Independent, structurally distinct transitions to microfruticose growth in the crustose genus Porina (Ostropales, Lecanoromycetes): new isidioid species from south-western Florida

Abstract

Porina is a widely distributed, species-rich genus of crustose, lichen-forming fungi, some with thalline outgrowths that have been recognized as isidia. We studied three taxa with thalli consisting chiefly of ascending isidioid structures occurring on trunks and branches of Taxodium in southwestern Florida, and provide details of their structure with light and electron microscopy. Two of these taxa we describe as new species: P. microcoralloides and P. nanoarbuscula. Genetic sequences (mtSSU) suggest that they are closely related to each other, yet they differ markedly in the size, morphology and anatomical organization of their isidioid branches as well as in the length of their ascospores. In the three Floridian taxa studied, the crustose portion of the thallus is partly endophloeodic and partly superficial, the latter often patchy, evanescent or inconspicuous, and completely lacks the differentiated anatomical organization characteristic of the isidioid structures arising from it. In Porina microcoralloides, the ascendant thallus consists of branched, coralloid inflated structures with phycobiont (Trentepohlia) unicells arranged at the periphery of a loose central medulla. Sparse fungal cells are interspersed and overlie the algal layer in places, but no differentiated cortex is present, leaving phycobiont cells more or less exposed at the surface. In the closely related Porina nanoarbuscula, the isidioid structures are much finer, more densely branched, and composed of a single, central file of roughly spherical Trentepohlia cells surrounded by a jacket of subglobose fungal cells. The ascospores of P. microcoralloides are more than twice the length of those of P. nanoarbuscula. Although thalli of these two Porina species occur in the same habitats and are sometimes found growing alongside each other, phylogenetic analysis of rbcL sequences suggest that they partner with distinct clades of Trentepohlia phycobionts. A third taxon examined, Porina cf. scabrida, is morphologically rather similar to P. microcoralloides, but the ascendant branches are bright yellow-orange, more cylindrical, and corticated by a thin layer of agglutinated fungal hyphae; perithecia were not seen. Analysis of mtSSU sequences places it distant from P. microcoralloides and P. nanoarbuscula phylogenetically. None of the Floridian taxa studied was particularly close to the European isidiate species Porina hibernica and P. pseudohibernica, which appeared as sister to each other in the analysis. While a particular type of isidiose structure may be reliably characteristic of specific taxa, similarities or differences in these structures do not seem to be useful indicators of phylogenetic proximity or distances among taxa. The morphological trends evident in Porina suggest that multiple transitions from crustose to isidioid or microfruticose growth have arisen repeatedly and in quite different ways within this single genus. At least some of the diverse structures treated within the broad concept of isidia may be representative of the developmental pathways by which fruticose growth forms may arise.

Further expansion of morphological variability in the Porinaceae (Ascomycota, Ostropales) with the placement of the enigmatic genus Gallaicolichen

“…1), which is consistent with recent phylogenetic reconstructions (Sobreira et al 2018;Lücking et al 2020;Orange et al 2020;Ertz & Diederich 2022;Sanders et al 2023). The 14 lineages highlighted in Sobreira et al (2018) are supported, along with five new clades added by Lücking et al (2020), Ertz & Diederich (2022) and Sanders et al (2023).…”

“…The 14 lineages highlighted in Sobreira et al (2018) are supported, along with five new clades added by Lücking et al . (2020), Ertz & Diederich (2022) and Sanders et al (2023).

Figure 1.Best-scoring maximum-likelihood (ML) tree of Porinaceae based on the mtSSU marker, showing the position of Gallaicolichen pacificus highlighted in green and the new sequences produced in the present study in bold.

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“…2018; Lücking et al . 2020; Orange et al 2020; Ertz & Diederich 2022; Sanders et al 2023). The 14 lineages highlighted in Sobreira et al (2018) are supported, along with five new clades added by Lücking et al .…”

“…2018; Lücking et al . 2020; Ertz & Diederich 2022; Sanders et al 2023), for a total of 122 sequences in the matrix. Four accessions of Coenogonium were selected as outgroup.…”

“…After confirmation of the newly generated sequences as members of the Porinaceae, a dataset of mtSSU sequences was assembled for resolving the exact placement of the newly sequenced taxa in a broader phylogeny of the family Porinaceae. Sequences retrieved from GenBank were selected mainly from previous studies of the family (Sobreira et al 2018;Lücking et al 2020;Ertz & Diederich 2022;Sanders et al 2023), for a total of 122 sequences in the matrix. Four accessions of Coenogonium were selected as outgroup.…”