Patterns of genetic variation in Pacific island land snails: the distribution of cytochrome b lineages among Society Island Partula

Molecular phylogeny of the Orthurethra (Panpulmonata: Stylommatophora)

We have undertaken a molecular analysis of the Orthurethra, one of the major groups of stylommatophoran land snails and slugs. Approximately 4000 nucleotides of the rRNA gene cluster [5.8S, internal transcribed spacer 2 (ITS2) and almost the full-length large subunit (LSU; 28S) gene] were sequenced for 40 orthurethran genera belonging to 19 families. Our phylogeny recovers three well-supported clades within the Orthurethra; the Azecidae, Chondrinidae + Truncatellinidae, and a main clade comprising all remaining orthurethran families. The first division in the Orthurethra separates the Azecidae from all other orthurethran taxa. Of those families represented by more than one genus, the Achatinellidae, Azecidae, Cerastidae, Partulidae and Vertiginidae are recovered as strongly supported monophyletic units, whereas the Chondrinidae, Enidae, Pupillidae and Valloniidae are unsupported in the tree. Although there is relatively little support for the deep-level relationships among the main orthurethran groups, some groupings are strongly supported. The sister-group relationship of the Cochlicopidae with the Amastridae is strongly supported in our molecular analyses, and there is also some support for the grouping of the Orculidae with the Pyramidulidae, and the Draparnaudiidae with the Gastrocoptidae. The findings of our molecular analyses support dividing the Orthurethra into three superfamilies: the Azecoidea, Chondrinoidea and Pupilloidea.

Repeated evolution of an undescribed morphotype of Rhagada (Gastropoda : Camaenidae) from the inland Pilbara, Western Australia

An undescribed small, banded morphotype of Rhagada land snails occurs widely in the rocky inland Pilbara region, Western Australia. Phylogenetic analysis of mitochondrial COI and 16S rRNA genes revealed that this novel morphotype is polyphyletic, comprising four distinct major clades, with divergences up to 21.4% at COI. These clades are apparently morphologically cryptic, with no obvious shell differences. Two of these species are associated with the major clade of Rhagada in the Pilbara mainland, one of which appears to be a variant of the larger, more globose species R. pilbarana, which occurs within 20km proximity. The other two small, banded species are phylogenetically distinct from each other and all other known Rhagada. This small, banded morphotype shows evidence for both plesiomorphy and homoplasy. The morphotype has evolved independently at least twice, and is associated with the reasonably uniform habitat and harsh conditions in the elevated hinterland of the inland Pilbara. The broad distribution of the inland, small, banded morphotype conforms to the pattern of broad-scale uniformity of shells of the more coastal species of Rhagada. Its repeated evolution, however, confirms that the morphological uniformity is not simply because of common ancestry, supporting the theory that shell form in Rhagada is adapted to a broadly homogenous environment. Shell morphology in this genus has been demonstrated on more than one occasion to have the potential to adapt to different available environments, and hence shells should be used with a degree of caution for taxonomic interpretation.

Evolutionary history of a vanishing radiation: isolation-dependent persistence and diversification in Pacific Island partulid tree snails

Background

Partulid tree snails are endemic to Pacific high islands and have experienced extraordinary rates of extinction in recent decades. Although they collectively range across a 10,000 km swath of Oceania, half of the family’s total species diversity is endemic to a single Eastern Pacific hot spot archipelago (the Society Islands) and all three partulid genera display highly distinctive distributions. Our goal was to investigate broad scale (range wide) and fine scale (within‐Society Islands) molecular phylogenetic relationships of the two widespread genera, Partula and Samoana. What can such data tell us regarding the genesis of such divergent generic distribution patterns, and nominal species diversity levels across Oceania?

Results

Museum, captive (zoo) and contemporary field specimens enabled us to genotype 54 of the ~120 recognized species, including many extinct or extirpated taxa, from 14 archipelagoes. The genera Partula and Samoana are products of very distinct diversification processes. Originating at the western edge of the familial range, the derived genus Samoana is a relatively recent arrival in the far eastern archipelagoes (Society, Austral, Marquesas) where it exhibits a stepping‐stone phylogenetic pattern and has proven adept at both intra‐and inter‐ archipelago colonization. The pronounced east–west geographic disjunction exhibited by the genus Partula stems from a much older long-distance dispersal event and its high taxonomic diversity in the Society Islands is a product of a long history of within‐archipelago diversification.

Conclusions

The central importance of isolation for partulid lineage persistence and diversification is evident in time-calibrated phylogenetic trees that show that remote archipelagoes least impacted by continental biotas bear the oldest clades and/or the most speciose radiations. In contemporary Oceania, that isolation is being progressively undermined and these tree snails are now directly exposed to introduced continental predators throughout the family’s range. Persistence of partulids in the wild will require proactive exclusion of alien predators in at least some designated refuge islands.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0202-3) contains supplementary material, which is available to authorized users.

Animal mitochondria, positive selection and cyto-nuclear coevolution: insights from pulmonates

Pulmonate snails have remarkably high levels of mtDNA polymorphism within species and divergence between species, making them an interesting group for the study of mutation and selection on mitochondrial genomes. The availability of sequence data from most major lineages – collected largely for studies of phylogeography - provides an opportunity to perform several tests of selection that may provide general insights into the evolutionary forces that have produced this unusual pattern. Several protein coding mtDNA datasets of pulmonates were analyzed towards this direction. Two different methods for the detection of positive selection were used, one based on phylogeny, and the other on the McDonald-Kreitman test. The cyto-nuclear coevolution hypothesis, often implicated to account for the high levels of mtDNA divergence of some organisms, was also addressed by assessing the divergence pattern exhibited by a nuclear gene. The McDonald-Kreitman test indicated multiple signs of positive selection in the mtDNA genes, but was significantly biased when sequence divergence was high. The phylogenetic method identified five mtDNA datasets as affected by positive selection. In the nuclear gene, the McDonald-Kreitman test provided no significant results, whereas the phylogenetic method identified positive selection as likely present. Overall, our findings indicate that: 1) slim support for the cyto-nuclear coevolution hypothesis is present, 2) the elevated rates of mtDNA polymorphims and divergence in pulmonates do not appear to be due to pervasive positive selection, 3) more stringent tests show that spurious positive selection is uncovered when distant taxa are compared and 4) there are significant examples of positive selection acting in some cases, so it appears that mtDNA evolution in pulmonates can escape from strict deleterious evolution suggested by the Muller’s ratchet effect.

Moorean tree snail survival revisited: a multi-island genealogical perspective

BACKGROUND

The mass extirpation of the island of Moorea's endemic partulid tree snail fauna, following the deliberate introduction of the alien predator Euglandina rosea, represents one of the highest profile conservation crises of the past thirty years. All of the island's partulids were thought to be extirpated by 1987, with five species persisting in zoos, but intensive field surveys have recently detected a number of surviving wild populations. We report here a mitochondrial (mt) phylogenetic estimate of Moorean partulid wild and captive lineage survival calibrated with a reference museum collection that pre-dates the predator's introduction and that also includes a parallel dataset from the neighboring island of Tahiti.

RESULTS

Although severe winnowing of Moorea's mt lineage diversity has occurred, seven of eight (six Partula; two Samoana) partulid tip clades remain extant. The extinct mt clade occurred predominantly in the P. suturalis species complex and it represented a major component of Moorea's endemic partulid treespace. Extant Moorean mt clades exhibited a complex spectrum of persistence on Moorea, in captivity, and (in the form of five phylogenetically distinct sister lineages) on Tahiti. Most notably, three Partula taxa, bearing two multi-island mt lineages, have survived decades of E. rosea predation on Moorea (P. taeniata) and in the valleys of Tahiti (P. hyalina and P. clara). Their differential persistence was correlated with intrinsic attributes, such as taxonomy and mt lineages, rather than with their respective within-island distribution patterns.

CONCLUSION

Conservation efforts directed toward Moorean and Tahitian partulids have typically operated within a single island frame of reference, but our discovery of robust genealogical ties among survivors on both islands implies that a multi-island perspective is required. Understanding what genetic and/or ecological factors have enabled Partula taeniata, P. hyalina and P. clara to differentially survive long-term direct exposure to the predator may provide important clues toward developing a viable long term conservation plan for Society Island partulid tree snails.

Prehistoric inter-archipelago trading of Polynesian tree snails leaves a conservation legacy

Inter-archipelago exchange networks were an important aspect of prehistoric Polynesian societies. We report here a novel genetic characterization of a prehistoric exchange network involving an endemic Pacific island tree snail, Partula hyalina. It occurs in the Society (Tahiti only), Austral and Southern Cook Islands. Our genetic data, based on museum, captive and wild-caught samples, establish Tahiti as the source island. The source lineage is polymorphic in shell coloration and contains a second nominal species, the dark-shelled Partula clara, in addition to the white-shelled P. hyalina. Prehistoric inter-island introductions were non-random: they involved white-shelled snails only and were exclusively inter-archipelago in scope. Partulid shells were commonly used in regional Polynesian jewellery, and we propose that the white-shelled P. hyalina, originally restricted to Tahiti, had aesthetic value throughout these archipelagoes. Demand within the Society Islands could be best met by trading dead shells, but a low rate of inter-archipelago exchange may have prompted the establishment of multiple founder populations in the Australs and Southern Cooks. The alien carnivorous land snail Euglandina rosea has recently devastated populations of all 61 endemic species of Society Island partulid snails. Southern Cooks and Australs P. hyalina now represent the only unscathed wild populations remaining of this once spectacular land snail radiation.

Tahitian tree snail mitochondrial clades survived recent mass extirpation

Oceanic islands frequently support endemic faunal radiations that are highly vulnerable to introduced predators [1]. This vulnerability is epitomized by the rapid extinction in the wild of all but five of 61 described Society Islands partulid tree snails [2], following the deliberate introduction of an alien biological control agent: the carnivorous snail Euglandina rosea[3]. Tahiti's tree snail populations have been almost completely extirpated and three of the island's eight endemic Partula species are officially extinct, a fourth persisting only in captivity [2]. We report a molecular phylogenetic estimate of Tahitian Partula mitochondrial lineage survival calibrated with a 1970 reference museum collection that pre-dates the predator's 1974 introduction to the island [4]. Although severe winnowing of lineage diversity has occurred, none of the five primary Tahitian Partula clades present in the museum samples is extinct. Targeted conservation measures, especially of montane refuge populations, may yet preserve a representative sub-sample of Tahiti's endemic tree snail genetic diversity in the wild.

The ovotestis: an underdeveloped organ of evolution

In animals that have separate sexes (gonochorists), many sperm are produced to fertilise a few eggs. As the male germline undergoes more mitoses, so the accumulated mutation frequency is elevated in sperm compared with ova, and evolution is 'male-driven'. In contrast, in many hermaphroditic animals, a single organ--the ovotestis--produces both ova and sperm. Since self-renewing cells in the ovotestis may give rise to both cell types throughout life, ova in hermaphrodites could in theory have undergone as many cell divisions as sperm. Here, I consider some possible effects of the ovotestis on evolution. In particular, I hypothesise that the accumulated mutation frequency of nuclear genes in hermaphrodites (including species that change sex) may reach twice that compared with gonochorists. There may be an even greater increase in the mitochondrial mutation frequency. Further developmental studies and the accumulation of comparative data should allow hypothesis testing. If the prediction is correct, then it may provide the most-straightforward explanation for the extraordinary diversity of mitochondrial DNA in some hermaphrodites, especially molluscs.

Pleistocene glaciation is implicated in the phylogeographical structure of Potamopyrgus antipodarum, a New Zealand snail

Pleistocene glaciation has been identified as an important factor shaping present-day patterns of phylogeographical structure in a diverse array of taxa. The purpose of this study was to use mitochondrial sequence data to address whether Pleistocene glaciation is also a major determinant of phylogeographical patterns in Potamopyrgus antipodarum, a freshwater snail native to New Zealand. We found that haplotypes were separated by no more than 3.7% sequence divergence, and major genetic divisions tended to occur on a north-south axis. These data fit the predictions of the hypothesis that isolation of P. antipodarum in glacial refugia at the northern and southern tip of the South Island of New Zealand during the Pleistocene glaciation underlies the present-day phylogeographical structure. Because sexual P. antipodarum occasionally produce asexual offspring, we also used these data to show that the appearance of asexuality is not phylogeographically constrained. This means that the maintenance of sex in P. antipodarum cannot be wholly due to limited contact between sexual and asexual lineages and must instead be linked to a selective advantage of sexual reproduction.