Sociality in the terrestrial epiphytic fern Platycerium bifurcatum

<p><strong>Throughout the history of life on Earth, major evolutionary transitions have led to greater levels of complexity and organization. One of the most intricate transitions is the emergence of eusociality, characterized by complex social organizations with overlapping generations, cooperative care of young, and reproductive and non-reproductive castes that drive reproductive division of labour. Eusociality has primarily been studied in animals, evolving independently in several lineages, including insects, crustaceans, and mammals. This complexity has captivated biologists for decades, but research has focused mainly on animal models, leaving unexplored the potential for analogous social tendencies in plants. In this thesis, we investigated whether the tropical epiphytic fern Platycerium bifurcatum exhibits traits traditionally associated with sociality. In Chapter 2, we examined how the arboreal environment affects asexual recruitment in staghorn ferns on Lord Howe Island, comparing fecundity and colony size between arboreal and terrestrial colonies. Glasshouse data were also compared to field results. Our findings showed that the arboreal environment favoured asexual recruitment, while terrestrial colonies grew solitarily. Chapter 3 focused on non-reproductive division of labour, specifically nest frond specialization. We found that nest fronds produced by different individuals in P. bifurcatum colonies specialize in specific tasks. Fronds at the colony apex captured nutrients through enlarged, highly lobed structures, while those in lower sections specialized in water storage and were small and circular. Additionally, apex fronds protected the colony from foreign seedlings. In Chapter 4, we conducted a reciprocal transplantation experiment to further investigate the impact of terrestrial versus epiphytic growth on differences in reproduction, frond production, longevity, and morphology. Staghorn ferns do not naturally grow in terrestrial soil, but in commercial settings, they do. Our results indicated that artificial terrestrial conditions led to greater frond production, survivorship, and larger fronds for both leaf types. However, reproductive mode was strongly influenced by habitat: arboreal habitats favoured asexual reproduction, while terrestrial plants sporulated more successfully during the experiment. Overall results indicate that, P. bifurcatum displays high degrees of specialization among individuals within colonies of various sizes. However, whether these plants meet all criteria to be considered eusocial remains unclear. This research opens new avenues for understanding social structures in plant species and their adaptations to different environments</strong></p>