Uncovering Lineages in the Spider Tortoise
Using genomics to clarify subspecies and guide conservation
The Spider Tortoise (Pyxis arachnoides) is one of the world’s smallest and most threatened tortoises, found only in a narrow strip of dry forest in southwestern Madagascar. Although scientists recognize three forms of this species, it is unclear whether these groups are true subspecies, distinct evolutionary lineages, or just local populations. This uncertainty matters for conservation because we cannot protect what we cannot clearly define.
Past studies relied on a single mitochondrial genetic marker. While useful, mitochondrial DNA tells only part of the story and can be misleading. Advances in genomic technology now let us compare thousands of regions across the genome to understand how populations are related, whether they interbreed, and how long they have evolved independently. These tools are especially valuable for tortoises, where physical traits such as shell shape often fail to reveal the true evolutionary history.
This project uses whole-genome sequencing to provide the most detailed look yet at the genetic structure of the Spider Tortoise. By collecting and analyzing genome-wide data from individuals across the species’ range, we can determine whether the three recognized subspecies—Southern Spider Tortoise (Pyxis arachnoides oblonga), Northern Spider Tortoise (Pyxis arachnoides brygooi), and Common Spider Tortoise (Pyxis arachnoides arachnoides)—are distinct lineages that should be managed separately. We also investigate the extent of interbreeding between neighboring populations and whether past isolation has led to unique genetic adaptations.
These questions have real-world consequences. The Spider Tortoise’s habitat has already shrunk by more than 70% because of deforestation and collection for the pet trade. If genetically distinct lineages exist, some may be much closer to extinction than others, but remain unprotected because conservation laws often ignore subspecies. Identifying clear conservation units ensures every unique lineage receives the attention, protection, and resources it needs.
Key Species
Key Species
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