Highly polymorphic microsatellite markers for Radix balthica (Linnaeus 1758)

We present data for eight polymorphic microsatellite markers isolated from a microsatellite-enriched DNA library for the freshwater snail Radix balthica. Three of them were specific for R. balthica while five also amplified polymorphic products in two congeneric species. Test application on populations from all over the species range has shown that these loci are highly informative for analysing population structure and estimating migration rates. Observed deviations from Hardy-Weinberg equilibrium are attributed to a mixed mating system.     

田螺科五种螺的核型研究

以早期胚胎细胞为材料,用火焰干燥法制片,对分布于我国湖北省武汉市近郊的常见田螺科(Viviparidae)五种螺的核型进行了分析。结果:两种圆田螺的染色体数和国外报道的同一属的种类的一致。而三种环棱螺的染色体数,则较国外报道的另两种的少得多。在铜锈环棱螺的核型中,其m组的第一对和sm组的第四对染色体上,具有明显的随体,出现频率甚高。     

Development of 10 microsatellite loci in the pulmonate snail Biomphalaria kuhniana (Mollusca, Gastropoda)

We characterized 10 variable microsatellite loci in the freshwater snail Biomphalaria kuhniana, as well as conditions for multiplexing and co-loading sets of loci. Two to five alleles were detected per locus over the two studied populations in Venezuela. High inbreeding coefficients suggest high selfing rates. Cross-species amplification provided some variability at eight and three loci in the other species belonging to the Biomphalaria straminea complex (B. straminea and B. intermedia), but was unsuccessful in more divergent species.     

Characterization of 15 microsatellite loci in the pulmonate snail Pseudosuccinea columella (Mollusca, Gastropoda)

We characterized 15 new variable microsatellites in the freshwater snail Pseudosuccinea (Lymnaea) columella, as well as conditions for multiplexing and simultaneously genotyping sets of loci. Two to six alleles were detected per locus over the six populations studied. Gene diversity ranged from 0.000 to 0.498, but essentially no heterozygous individuals were observed. This resulted in extremely high F(IS) estimates, and therefore high selfing rates. The F(ST) estimates ranged from 0.18 to 1 among populations, but was generally high. These markers will constitute efficient tools for investigating the population structure of this invasive species. Cross-species amplification was on the whole unsuccessful.     

Characterization of 15 polymorphic microsatellite markers in the freshwater snail Aplexa marmorata (Mollusca, Gastropoda)

We describe 15 new variable microsatellites in the freshwater snail Aplexa (Physa) marmorata, as well as conditions for multiplexing and simultaneous genotyping sets of loci. Two to 11 alleles were detected per locus over the five populations studied, and up to eight alleles per locus and population. The observed heterozygosity was extremely low, most probably as the consequence of high selfing rates. The genetic differentiation among populations was large. These markers will constitute efficient tools for investigating the population structure in fragmented habitats. Cross-species amplification in three Physa species (P. acuta, P. fontinalis and P. gyrina) was rather unsuccessful.     

How do gastropods grow synchronized shell sculpture? Effect of experimental varix manipulations on shell growth by Ceratostoma foliatum (Muricidae: Ocenebrinae)

Gastropod shells display striking patterns in both color and sculpture, but rather little is known about the developmental mechanisms that produce those patterns. Here, we tested a physical feedback hypothesis for how snails control spatial patterning of shell sculpture. Varices—a form of synchronized, blade‐like axial sculpture—are produced at regular intervals around the shell and often aligned closely between adjacent whorls. Older varices were believed to provide a spatial cue about where to position a new varix. To test this hypothesis, we manipulated physical cues by cutting off varices or attaching new ones to the body whorl of individuals of Ceratostoma foliatum, and then allowing snails to grow a new varix. We found that previous varices on the shell were neither necessary nor sufficient to induce a new varix at a particular location. However, the position of older varices did appear to affect the fine tuning of subsequent varix placement. The results of our experiments therefore suggest that varix synchrony arises mainly from some internal mechanism that yields a standardized amount of spiral growth per growth spurt. We also found that shell damage can induce varix production in unusual or aberrant locations during subsequent shell growth.     

Further twists in gastropod shell evolution

The manner in which a gastropod shell coils has long intrigued laypersons and scientists alike. In evolutionary biology, gastropod shells are among the best-studied palaeontological and neontological objects. A gastropod shell generally exhibits logarithmic spiral growth, right-handedness and coils tightly around a single axis. Atypical shell-coiling patterns (e.g. sinistroid growth, uncoiled whorls and multiple coiling axes), however, continue to be uncovered in nature. Here, we report another coiling strategy that is not only puzzling from an evolutionary perspective, but also hitherto unknown among shelled gastropods. The terrestrial gastropod Opisthostoma vermiculum sp. nov. generates a shell with: (i) four discernable coiling axes, (ii) body whorls that thrice detach and twice reattach to preceding whorls without any reference support, and (iii) detached whorls that coil around three secondary axes in addition to their primary teleoconch axis. As the coiling strategies of individuals were found to be generally consistent throughout, this species appears to possess an unorthodox but rigorously defined set of developmental instructions. Although the evolutionary origins of O. vermiculum and its shell's functional significance can be elucidated only once fossil intermediates and live individuals are found, its bewildering morphology suggests that we still lack an understanding of relationships between form and function in certain taxonomic groups.     

Tuning in to multiple predators: conflicting demands for shell morphology in a freshwater snail

1. We examined the response to chemical cues from fish and crayfish, two predators with contrasting feeding modes, and their single and combined effect on shell morphology in the freshwater snail Radix balthica. 2. Snails were subjected to four treatments: tench (Tinca tinca), signal crayfish (Pacifastacus leniusculus), a combination of tench and signal crayfish and no predators (control). Shell shape, crushing resistance and shell thickness were quantified. We also analysed whether shape or shell thickness contributes most to crushing resistance. 3. Chemical cues from the fish induced a rounder shell shape in R. balthica, a thicker shell and a higher crushing resistance, whereas crayfish chemical cues had no effect on shell morphology, shell thickness or crushing resistance. Shell shape contributed more to crushing resistance than shell thickness. 4. The combined predator treatment showed an intermediate response between the fish and crayfish treatments. Shell roundness was reduced compared with the fish treatment, but the reduced crushing resistance that comes with a less rounded shell was compensated by an increased investment in extra shell material, exceeding that of the fish treatment. 5. Our study extends previous studies of multipredator effects on phenotypically plastic freshwater snails by showing that the snails are able to fine‐tune different elements of morphology to counter predator‐specific foraging modes.     

Development of polymorphic microsatellite loci in the hermaphroditic freshwater snails Drepanotrema surinamense and Drepanotrema depressissimum

We characterized new variable microsatellites in two congeneric species of hermaphroditic freshwater snails (Drepanotrema depressissimum and D. surinamense), as well as conditions for multiplexing and simultaneously genotyping sets of loci. D. depressissimum had high mean gene diversity (> 0.8) and large number of alleles (= 10.9) per population. Most loci and populations were at Hardy–Weinberg equilibrium. The F_ST estimates were low among lesser Antilles populations and larger with a Venezuelan population. Far less diversity was found in D. surinamense with mean number of alleles and gene diversity per population of 2.8 and 0.34, respectively. Very few heterozygous individuals were observed. The most likely explanation is a high selfing rate (> 0.825) in this species. Unsurprisingly, the F_ST estimates among populations were much higher than in D. depressissimum. Cross‐species amplification in three congeneric species was on the whole unsuccessful.     

Evolutionary aspects of population structure for molecular and quantitative traits in the freshwater snail Radix balthica

Detecting the action of selection in natural populations can be achieved using the QST-FST comparison that relies on the estimation of FST with neutral markers, and QST using quantitative traits potentially under selection. QST higher than FST suggests the action of directional selection and thus potential local adaptation. In this article, we apply the QST-FST comparison to four populations of the hermaphroditic freshwater snail Radix balthica located in a floodplain habitat. In contrast to most studies published so far, we did not detect evidence of directional selection for local optima for any of the traits we measured: QST calculated using three different methods was never higher than FST. A strong inbreeding depression was also detected, indicating that outcrossing is probably predominant over selfing in the studied populations. Our results suggest that in this floodplain habitat, local adaptation of R. balthica populations may be hindered by genetic drift, and possibly altered by uneven gene flow linked to flood frequency.