Understanding Symbiosis Through the Lens of Squamellaria
The study of symbiotic relationships in nature often reveals fascinating insights into how different species interact and coexist. A recent discovery involving plants from a remote Pacific island has provided new understanding into how unrelated and potentially antagonistic partners can form long-term mutualistic relationships with a single host.
Scientists researching ant plants in Fiji have uncovered a unique mechanism that allows a host plant to maintain peace among various ant species that live within it. This process involves the Squamellaria plant, which creates separate chambers inside its tuber. Each chamber is equipped with an entry point from the outside but remains isolated from other chambers, preventing conflicts between the ants that inhabit them.
This research, conducted by scientists at Washington University in St. Louis and Durham University in the United Kingdom, was published in the journal Science. According to Guillaume Chomicki, the lead author of the study, compartmentalization serves as a solution to prevent deadly conflicts between ant species. However, this adaptation benefits the plant rather than the ants, as more ants living inside the tuber means more nutrients for the plant.
Susanne S. Renner, a biologist at WashU and senior author of the study, emphasized the importance of fieldwork in uncovering these insights. She noted that dried Squamellaria tubers, which have been stored in herbaria or natural history collections since the 1880s, do not provide the same level of detail as direct observation in the wild. To fully understand the structure of these tubers, researchers needed to see and identify the ants and recognize the presence of entirely separate compartments, similar to a condo building.
In biology, symbiosis refers to a close and long-term interaction between two different species. When this relationship is mutually beneficial, it is known as mutualism. While many symbioses involve a host species having multiple mutualist partners, theory suggests that unrelated partners can lead to destabilizing conflict through competition for resources.
Squamellaria exemplifies this dynamic. Previously, Chomicki and Renner documented how certain specialized ants farm the plant by planting its seeds and fertilizing seedlings with their feces. These ants then nest in the hollow chambers, or domatia, that form on each plant after the cotyledons unfold. The farming activity is crucial for the plant’s survival, as it is an epiphyte that grows on rainforest tree branches and relies on ants to establish itself in the canopy.
During a field excursion in the Waisali Rainforest Reserve in 2014, Chomicki discovered a Squamellaria plant with a domatium about 8 inches in diameter, containing two separate ant colonies. This observation led to further investigation into how the plant enables multiple ant species to coexist within its chambers.
Experiments revealed that removing the partition walls between ant colonies resulted in immediate and violent conflicts. The scientists measured the number of conflicts and found high worker mortality in both ant colonies. Additionally, feeding experiments demonstrated that the plant benefits from hosting multiple ant species, receiving nutritional advantages from their activities.
Computed-tomography (CT) scanning played a crucial role in revealing the compartmentalization of the domatia. Dissecting the structures alone made it difficult to determine which chambers were connected or isolated. By creating 3D models from CT scan data, researchers were able to visualize the internal structure of the plant, leading to significant discoveries.
The findings highlight a conflict-reduction mechanism that allows hosts to benefit from multiple, unrelated symbionts. Renner and Chomicki noted that while the concept of conflict between unrelated partners has been theorized, the Squamellaria-ant relationship provides a clear example of how such conflicts can be mitigated through evolutionary strategies like compartmentalization.
This research not only sheds light on the intricate interactions between plants and ants but also offers broader implications for understanding the ecology and evolution of species interactions. The study demonstrates that even unique biological systems can illuminate general principles that apply across diverse ecosystems.
