Integrative taxonomy reveals hidden fungal and algal diversity in foliicolous Porinaceae (Ascomycota, Ostropales) from Madagascar and the Indian Ocean Islands

Porinaceae is one of the most common families of foliicolous lichens in tropical regions. Despite its abundance, major gaps remain in our understanding of its diversity and phylogeny. This study addresses these knowledge gaps and examines photobiont selectivity in Madagascar and the Indian Ocean Islands. In total, 114 lichen specimens from Madagascar and the neighbouring islands of Mauritius and Mayotte were sequenced. Mycobiont diversity was investigated using the mtSSU and ITS regions, while photobiont diversity and selectivity were assessed using the rbc L marker. Phenotypic traits, including ascospore and perithecial characteristics, were evaluated alongside molecular data to support species delimitation. Thirty-seven Porinaceae taxa were recognised using an integrative taxonomic approach, including six species newly described here: Porina cincta Debaene & Ertz, P. complanata Debaene & Ertz, P. madagascariensis Debaene & Ertz, P. mayottensis Debaene & Ertz, P. parva Debaene, Diederich & Ertz, and P. trichocoronata Debaene & Ertz. The rbc L data recovered 25 algal haplotypes distributed across six major clades, four of which were newly recovered. Despite limited morphological differentiation, the mycobiont exhibited unexpectedly high genetic diversity, and some traditionally diagnostic characters, such as ascospore septation, proved to be unreliable at times. These patterns were accompanied by strong phylogeographic structuring and high levels of island endemism. Bipartite network analyses revealed complex symbiotic interactions, with both highly specific and more flexible mycobiont-photobiont associations, alongside substantial photobiont diversity. Altogether, this study emphasises the need for extensive DNA multilocus sampling and type-linked material, combined with thorough phenotypic assessments, to refine species and genus concepts within Porinaceae .