Barriers or Bridges? The Role of Streams in Shaping the Genetic Structure of the Australian Land Snail Sauroconcha grayi (Gastropoda, Camaenidae)

Abstract

Aim

Short-range endemic (SRE) terrestrial invertebrates, such as land snails, are of significant conservation concern due to their limited dispersal abilities and high susceptibility to habitat fragmentation. A central debate in landscape genetics, the River Barrier Hypothesis, posits that river systems act as major obstacles to gene flow, yet empirical evidence remains mixed, with some studies suggesting rivers may instead facilitate movement. To test the River Barrier Hypothesis by evaluating whether river systems act as barriers to gene flow in a short-range endemic land snail, and to assess how landscape heterogeneity shapes population genetic structure and divergence. Our study can inform efforts to conserve the genetic diversity of SRE.

Location

Sydney Basin, Hawkesbury and Hunter Valley, New South Wales, Australia.

Taxon

Ourimbah Forest Snail (Sauroconcha grayi), Camaenidae.

Methods

This study utilizes genome-wide restriction site-associated DNA sequencing (RAD-seq) to investigate the fine-scale population structure of an endemic land snail in central eastern New South Wales and estimate migration rates across a heterogeneous landscape. We also tested whether the river influences the genetic connectivity across different heterogeneous topographies using isolation by distance and isolation by resistance.

Results

Structure analyses of genome-wide single nucleotide polymorphisms (SNPs) identified three spatially distinct genetic clusters, which are delimited by major hydrological features, specifically the Hunter and Hawkesbury Rivers. Patterns of genetic differentiation were heterogeneous across the landscape, revealing variation that corresponded with underlying topographic context and hydrological configuration. We identified rugged terrain areas where rivers corresponded with marked genetic discontinuities, indicating their role as barriers to gene flow, and plain areas where these barrier effects were reduced or absent.

Main Conclusions

The results conditionally support the River Barrier Hypothesis. Landscape heterogeneity, particularly interactions between topography and hydrology, plays a critical role in structuring genetic variation in short-range endemic taxa. Conservation strategies for S. grayi in rugged terrain should prioritize networks of multiple small reserves to preserve geographically structured and genetically distinct lineages.