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Chiral Instability in Alopia snails

Alopia, an endemic snail genus of the Carpathian Mts. (E Europe), is unique by the comparable representation of sinistrally and dextrally coiled taxa, and the existence of enantiomorph taxon pairs that seemingly differ only in their chirality.

Zoltán Fehér

on 21 February 2013

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Transcript of Chiral Instability in Alopia snails

Zoltán Fehér — Hungarian Natural History Museum
Miklós Szekeres — Biological Research Center of the Hungarian Academy of Sciences Chiral instability as an important driver of speciation in the Carpathian rock-dwelling gastropod genus Alopia Characteristic features of
the Clausiliidae family Conclusions Results Clausilium plate makes this family unique
This is a spoon-like calcareous structure Alopia glauca
www.eol.com Diversification of the clausiliids
during the Tertiary Period Subtropical periods of the Pliocene were dominated by large size species Southeastern Europe and Western Asia in the Upper Pliocene Andreescu et al. (2012) Quaternary International During the Pliocene, the Alopiinae subfamily has evolved in the eastern Mediterranean region, in parallel with the extinction of large size clausiliids Smaller, more elongated shell shape
The wall of the shell is thinner
Shell surface often white (light protection) and ribbed (against drought)
Often strong body pigmentation Among Alopiinae, the rock-dwelling life form became predominant Most of the Mediterranean Alopiinid species (>90%) practice 'negative hibernation', i.e. they are inactive during the summer The Alopiinae subfamily Present-day distribution One species is a Holocene success story, contributing about half of the subfamily's distribution area
Most of the genera are limestone rock-dwelling groups with well defined, narrow geographical ranges. Distribution of Alopia and some closely related genera Highest alopiinid diversity is found in the Eastern Mediterranean area
Europe was conquered through the Balkan Peninsula.
Alopia is the northernmost of many genera that have compact ranges What makes Alopia so peculiar? Unusually among clausiliids, there are a large number of dextral forms in Alopia (23 of the 78 known taxa)
Six of these dextral taxa has mirror images, i.e. oppositely coiled but otherwise identical morphs (a.k.a. enantiomorph taxon pairs)
Controversial taxonomic concepts. (Due to the disagreement over the homologous or homoplastic origin of the dextral subspecies - Bielz (1861) and Wagner (1914) vs. Kimakowicz (1894)) Enantiomorph taxon pairs Enantiomorph pairs always occur in contiguous ranges But sympatric occurrence is very rare Sinistral and dextral subspecies of A. lischkeana in the Piatra Craiului Mts. Objectives To explain the unusually high number of dextral forms in Alopia. There are two competing hypotheses - one early reversion vs. multiple independent reversion events. Which one is true? One early reversion or
multiple independent reversions? Based on a 655 bp coding segment of mitochondrial cytochrome c-oxidase subunit I (COI)
Best studied marker within the Alopiinae
Unambiguously alignable sequences
Fairly variable to reveal intra-generic relationships (in Alopia, 193 variable positions, i.e. 27 %)
We analysed 90 samples, representing 84 haplotypes
Herilla, Medora and Albinaria were used as outgroups
Bayesian, ML, MP and NJ trees
Divergence time estimation: constrained Bayesian tree, two analyses using 1% and 8.6% clock rates Molecular
phylogenetic reconstruction Ingroup topology was unambiguous, but it was difficult to define the rooting point Estimated times of Alopia and Carinigera diversification Due to low dispersal capabilities and the fragmented distribution of rock-dwelling gastropods, allopatric (a.k.a. geographic) speciation results in a large number of narrow-range, morphologically distinguishable morphotypes.
These morphotypes are traditionally considered as subspecies. To reveal the phylogenetic relationships between enantiomorph taxon pairs. Are they closely related or do they exemplify extreme morphological convergence? To draw taxonomic conclusions. How should we consider coiling direction in taxon delimitation? Which of the two contrasting taxonomic concepts reflects phylogeny better? Dextral taxa do not form a monophyletic group Close evolutionary relationships between enantiomorph taxa (0-2 bp differences) indicate recent diversification events Bayesian unrooted tree Positions of dextral taxa within the tree Phylogenetic relationships between enantiomorph pairs Compared to related genera, Alopia radiation seems to be a more recent event (e.g. compared with Carinigera) COI sequence divergence in Alopia and Carinigera The higher clock rate (8.6 %) seems to be in better accordance with the scenario based on the paleogeographic history, i.e. Alopia colonized the Carpathians later then the Upper Pliocene Clausiliids are chacterized by spindle-shaped shell
In contrast to most of the gastropods they are sinistrally coiled Rock-dwelling Alopiinid gastropods live on isolated limestone outcrops in topographically complex terrain, therefore populations occur in isolated patches Large number of taxa Distribution of Alopia in the Carpathians To make - at least a relative - divergence time estimation. How old is the Alopia radiation compared to related taxa? Taxonomy Developmental genetics Evolution Phylogenetic background for a revision Coiling direction is a homoplasious character. It has no sense to classify inversely coiled but otherwise morphologically similar morphotypes into distinct species.
A comprehensive systematic revision of the genus will be provided in a forthcoming taxonomic publication. Multiple independent chiral inversions indicate that genetic determination of chirality is less stable in Alopia than in other genera, offering Alopia species as attractive experimental models for studying the genetic background of chiral stability A prospective model organism? A possible way to evolve new taxa Chiral inversions occur frequently
Restricted interchiral mating (restricted gene flow)
Inverted individuals occasionally succeed in establishing stable subpopulations
This gradually leads to territorial separation and, in time, the formation of isolated sinistral and dextral populations
Enantiomorphs are now in this phase of the speciation
It is followed by further phenotypic and spatial divergence

In addition to the normal mechanisms of allopatric speciation, chiral instability is another factor that contributed to the evolution of large number of taxa in this genus Clausiliids are equipped with an intricate closing apparatus
This is situated inside the last whorl and consists of multiple lamellae and plicae that are useful morphological markers for taxonomists Clausilium plate is situated inside the last whorl and efficiently seals the entrance when the snail retreats The Alopia genus Lives on exposed limestone cliffs between 500 m and 2500 m above sea level
Distributed in the Carpathians (there is one remote occurrence in the Northern Carpathians, but the diversity hot-spot is in the Southern Carpathians)
Due to the paleogeographic history, it is supposed that Alopia colonized the Carpathians not earlier then the Upper Pliocene
Rich in taxa: 23 species and 78 subspecies (according to Fauna Europaea) An almost typical rock-dwelling genus But not entirely typical
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