Dextral Clausiliidae (Gastropoda, Stylommatophora), an evolutionary problem

Hartmut Nordsieck

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, 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. Dextrality obviously evolved secondarily within the family and several times independently.

List of dextral groups

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. The dextrality within the subgenus S. (Excussispira) evolved more than once (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; n. sp. Thailand (near Trang).
O. cuongi + O. eregia and O. duci + O. miranda are closely related. The dextrality within the subgenus O. (Oospira) evolved at least three times.
Oospira (Formosana) O. Boettger 1877:
antilopina Heude 1885; kiangshiensis Gredler 1892; moschina group: dextrogyra Bavay & Dautzenberg 1909, kongshanensis H. Nordsieck 2007, moschina Gredler 1888, moschinella H. Nordsieck 2007, n. sp. Guangxi, ooharai H. Nordsieck 2007, psilodonta Heude 1889; semprinii Gredler 1884.
O. antilopina + O. kiangshiensis + O. semprinii are closely related (Dextroformosana Boettger & Schmacker 1894), and the moschina group may be monophyletic (Nordsieck 2007b: 219), thus dextrality within the subgenus O. (Formosana) evolved at least two times.
Oospira (Leptacme) Ehrmann 1927:
sykesi Bavay & Dautzenberg 1899.
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 by A., N. & P. Reischütz (2010).

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 (Nordsieck 2005: 204).

Alopiinae A. J. Wagner 1913

Albinaria Vest 1867:
For Albinaria sensu lato see Nordsieck (2007a: 44 ff.).
[Cristataria Vest 1867]:
colbeauiana L. Pfeiffer 1861.
A closely related normally (left-) coiled species is A. leprevieri inversa Szekeres (Szekeres 1998: 169). It was described as subspecies of A. colbeauiana, but differs from it in some shell characters.
[Albinaria]:
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 (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. (2006: 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 two times.
Leucostigma A. J. Wagner 1919:
candidescens Rossmässler 1835 (three dextral subspecies, looking like mirror images of certain normally (left-) coiled subspecies).
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; lischkeana Charpentier 1852; livida Menke 1828; meschendorferi Bielz 1858; nefasta M. Kimakowicz 1894.
Six dextral species occurring in four mountain-ranges of the southern Carpathians live side by side with normally (left-) coiled counterpart species (Nordsieck 2008: 15). As far as known, normally coiled counterparts of A. bielzii and A. meschendorferi do not exist.
Most dextral species of Alopia are not closely related to other ones; thus, dextrality may have evolved several times.
Summing up, it is concluded that dextrality within the subfamily Alopiinae evolved several times.

Dextral subspecies of Leucostigma candidescens

Until now, 42 species of Clausiliidae are known as dextral, which is proportionally about 3% of the described living species. Thus, dextrality is a rare phenomenon within the family.
Dextral species occur only in certain subfamilies and in certain groups within those:
Phaedusinae: Synprosphymini, oospiroid groups of Phaedusini;
Serrulininae;
Neniinae;
Alopiinae: Medorini, Alopiini.
Dextral species are not known to occur in some more derived 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.

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 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 only dextrally coiled offspring.

In the given survey of dextral Clausiliidae three groups are recognized: 1. dextral subspecies of normally (left-) coiled species (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).
Oppositely coiled subspecies of Leucostigma candidescens have allopatric ranges. Only in one case, syntopy of a dextral subspecies with a normal (sinistral) one has been observed (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 species pairs of Alopia which occur together in four mountain-ranges of the southern Carpathians (Nordsieck 2008: 15). In their characters these species pairs are no perfect mirror images, but exhibit slight differences in shell and, as far as examined, also in genital characters. Some species pairs of Alopia also differed in the allozyme ("esterase") electropherograms which they produced (Grossu & Tesio 1972: 337-341). Unlike subspecies, their ranges are often not coherent and separate from that of the counterparts, but mosaic-like meshed together (Nordsieck 2007a: 104, 2008: 15-16). Therefore, these taxa are regarded as species.
The Alopia species pairs 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 is known that some oppositely coiled species of Alopia hybridize to a certain extent. The hybridization is phenotypically manifest when the clausilial apparatus of both counterparts differs considerably (Szekeres 1976: 390, Nordsieck 2007a: 105-106).
Syntopic occurrences and hybridization of oppositely coiled 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 were dominant, a certain percentage of left-coiled individuals should appear in the populations of the dextral species. But this is not the case. All known populations of dextral species which occur parapatrically to those which by contact with the sinistral counterpart contain hybrids are uniquely dextrally coiled. Therefore, I assume that in dextral Alopia species the dextral allele is dominant.

References

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