Dextral Clausiliidae (Gastropoda, Stylommatophora), an evolutionary problem
Hartmut Nordsieck (VIII.2013)
I. General remarks
Land snails have asymmetrical organs, therefore they are chiral, sinistral or dextral. This is externally visible in the coiling of the shell and the position of the genital opening. Chirality is determined by a single gene (locus) with two alleles, sinistral and dextral. The alleles are expressed by the direction of spiral cleavage of the fertilized oocyte, leiotropic or dexiotropic. One allele is dominant over the other, that is landsnails are left-chiral (left-coiled, with left situs of genitalia) or right-chiral (right-coiled, with right situs of genitalia). The genotype of the egg-producing parent determines the chirality of the offspring ("delayed inheritance" = maternal effect).
Clausiliidae are in general sinistral = left-coiled, with left position of genital opening. The sinistral allele is dominant over the dextral one (Degner 1952).
The majority of other stylommatophoran groups is dextral = right-coiled, with dextral allele dominant over the sinistral one, thus dextrality may be plesiomorphic in the Stylommatophora on the whole. The shift in direction of coiling in the ancestors of Clausiliidae must have been associated with a shift in dominance of the chirality alleles. Clausiliidae have with high probability a monophyletic origin (Nordsieck 2007a, Uit de Weerd & Gittenberger 2013), therefore sinistrality may be plesiomorphic within the family.
Some groups of Clausiliidae are dextral, externally visible in the right coiling. Such groups have been found in several subfamilies. Obviously, dextrality evolved secondarily within the family and several times independently.
II. List of dextral groups
In comparison to that of Gittenberger et al. (2012), the following list is updated and more detailed.
Phaedusinae A. J. Wagner 1922
Synprosphyma (Excussispira) Lindholm 1925:
fargesianella (Heude 1885), inversa (Heude 1886), pseudinversa H. Nordsieck 2001.
A normally (left-) coiled species closely related to S. inversa is S. retorta (Heude 1890). With high probability, the dextrality within the subgenus S. (Excussispira) evolved at least twice (Nordsieck 2001: 39).
Oospira (O.) Blanford 1872:
bouddah (Bavay & Dautzenberg 1912), cuongi (Maassen & Gittenberger 2007), duci Maassen & Gittenberger 2007, eregia (Szekeres 1969), miranda (Loosjes & Loosjes-van Bemmel 1973), oviformis H. Nordsieck 2011, raehlei H. Nordsieck 2013.
O. bouddah and O. oviformis, O. cuongi and O. eregia and O. duci and O. miranda from Vietnam are closely related. Thus, dextrality within the subgenus O. (Oospira) could have evolved at least four times.
Oospira (Formosana) O. Boettger 1877:
antilopina (Heude 1885), kiangshiensis (Gredler 1892), semprinii (Gredler 1884); moschina group: dextrogyra (Bavay & Dautzenberg 1909), elegans H. Nordsieck 2012, kongshanensis H. Nordsieck 2007, liujinae Maassen 2008, moschina (Gredler 1888), moschinella H. Nordsieck 2007, ooharai H. Nordsieck 2007, psilodonta (Heude 1889).
O. antilopina, O. kiangshiensis and O. semprinii are closely related (Dextroformosana Boettger & Schmacker 1894). The moschina group may be monophyletic (Nordsieck 2007b: 219), thus dextrality within the subgenus O. (Formosana) could have evolved at least twice.
Oospira (Leptocochlea) Grego & Szekeres 2012:
sykesi (Bavay & Dautzenberg 1899).
Miraphaedusa (Falsiluna) Grego & Szekeres 2011:
harryleei (Grego & Szekeres 2011).
Sinigena Lindholm 1925:
bisdelineata (Heude 1885), vincotiana (Heude 1885).
Streptodera Lindholm 1925:
trachelostropha (Moellendorff 1885).
Summing up, it is concluded that dextrality within the subfamily Phaedusinae evolved several times.
Serrulininae Ehrmann 1927
Tsoukatosia Gittenberger 2000:
christinae A. & P. Reischütz 2003, liae Gittenberger 2000, subaii Hunyadi & Szekeres 2009.
Survey and indication of a fourth species are given by A., N. & P. Reischütz (2010).
Garnieriinae C. Boettger 1926
Euryauchenia H. Nordsieck 2007:
demangei (Bavay & Dautzenberg 1909) (d. n. subsp., looking like a mirror image of the normally (left-) coiled = enantiomorph subspecies d. demangei, figs. 1-2).
Because of differences in genital morphology, Euryauchenia is separated from Tropidauchenia Lindholm 1924 as an independent genus.
The new subspecies will be described in the next future (Nordsieck in press).
Neniinae Wenz 1923
Incaglaia Pilsbry 1949:
dextroversa (Pilsbry 1949).
I. dextroversa is an independent species, closely related to the normally (left-) coiled species I. adusta (O. Boettger 1880) (Nordsieck 2005: 204).
Alopiinae A. J. Wagner 1913
Albinaria Vest 1867:
For Albinaria sensu lato = AICC-group see Nordsieck (2007a: 44 ff.).
[Cristataria Vest 1867]:
colbeauiana (L. Pfeiffer 1861).
A closely related normally (left-) coiled species is A. leprevieri inversa (Szekeres 1998) (Szekeres 1998: 169). It was described as a subspecies of A. colbeauiana, but like other subspecies of A. leprevieri (Pallary 1929) differs from it in some shell characters.
dextrorsa (O. Boettger 1877); voithii group (Laconica O. Boettger 1878): gerolimena H. Nordsieck 1974, menelaus (E. Martens 1873), voithii (Rossmässler 1836).
A normally (left-) coiled species closely related to A. dextrorsa is A. torifera (O. Boettger 1885) (Nordsieck 1972: 10-12, Uit de Weerd et al. 2006: 157). It differs from A. dextrorsa in some shell and genital characters. The conclusion of Uit de Weerd et al. (: 161) that dextrality within A. dextrorsa evolved more than once appears premature, because the species is not yet revised.
The voithii group may be monophyletic (Nordsieck 1999: 8-9, note 6), thus dextrality within [Albinaria] evolved at least twice.
Leucostigma A. J. Wagner 1919:
candidescens (Rossmässler 1835) (l. convertita (Flach 1907), l. paraconvertita H. Nordsieck 2011, l. dextromira H. Nordsieck 2011, looking like mirror images of certain normally (left-) coiled = enantiomorph subspecies, figs. 3-4).
The dextrality within the species evolved at least twice (Nordsieck 2011).
Alopia (A.) H. & A. Adams 1855:
bielzii (L. Pfeiffer 1849), fussi (M. Kimakowicz 1894), helenae R. Kimakowicz 1928, hildegardae R. Kimakowicz 1931, hirschfelderi H. Nordsieck 2013, lischkeana (Charpentier 1852), livida (Menke 1828), meschendorferi (Bielz 1858), nefasta (M. Kimakowicz 1894).
Six of the dextral species occurring in four mountain-ranges of the southern Carpathians live side by side with normally (left-) coiled counterpart = enantiomorph species (Nordsieck 2008: 15). Normally coiled counterparts of A. bielzii and A. meschendorferi have not been found.
Dextral species of Alopia differ from each other as much as sinistral ones. COI analysis has shown that dextrality has evolved several times (Fehér et al., 2013).
Summing up, it is concluded that dextrality within the subfamily Alopiinae evolved several times.
Until now, 45 described Clausiliidae species are known as dextral, which is proportionally about 3% of the described living species. Thus, dextrality is a rare phenomenon within the family.
Until now, dextral species have been found only in certain subfamilies and in certain groups within these subfamilies:
Phaedusinae: Synprosphymini, oospiroid groups of Phaedusini;
Alopiinae: Medorini, Alopiini.
Dextral species are not known to occur in some other groups, such as phaedusoid group of Phaedusini, Delimini of Alopiinae, and the whole Clausiliinae subfamily group.
It is interesting to note that Cretaceous (Campanian) clausiliids from Catalonia are both sinistral and dextral (Nordsieck 2000: 7, note 4). In Tertiary groups, only three species of the Disjunctaria group (Serrulininae?, Nordsieck 2000: 7-8, note 5) from Middle Eocene are dextral. There are no dextral species among the post-Eocene Clausiliidae from central and western Europe.
The list shows, that among the Clausiliidae reversals to dextrality occurred certainly more often than fourteen to twenty times, as assumed by Gittenberger et al. (2012: 2-3).
III. Evolution of dextrality
In populations of normally (left-) coiled clausiliid species mutations of the sinistral allele to the dextral one can happen, and dextral mutants can appear. When the frequency of the dextral allele has increased, dextral specimens occur in the population to a certain percentage. This is the case in the population of Alinda biplicata (Montagu 1803) from Lauenburg, Germany, where several dextral specimens have been found (Degner 1952). But this marks only the first step of evolution towards dextrality. Contrary to that population of A. biplicata, all known populations of dextral subspecies and species are uniquely dextrally coiled and produce as far as observed only dextrally coiled offspring.
In the given survey of dextral Clausiliidae three groups are recognized:
1. dextral subspecies of normally (left-) coiled species (Euryauchenia demangei, Leucostigma candidescens);
2. dextral species, closely related to normally coiled species and looking like their mirror images (about one third of the dextral species listed above);
3. dextral species not closely related to normally coiled species (the majority of the species listed above).
The three groups could represent stages of the evolution of dextrality.
Dextral subspecies can only originate by fixation of the dextral allele in an isolated population.Van Batenburg & Gittenberger (1996: 284-285) have shown by computer experiments that this fixation is easier when the dextral allele is dominant. This dominance would only be proven by crossing-experiments with sinistral mutants. Clues pointing towards that dominance, however, are given by the relations of the oppositely coiled taxa in the field.
Successful mating between oppositely coiled clausiliids is possible, as is proven by the hybridization between oppositely coiled species of Alopia and observations of relevant copulations (Nordsieck 2007a: 103-106, see website art. 2012).
Oppositely coiled subspecies of Leucostigma candidescens have allopatric ranges. Only in one case, syntopy of a dextral subspecies with the enantiomorph subspecies has been observed (c. dextromira, Nordsieck 2011).
The oppositely coiled species pairs of Albinaria (colbeauiana-inversa, dextrorsa-torifera) have allopatric ranges. They differ somewhat in their characters.
This is also true for the enantiomorph species of Alopia which occur together in four mountain-ranges of the southern Carpathians (Nordsieck 2008). In their characters these species are no perfect mirror images, but exhibit slight differences in shell and, as far as examined, also in genital characters. In general, they also differ in the COI sequences (Fehér et al. 2013). Unlike subspecies, their ranges are often not coherent and separate from that of the enantiomorph species, but mosaic-like meshed together (Nordsieck 2007a: 104, 2008: 15-16). Therefore, these taxa are regarded as species.
The enantiomorph Alopia species have only few syntopic occurrences. Among the localities, where I have collected the respective species, in about 10% the oppositely coiled species occurred syntopically. In nearly half of those syntopic occurrences, I found the less frequent species to a portion of 10% or more (Nordsieck 2008: 16-17).
It could be shown that some enantiomorph Alopia species hybridize to a certain extent. The hybridization is phenotypically manifest, if the clausilial apparatus of the enantiomorphs differs considerably (Szekeres 1976: 390, Nordsieck 2007a, 2012, see website art. 2012). It must be expected that the differences in COI sequences (Fehér et al. 2013) can be cancelled by hybridization (see website art. 2013).
Syntopic occurrences and hybridization of enantiomorph species permit introgression of alleles from one species into the other. This is also true for the chirality alleles. The dextral species achieves the sinistral allele by introgression and vice versa. If the sinistral allele would be dominant also in dextral species, a certain percentage of left-coiled individuals should appear in the populations of the dextral species close to sinistral ones, but this is not the case.
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