Phylogeography and conservation genetics of the Iowa pleistocene snail

The Iowa Pleistocene snail, Discus macclintocki, is an endangered species that survives only in relictual populations on algific (cold-air) talus slopes in northeast Iowa and northwest Illinois in the central region of the USA. These populations are believed to have been isolated since the temperatures began to warm at the end of the last glacial period around 16 500 years ago. DNA sequencing of the 16s rRNA gene of the mitochondria was used to determine the genetic relationship among 10 populations and the genetic diversity within these populations. Genetic diversity is extremely high within this species with 40 haplotypes spread across the 10 populations sampled within a 4000 km2 region. Phylogenetic analyses showed that haplotypes formed monophyletic groups by the watershed on which they were found, suggesting that watersheds were important historical avenues of gene flow. Genetic distances were strongly related to the geographical distance among all populations, but this relationship was dependent on the scale being considered.     

Phylogeography of Ptychadena mascareniensis suggests transoceanic dispersal in a widespread African-Malagasy frog lineage

Aim The Mascarene ridged frog, Ptychadena mascareniensis, is the only African amphibian species thought to occur on Madagascar and on the Seychelles and also Mascarene islands. We explored its phylogenetic relationships and intraspecific genetic differentiation to contribute to the understanding of transoceanic dispersal in amphibians. Methods Fragments of the mitochondrial 16S rRNA gene were sequenced from specimens collected over most of the distribution area of P. mascareniensis, including populations from Madagascar, Mascarenes and Seychelles. Results We identified five deeply divergent clades having pairwise divergences >5%, which probably all represent cryptic species in a P. mascareniensis complex. One of these seems to be restricted to Madagascar, the Mascarenes and the Seychelles. Sequences obtained from topotypic material (Réunion) were identical to the most widespread haplotype from Madagascar. The single Mauritian/Seychellean haplotype differed by only one mutation from a Malagasy haplotype. Main conclusions It is likely that the Mascarene and Seychellean populations were introduced from Madagascar by humans. In contrast, the absence of the Malagasy haplotypes from Africa and the distinct divergences among Malagasy populations (16 mutations in one divergent hapolotype from northern Madagascar) suggest that Madagascar was populated by Ptychadena before the arrival of humans c. 2000 years ago. Because Madagascar has been separated from Africa since the Jurassic, this colonization must have taken place by overseas rafting, which may be a more widespread dispersal mode in amphibians than commonly thought.     

Phylogeography of the land snail Albinaria hippolyti (Pulmonata: Clausiliidae) from Crete, inferred from ITS-1 sequences

The polytypic Cretan land snail Albinaria hippolyti has a range that is partly fragmented and partly subdivided by hybrid zones. For this reason, it has served as a model species for investigating speciation and radiation in Mediterranean Clausiliidae. The first internal transcribed spacer ( ITS-1 ) of the nuclear ribosomal DNA was sequenced in 20 populations of A. hippolyti and phylogenetically analysed using maximum parsimony. We employed a novel method involving logarithmic weighting of gaps and topological constraints based on bootstrap values. The resulting phylogeography suggests that the species has undergone a recent cycle of range expansion and range reduction. Speciation cannot be linked to major geological vicariance events in the Miocene and Pliocene, as has been suggested previously. The subspecies A. h. arthuriana appears unrelated to other A. hippolyti subspecies, which supports recent suggestions, based on morphology, to regard it as a separate species.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 317–326.     

Population structure and coil dimorphism in a tropical land snail

Tree snails of the subgenus Amphidromus s. str. are unusual because of the chiral dimorphism that exists in many species, with clockwise (dextrally) and counter-clockwise (sinistrally) coiled individuals co-occurring in the same population. Given that mating in snails is normally impeded when the two partners have opposite coil, positive frequency-dependent selection should prevent such dimorphism from persisting. We test the hypothesis that a strong population structure with little movement between tree-based demes may result in the fixation of coiling morphs at a very small spatial scale, but apparent dimorphism at all larger scales. To do so, we describe the spatial structure in a Malaysian population of A. inversus (Müller, 1774) with 36% dextrals. We marked almost 700 juvenile and adult snails in a piece of forest consisting of 92 separate trees, and recorded dispersal and the proportions of dextrals and sinistrals in all trees over a 7-day period. We observed frequent movement between trees (155 events), and found that no trees had snail populations with proportions of dextrals and sinistrals that were significantly different from random. Upon recapture 1 year later, almost two-thirds of the snails had moved away from their original tree. We conclude that population structure alone cannot stabilise the coil dimorphism in Amphidromus.     

Selective increase of a rare haplotype in a land snail hybrid zone

In hybrid zones, which are regions where genetically differentiated populations of organisms meet and produce hybrids, allozyme studies have often revealed unexpected alleles. The cause of this ''hybrizyme'' or ''rare allele'' phenomenon has been elusive, although it has been variously ascribed to natural selection or increased mutation rates. If the latter hypothesis is correct, selectively neutral markers should demonstrate increased variability in contrast to expressed markers such as allozymes. In this study, we screened selectively neutral variation in an intron of the calmodulin (CaM) gene in a hybrid zone between two subspecies of the Greek land snail Albinaria hippolyti. In previous allozyme studies, this hybrid zone has been shown to exhibit the rare allele phenomenon. We used a variant of the single-strand conformational polymorphism (SSCP) technique to detect seven haplotypes in both parental taxa. In the zone, one of these occurs at unexpectedly high frequencies. Since no additional mutants were found, we concluded that this is the result of selection.     

Behavioral responses to a change in stimulus: transport vs. attack behavior in a carnivorous land snail, Haplotrema concavum

Abstract. Haplotrema concavum , a carnivorous land snail, responds differently to the same prey when hunting versus when transporting food. The ability of these animals to distinguish between fresh, unmanipulated or uneaten prey and manipulated or partially eaten prey was tested by switching prey items while the predator was in the process of transporting the prey. Predators responded to a switch from a manipulated egg of Anguispira alternata to an unmanipulated egg by suspending transport behavior while the new egg was manipulated. Seven of ten predators responded to a switch from an egg of A. alternata to a conspecific ( H. concavum ) egg by stopping transport and abandoning the less-preferred food. Predators responded to a switch from partially eaten hatchlings of A. alternata to fresh but injured hatchlings by stopping transport after the retrieval stage to eat the new hatchling, after which transport was resumed. Predators which were not engaged in transport behavior attacked rather than retrieved fresh, injured hatchling prey. These results suggest that by manipulating its prey, the predator alters the sensory stimulus of the prey by marking the prey with predator mucus which, in turn, elicits the transport behavior by the predatory snail.     

Gene regulation of shell banding in a land snail from Israel

The Israeli land snail, Xeropicta vestalis, offers a particularly clear example of gene regulation in relation to natural selection, in that within each population the appropriate phenotype is generated only at the correct part of the animals life cycle, and a contrasting phenotype develops when the forces of natural selection change. In the mountains of Jerusalem, where the winter is cold, the shells are dark. Westwards, towards the coastal plain where the winter is warmer, the shells gradually become paler. As dark shells absorb more radiation than pale ones, this clinal variation in morph frequencies can be explained in thermal terms. (Banded shells are also more cryptic than non-banded shells, so that in the mountains visual selection by predators may be an additional force which favours dark shells.) Xeropicta vestalis is an annual, semelparous species: the snails hatch in winter, become mature within one year, reproduce, and then die. In the coastal plain the snails are active throughout most of the year, and they have a long period in which to grow to reach adult size. In the mountains and hills, on the other hand, the snails are active for only a very short period. They spend most of their lives as small snails, in a state of aestivation. Xeropicta vestalis must be dark in mountains because when it finally awakens, it must very rapidly and hastily reach reproduction size. A dark shell, by speeding up temperature-dependent processes in this critical stage, assists the snail to mature rapidly. Shell darkness varies with age: in the mountains and hills the shells are moderately dark when they hatch, but become darker whilst growing in early winter. In the coastal plain also, the snails are moderately dark when they hatch; but here they become paler, whilst growing in winter and spring. In both cases, each snail is darker in the colder months and paler in the hot ones. A strategy of gene regulation of shell colour is thus favoured when the subsequent forces of the environment are very contrasting in their direction, very severe—yet also very predictable.     

A predatory land snail invades central-western Argentina

The pulmonate land snail, Rumina decollata, is a highly invasive gastropod adapted to arid conditions, and native from the Mediterranean area. It was recorded for the first time in Argentina in 1988, in the northeastern Pampas of the Buenos Aires Province, a region characterized by a humid mesothermal climate with no water deficit. In the present contribution, we report the finding of populations of this species in the semiarid region of La Pampa and Mendoza provinces in central-western Argentina. The present findings extend the invasive distributional range of R. decollata westwards through more than 1000 km from the site where it was detected in 1988. Although no damages to agricultural areas have been reported to the moment in Argentina, the presence of R. decollata in semiarid areas warns us about its potential as an important plague in the future.     

Differences in habitat quality explain nestedness in a land snail meta-community

We set up two alternative hypotheses on how environmental variables could foster nestedness; one of “nested habitats” and another of “nested habitat quality”. The former hypothesis refers to situations where the nestedness of species depends on a nestedness of discrete habitats. The latter considers situations where all species in an assemblage increase in abundance along the same environmental gradient, but differ in specialisation or tolerance. We tested whether litter‐dwelling land snails (terrestrial gastropods) in boreal riparian forest exhibited a nested community structure, whether such a pattern was related to differences in environmental variables among sites, and which of the two hypotheses that best could account for the found pattern. We sampled litter from 100 m² plots in 29 mature riparian forest sites along small streams in the boreal zone of Sweden. The number of snail species varied between 3 and 14 per site. Ranking the species‐by‐site matrix by PCA scores of the first ordination axis revealed a similarly significant nested pattern as when the matrix was sorted by number of species, showing that the species composition in this meta‐community can be properly described as nested. Several environmental variables, most notably pH index, were correlated with the first PCA axis. All but two species had positive eigenvectors in the PCA ordination and the abundance increased considerably along the gradient for most of the species implying that the hypothesis of “nested habitats” was rejected in favour of the “nested habitat quality” hypothesis. Analyses of nestedness have seldom been performed on equal sized plots, and our study shows the importance of understanding that variation in environmental variables among sites can result in nested communities. The conservation implications are different depending on which of our two hypotheses is supported; a conservation focus on species “hotspots” is more appropriate if the communities are nested because of “nested habitat quality”.     

Rapid,habitat-related evolution of land snail colour morphs on reclaimed land

I made use of the known dates of reclamation (and of afforestations) in the IJsselmeerpolders in The Netherlands to assess evolutionary adaptation in Cepaea nemoralis. At 12 localities (three in each polder), I sampled a total of 4390 adult individuals in paired open and shaded habitats, on average 233 m apart, and scored these for genetic shell colour polymorphisms. The results show (highly) significant differentiation at most localities, although the genes involved differed per locality. Overall, though, populations in shaded habitats had evolved towards darker shells than those in adjacent open habitats, whereas a ‘Cain & Sheppard'' diagram (proportion yellow shells plotted against ‘effectively unbanded'' shells) failed to reveal a clear pattern. This might suggest that thermal selection is more important than visual selection in generating this pattern. Trait differentiation, regardless of whether they were plotted against polder age or habitat age, showed a linear increase of differentiation with time, corresponding to a mean rate of trait evolution of 15–31 kilodarwin. In conclusion, C. nemoralis is capable of rapid and considerable evolutionary differentiation over 1–25 snail generations, though equilibrium may be reached only at longer time scales.     

Discrete movements of foot epithelium during adhesive locomotion of a land snail

During the adhesive locomotion of land snails a series of short dark transverse bands, called pedal or foot waves, is visible ifa moving snail's ventral surface is observed through a sheet of glass. Moreover, the mucus secreted from the pedal glands and some pedal epithelial cells forms a thin layer which acts as a glue augmenting adherence, while also acting as a lubricant under the moving parts of the snail's foot. The relationships between velocity and the frequency of pedal waves as well as changes in the volume of small air bubbles under foot waves were analyzed by means of digital recordings made through a glass sheet on which the snails were moving. On the ventral surface of a moving snail foot, the adhering parts of the foot constituted about 80% of the total area, while several moving parts only about 20%. The single moving region of the foot (the pedal wave) amounted to about 3% of snail length. The epithelium in the region of the pedal wave was arched above the substrate and was also more wrinkled than the stationary epithelium, which enabled the forward motion of each specific point of epithelium during the passage of a pedal wave above it. The actual area of epithelium engaged by a pedal wave was at least 30% greater than the area of the epithelium as recorded through a glass sheet. In the region of the pedal wave, the tiny subepithelial muscles acting on the epithelium move it up in the front part of the wave, and then down at the end of the wave, operating vertically in relation to the substrate. In the middle part of the wave, the epithelium only moves forward. In summary, during the adhesive locomotion of snails, the horizontal movement of the ventral surface epithelium proceeds as temporally separate phases of upward, forward and downward movement.     

Nonrandom variation of morphological traits across environmental gradients in a land snail

Morphological variation is often attributed to differential adaptations to diverse habitats, but adaptations to a similar environment do not necessarily result in similar phenotypes. Adaptations for water and heat budget are crucial for organisms living in arid habitats, and in snails, variation in shell morphology has been frequently attributed to selection by stressful environmental factors. However, their phenotypic divergence often is not accompanied by a relevant niche differentiation and consistent relationships with environmental correlates are lacking. In the pulmonate genus Albinaria, there is great size and shape variation between and within species, and there are two major shell sculpture morphotypes, ribbed and smooth. We used 62 populations of 28 Albinaria species, taking into account their phylogeny, to examine the variation of shell traits (sculpture, size, shape), their effect on water and heat budget, and their association with geographical and climatic gradients. We found unambiguous size and shape discrimination between the two morphotypes. Ribbed shells are lighter, taller, and slimmer and have a smaller aperture than the smooth ones. Moreover, significant correlations between shell traits and heat/moisture budget and climate/geography were revealed. Ribbed and taller shells retain more water on their shell surface, and on the other hand, smooth shells exhibit lower water permeability. Therefore, two strategies are being used to prevent water loss, active retention or resistance to loss. Consequently, different alternative solutions evolved and were retained as responses to the same stressful factor by the two distinct shell morphotypes. Larger shells occur in southern latitudes, mostly on islands, and at sites where there is a shortage of rainfall. Therefore, the variation of the examined traits is nonrandom with respect to location and to climate and their evolution can be attributed to selection by environmental factors, with water availability being the key driving agent of body-size variation.     

Reproductive cycle of Leptaxis caldeirarum,a locally endangered Azorean land snail

Summary

The land snail Leptaxis caldeirarum(Morelet and Drouet, 1857), a hermaphrodite Hygromiidae species, endemic from São Miguel Island (Azores), is geographically limited to a range of 31 km², occupying an area of about 14 km² A preliminary study confirmed the locally endangered status of the species, raising the possibility that it could reach the “critically in danger” status, the most serious category in IUCN. A population from Ramal dos Mosteiros was studied all year round in order to elucidate its reproductive cycle and to test the validity of the total weight of the animal and the maximum diameter of the shell as diagnostic parameters for maturation. Gonadal maturation of L. caldeirarum proceeds from late winter to early summer. Intense gametogenesis takes place from January until May and the snail is apparently in a reproductive condition to copulate from late spring onwards. The strong correlation observed between the relative volumetric density of mature gametes and the maximum diameter of the shell suggests that the reproductive status of the species can be inferred from the latter parameter. This may be of major importance in future studies, minimizing potential damage to already unstable populations, and facilitating the development of conservation strategies.     

Self-fertilization in the white-lipped land snail Triodopsis Albolabris

The breeding system of the polygyrid land snail Triodopsis albolabris was studied in laboratory colonies through the use of allozyme-genetic markers. Isolated virgin individuals self-fertilized only after several months of apparent self-sterility. Isolated pairs appeared to reproduce solely by outcrossing. Overall, the normalized reproductive success of paired individuals was about 86 times greater than that of isolates.
Nine natural populations of this snail were surveyed at eight allozyme loci. All populations were highly polymorphic, and heterozygosity was high and agreed with Hardy-Weinberg expectations. This indicates that selfing is not common in established populations. We conclude that T. albolabris avoid inbreeding by selfing if at all possible. However, if the probability of finding a mate is low (assessed by several months without finding one), lone individuals self-fertilize.     

Phylogeography of the endangered tideland snail Batillaria zonalis in the Japanese and Ryukyu Islands

The phylogeography of the endangered tideland snail Batillaria zonalis in the Japanese and Ryukyu Islands was analyzed on the basis of nucleotide sequences of a mitochondrial gene for cytochrome oxidase c subunit I (COI). Extremely low genetic diversity was found in populations at both the northern and southern boundaries of the geographic distribution of this species in Japan, i.e., Sendai Bay and Iriomotejima Island, respectively, which might be attributed to the population bottleneck due to historical environmental variations and/or the recent foundation of populations in the marginal part of the inhabitable range. Most populations contained unique rare haplotypes, and significant genetic differentiation on the whole was shown, while no clear geographic genetic structure was detected between the Japanese and Ryukyu Islands or over the distribution area of B. zonalis in Japan, with the exception of significant genetic divergences in Ago Bay in the central part of Honshu and the southern part of Okinawajima Island.