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

Abstract

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

Revision of Partulidae (Gastropoda, Stylommatophora) of Palau, with description of a new genus for an unusual ground-dwelling species

“…The vas deferens of Palaopartula remains narrow for its entire length whereas the vas deferens of Partula broadens before entering the apical chamber. Molecular data also do not support placement of Palaopartula within Partula (Lee et al 2014) but link it instead with Samoana , from which it differs radically in shell and genital morphology. Accordingly, we here resurrect Palaopartula .…”

“…7). The family is particularly diverse in the Society Islands, where they are among the largest and best-studied land snails (Crampton 1916, 1932, Murray and Clarke 1980, Cowie 1992, Lee et al 2014). However, many species have declined or are extinct as a result of the loss of native lowland forest and human introduction of alien species, especially predators like Euglandina rosea , released in disastrous and unsuccessful biocontrol efforts (Murray et al 1988).…”

“…The sequence is deposited in GenBank under accession number KX685957. Comparative COI sequences were provided by the authors of Lee et al (2014). We conducted a phylogenetic analysis of our taxon versus other partulids using Bayesian analysis in MrBayes (Ronquist and Huelsenbeck 2003) as implemented in Geneious version 6.1.7 (http://www.geneious.com, Kearse et al 2012).…”

“…We thank Ron Crombie for providing GPS coordinates along the trail to the German lighthouse and for information on the type locality and Palau in general; Ron Leidich for kindly providing habitat photos; Don Cameron for advice on Greek etymological options; Taehwan Lee and Diarmaid O'Foighil for a copy of Yoshio Kondo’s thesis and information from their Partula study (Lee et al 2014) before its publication; John Megahan for constructing Fig. 7; Paul Callomon, ANSP for loan of specimens; Amy Heatherington and Kimberly Backer-Kelley for assistance with Scanning Electron Microscopy; and Verity Mathis for assisting with photo stacking.…”

“…In comparing this snail to its closest relatives it became obvious that Palauan snails currently assigned to Partula in fact do not accord well morphologically with that genus. Furthermore, they have been found to comprise a separate clade of partulids evolutionarily independent of lineages that are properly assigned to Partula (Lee et al 2014). To facilitate comparisons with our new genus, we first resurrect Palaopartula Pilsbry for the three known species of Palauan partulids.…”