The Roccellaceae are a family of mostly lichen-forming fungi in the order Arthoniales, established by the French botanist François Fulgis Chevallier in 1826. Species in the family exhibit various growth forms, including crustose and fruticose (shrub-like) thalli, and diverse reproductive structures. Roccellaceae species typically have disc-like () or slit-like () fruiting bodies, often with distinct blackened () margins. Molecular phylogenetics studies have revealed considerable genetic diversity and complex evolutionary histories within the family.
Convergent evolution in the Roccellaceae has led to multiple independent developments of similar traits, particularly in growth forms. Molecular studies have divided the family into two main phylogenetic groups, each characterised by distinct internal structures. These findings have prompted major taxonomic revisions, clarifying their evolutionary relationships. Roccellaceae produce lichen substances from different chemical classes, some of which have been historically important as sources of purple or red dyes like orchil.
Roccellaceae species have a nearly worldwide distribution but predominantly inhabit tropical and subtropical regions. Species typically grow on rocks or on tree bark, although some grow on leaves. They form symbiotic relationships with green algae of the genus Trentepohlia, and occupy diverse habitats, especially coastal and near-coastal environments. Five species in the family have been assessed for the IUCN Red List.
Systematics
Historical taxonomy
The family was circumscribed by the French botanist François Fulgis Chevallier in his 1826 work . His original included basic morphological characters of the type genus, Roccella: "Apothecia shield-like, stalked or embedded in the thallus, with a coloured that is (firm and somewhat flexible), initially concave and eventually becoming flat and margined, with a thin margin. Thallus erect or (hanging), branched, tufted, cartilaginous-homogeneous (uniformly firm and somewhat flexible), and in many cases compressed". Chevallier included the genus Roccella in the family, which was erected in 1805 by the Swiss botanist Augustin Pyramus de Candolle with Roccella fuciformis as the type species.
Three genera in the Roccellaceae were previously placed in distinct families. These are the Lecanactidaceae for genus Lecanactis (Ernst Stizenberger, 1862); the Chiodectonaceae for genus Chiodecton (Alexander Zahlbruckner, 1905); and Dirinaceae for genus Dirina (Zahlbruckner, 1905). These families have since been synonymised with Roccellaceae.
Etymology
Following standard practice in botanical nomenclature, the name Roccellaceae is derived from the type genus Roccella, with the suffix denoting its rank as a family. The genus name Roccella probably originates from the Italian word , a common name for lichens, itself derived from the Latin (meaning ) with the diminutive suffix -ella.
Classification
Roccellaceae is the largest family in the order Arthoniales by number of genera. Its status as a distinct family was firmly established by genetic evidence, first demonstrated in a large-scale phylogenetic study by Damien Ertz and Anders Tehler in 2011. This study distinguished Roccellaceae from Opegraphaceae and laid the groundwork for further research. Additional molecular phylogenetic studies have since reinforced its position as a separate family within the order.
The family is part of one of the three major evolutionary lines identified within Arthoniales, specifically the lineage that includes Lecanographaceae, Opegraphaceae, Roccellaceae, and Roccellographaceae. These four families are consistently grouped together in phylogenetic analyses, suggesting they are closely related. However, the exact relationships among these families are not fully resolved, partly because the deeper branches within the Roccellaceae (the basal nodes) tree lack strong evidence, making the connections uncertain. This phylogenetic uncertainty may explain some differences in tree structure among various studies.
Within Roccellaceae in the strict sense, Ertz and Tehler identified five main lineages: the sister pair Dirina and Roccella; Chiodecton natalense and Lecanactis (grouped together but not identified as a sister pair); Roccellina; Syncesia; and Dendrographa. Roccellaceae was the only family in their analysis to include large genera that had been thoroughly sequenced, such as Roccella and Roccellina.
Roccellaceae is distinct from Arthoniaceae, the largest family in the order. This distinction is supported by both genetic and morphological differences, particularly in ascus type. Roccellaceae typically have cylindrical asci, whereas Arthoniaceae have more or less spherical () to club-shaped () asci. Some synapomorphies for Roccellaceae include the reduction of the , the protective layer around the hymenium, (with a reversal in Lecanactis) and the loss of the gelatinous sheath surrounding ascospores. These characteristics are significant because they provide morphological support for the family's monophyly, complementing the molecular data. The reduction of the proper exciple suggests a shared evolutionary trend within the family, possibly related to environmental adaptations or reproductive strategies. The absence of a gelatinous sheath around ascospores, a feature often associated with spore dispersal and protection, indicates a unique developmental pathway in Roccellaceae compared to related families. The reversal of the reduced exciple in Lecanactis demonstrates that even synapomorphies can exhibit exceptions within a monophyletic group.
Some genera previously assigned to Roccellaceae, such as Enterographa and Erythrodecton, have been shown to belong to a separate, well-supported clade distinct from Roccellaceae. This was first suggested by Ertz and Tehler in 2011 and later confirmed by subsequent studies. While Roccellaceae includes a diverse range of species, no former members of the genus Arthonia are currently classified within it. This taxonomic separation aligns with morphological and genetic distinctions between the families.
Molecular phylogenetics
Phylogenetic studies of Roccellaceae have made substantial progress in recent decades. Early work in the 1990s used cladistic methods based on morphological and chemical characteristics. Tehler's 1990 study was among the first to apply cladistic analysis to relationships within Arthoniales and Roccellaceae, using type species of genera as the end points of the evolutionary tree. This early work supported the monophyly of Roccellaceae within Arthoniales, based on synapomorphies such as cortex plectenchyma. It also highlighted the ecological association of many Roccellaceae species with coastal habitats and suggested multiple origins of the fruticose (shrub-like) habit within the family, with reversals to crustose forms in some lineages.
Molecular phylogenetic studies of Roccellaceae began in the late 1990s, with early work utilising both internal transcribed spacer (ITS) and small subunit ribosomal DNA (SSU rDNA) sequences. These initial studies revealed that fruticose and crustose growth forms had evolved multiple times within the family. They also suggested a broader circumscription of Roccellaceae, incorporating genera previously assigned to other families such as Lecanactis, Schismatomma, and Syncesia. However, these studies also highlighted challenges in molecular analysis of the family, particularly in aligning the highly variable ITS regions. The usefulness of ITS sequences was found to vary among genera, proving more informative for some (e.g., Roccella, Dendrographa) than others (e.g., Roccellina, Dirina).
Building on these early findings, more recent molecular studies, particularly those using nuclear large subunit ribosomal RNA (LSU) and RNA polymerase II second largest subunit (RPB2) gene sequences, have provided insights into the phylogenetic relationships within Roccellaceae. These studies have revealed that the family is divided into two main clades, each with distinct characteristics. The first clade, the "pale hypothecium clade", includes genera such as Llimonea, Combea, Pentagenella, Hubbsia, and Ingaderia. This group is characterised by species with pale-coloured and has a range of growth forms from crustose to fruticose.
The second major clade, known as the "carbonaceous hypothecium clade", encompasses genera such as Roccella, Dirina, Roccellina, Lecanactis, Schismatomma, Dendrographa, and Syncesia. Species in this group are distinguished by their black, hypothecia. Like the first clade, this group also displays a variety of growth forms from crustose to fruticose. The carbonaceous hypothecium clade further divides into two groups: the Dirina-Roccella subgroup and the Lecanactis-Schismatomma-Dendrographa-Syncesia-Roccellina subgroup.
A 2014 study, based on analyses of 341 sequences representing about 114 species, has further refined the understanding of these relationships. The genus Cresponea was identified as possibly sister to the rest of the Roccellaceae. The first major clade was expanded to include genera such as Dichosporidium, Enterographa, Erythrodecton, and Mazosia, in addition to Cresponea. In the second major clade, Sigridea californica was resolved as sister to the rest of the group, with the genera Gyrographa and Psoronactis forming a well-supported group near the base of this clade.
These phylogenetic findings have important implications for understanding the evolution of morphological traits within the family, particularly the development of different growth forms. The research has shown that the fruticose growth form has evolved independently multiple times within the family. This convergent evolution is particularly evident in the genus Roccellina, where the fruticose form has appeared several times.
The molecular data have also led to major taxonomic revisions within the family. Several species previously classified in other genera, particularly Opegrapha, have been transferred to Roccellaceae based on these analyses. Additionally, some genera have been shown to be polyphyletic (derived from more than one evolutionary ancestor), leading to the description of new genera to accommodate distinct lineages.
Evolution of growth forms
The evolution of growth forms within the Roccellaceae exemplifies convergent evolution. Molecular studies show that the fruticose growth form has evolved independently multiple times within the family. This phenomenon is particularly evident in the genus Roccellina, where the fruticose form has emerged at least three separate times from crustose ancestors. Similarly, the small genus Pentagenella shows evidence of both evolving the fruticose form and reverting to a crustose state. These repeated evolutions challenge the use of growth form as a primary characteristic for classification and highlight the adaptability of these lichens to various environments, suggesting that the fruticose form offers specific habitat advantages.
Other research further illuminates the evolution of other morphological characters within the family. For instance, the (blacked and charcoal-like) , a feature traditionally used to define certain genera, has a complex evolutionary history. Some species now placed in Roccellaceae based on molecular data possess a carbonaceous excipulum, despite it not being typical of the family. This suggests multiple gains or losses of this feature throughout the evolution of Roccellaceae.