occur at the boundaries of the ranges of the neighbouring species. They consist of uniform populations and have a more or less extended range, or of less uniform populations between populations of the neighbouring species, or are individuals in syntopic populations of these species. Only the first-mentioned forms are regarded as subspecies of the species, with which they share most characters.
The assumed hybrid forms are the following:
Central Dalmatia (see Nordsieck 2007: chapter VI, map 1):
Intermediate between
Dblanda conspurcata and Dlatilabris = Dbblanda, from Makarska coastland;
Dbconspurcata and Dpachystoma = Dbschmidti, in Vrlika;
Dbconspurcata and Dpachystoma, in Drniš and Petrovo Polje;
Dpachystoma and Dalbocincta, in Drniš;
Dpachystoma and Dlatilabris = Dpsatricensis, between Hrvace and Maljkovo;
Dpachystoma and Dbconspurcata = Dpvicariella, near Muć;
Dlatilabris and Dpachystoma = Dlangusticollis, between Sinj and Muć;
Dlatilabris and Dbconspurcata, in Dicmo Polje;
Dlatilabris and Dbconspurcata, = Dlduarensis, inland of Omiš;
Dalbocincta and Dbconspurcata, in Čikola valley near Drniš;
Dalbocincta and Dbconspurcata = Dasororia, in middle Krka valley;
Dsemirugata and Dbilabiata, in western part of Hvar island.
High Croatia:
Intermediate between
Dlatilabris and Dbinotata, in Gračac.
It is striking that most assumed interspecies hybrid forms originate from species of the Dblanda group. Because of shell and genital differences there is no doubt about their species rank. In the majority of cases Dbconspurcata is involved, a species widespread in central Dalmatia, at least in part distributed by man. The meeting of this species with Dpachystoma in the villages of Drniš and Vrlika (in Drniš also with Dalbocincta), which I investigated in 1965 and 1966, is especially interesting. In these villages I found in several populations both stem species and hybrid forms. In Vrlika Dbconspurcata is modified by introgression from Dpachystoma to a subspecies intermediate between both species (described as Dbschmidti H. Nordsieck). I assume that Dbconspurcata was introduced by man and hybridized there with the autochthonous Dpachystoma (in Drniš also with Dalbocincta from Čikola valley, via Dpachystoma which is intermediary in size).

Barcania:

Brandt (1956, 1958), in his papers on the clausiliids of Cyrenaica (Barcania), described some subspecies which are intermediate between B. kaltenbachi and other B. species:
Bkaltenbachi albaensis (1956: 131) from the eastern part of the species range, intermediate between Bkaltenbachi and Bapolloniana;
Bchaligi tigaensis (1958: 5) from the western part of the species range, intermediate between Bkaltenbachi and Bchaligi;
Bksusaensis (1958: 6), restricted to Wadi Haulla, intermediate between Bkaltenbachi and Bklugi.
B
klugi from Wadi en Nsuria had been described as separate species (1956: 133), because it differs in shell characters from the surrounding Bkaltenbachi by being similar to Bbengasiana. After the discovery of the ribbed subspecies Bklugi streyi (1958: 6) from Wadi Sneides, it was downgraded to a subspecies of Bkaltenbachi, because west of Wadi Sneides Bkaltenbachi with transitions to Bklstreyi had been found. The examination of the material available to me (SMF) showed that in Wadi en Nsuria Bkaltenbachi and Bkl. streyi occur syntopically, without transitions, while in other valleys (Wadi Sneides, Wadi Haulla) transitional populations exist, in Wadi Haulla even regarded as subspecies Bksusaensis (see above). Obviously there is interspecies hybridization between Bkaltenbachi and Bklugi.
Bsasaensis from Wadi Zaza, which has a smooth shell like Bkaltenbachi, is regarded as separate species (1956: 126). In both wadis neighbouring Wadi Zaza the ribbed subspecies Bscostellata has been found. In one of these wadis it is more coarsely ribbed, like Bbengasiana, with which it occurs syntopically (Nordsieck 2019). Obviously there is interspecies hybridization between Bsasaensis and Bbengasiana.

Clausiliinae:

Clausilia (Strobeliella):

In Nordsieck (1966: 30, 2013: 52) I described a population of Cexoptata from Bracca Serina, Val Serina, Bergamasque Alps, which occurred together with Cwhateliana (20% of the specimens) and a small percentage of transitional specimens (8%). Both taxa are clearly different in shell and genital morphology, so that an introgression at secondary contact was concluded. Therefore, until now I treated Cexoptata as a separate species and its hybridization with Cwhateliana as interspecies hybridization.
Because of the gene flow stated in a DNA study, Hausdorf & Nägele (2016) classified Cexoptata in spite of the morphological differences as subspecies of Cwhateliana.

Micropontica (Baleopsina):

The subgenus includes the species Mcircassica with N-type CA and Mcaucasica with an incipient G-type CA. A form of Mcaucasica, with a transitional CA, has been described as Mcinterjecta . This name is used by Koch et al. (2016) in a DNA study for the M. (Baleopsina) populations from the Lagonaki Mountains , northwestern Caucasus, which have characters intermediate between both species and form a genetic unit of their own, which shall have originated by interspecies hybridization. This unit, however, includes specimens with very different morphological characters (e. g., Mannae with strongly developed N-type CA and pure Mcaucasica).

Mentissa:

Baidashnikov (1990: 29-30) stated that Mvelutina from S. W. Crimea, separated by him as independent species from Mgracilicosta because of differences in shell and genitalia, is connected with that species by transitional forms at the boundary of the range in Ai-Petri and the high plateau of Čatir Dag. He therefore (: 31) considered Mvelutina as a „semispecies“. Neiber et al. (2019), after a DNA study of Mentissa, treated Mvelutina in spite of the differences as subspecies of Mgracilicosta, because in their tree representing a subclade of the Mgracilicosta clade. On the other hand, they found gene flow between the species Mgracilicosta and Mcanalifera.

Interspecies hybridization and ecology

In all described examples of interspecies hybridization rock-dwelling clausiliids, mostly from southern Europe, are concerned. Within the genera which are widespread in the woodlands of remaining Europe, Cochlodina (Alopiinae), Macrogastra and Clausilia (C.) (Clausiliinae), no case of interspecies hybridization is known, though a great amount of material of the species belonging to has been examined. An exception could be the assumed hybrids of Cochlodina laminata and Cdubiosa, two closely related species, which I have found in the southern Alps.
While the species of rock-dwelling clausiliids have in general similar copulatory organs, which make possible interspecies hybridization, in the forest-dwelling species these organs are more or less different. Contrary to the rock-dwelling clausiliids, which as a rule occur allopatrically, for forest-dwelling species, which often occur syntopically, interspecies hybridization might be disadvantageous, because they have different ecological niches.

III. Conclusions for the species concept

For future research it is predicted that there will arise severe problems, when authors will try to reconstruct the phylogeny within clausiliid genera from southern Europe, in which interspecies hybridization is common. It will certainly not be sufficient to propose phylogenies and new systems without considering this phenomenon.
On the other hand, uniting species which demonstrably hybridize to megaspecies will also not be an satisfactory solution. The arguments are as follows:
1. Imbalance within a genus: Investigations which prove interspecies hybridization are as a rule focussed within a genus to certain cases with two or few species concerned; in uniting these species the other cases with similar preconditions in the same genus are not considered. If one unites certain species with interspecies hybridization within a genus to megaspecies, one must do this also with the other species with such hybridization in order to avoid an inappropriate imbalance. A good example is the Alopia livida group from Bucegi Mountains, Romania (Koch et al. 2020, see part II).
2. Imbalance in comparison with other genera: To treat species of a genus (e. g., AlbinariaAlopiaDelima) which hybridize, as subspecies, which are on the other hand morphologically and from their distributional relations as well characterized as other species of another genus (e. g., Montenegrina), in which species because of isolation mostly cannot hybridize, or other species of another genus (e. g., Cochlodina), in which species do not hybridize, leads to another inappropriate imbalance. The ability to hybridize is a character of the species of certain groups; species of these groups must remain comparable with species of groups without that ability.
3. Obscuring of diversity: A further consequence of these unions is a loss of information by obscuring the diversity. If within these megaspecies subspecies of the former species and the former species, which now are subspecies, are recorded, these have different rank. This leads inevitably to a neglecting of the subspecies of lower rank, with consequences for their perception as taxa and for their eventual protection, the same problem as discussed for neglected subspecies of polytypic Albinaria species.
Therefore, species of groups which hybridize, but are on the other hand comparable with species of other groups which cannot or do not hybridize, should not be united to megaspecies, but remain to be treated as species.

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