Construction of the lichenized aerial mycelium in Botryolepraria (Verrucariales; Eurotiomycetes; Ascomycota): A somatic structure unique among fungi.

PREMISE

Lichen-forming fungi of genus Botryolepraria build no compact thalli, yet elevate and display algal symbionts upon their open, aerial mycelium. Although Botryolepraria occurs worldwide, the construction of its unique somatic form has not been examined in detail. We applied light microscopy and SEM to better understand how it is built and stabilized and how phycobionts are distributed during development.

METHODS

Specimens were examined with light microscopy, conventional SEM, and cryo-field emission SEM. Symbiont identity was corroborated by obtaining and comparing nucleotide sequences with those in the NCBI database.

RESULTS

Hyphal branches grew centripetally toward clusters of algal symbionts, while other branches grew centrifugally outward before further bifurcating to produce additional hyphal branches that reoriented centripetally toward algal clusters. Anastomosis of hyphae, tip to tip or laterally via short bridging connections, occurred frequently. The lichen was irregularly but often densely covered with thread-like hydrophobic materials that resemble certain forms of plant epicuticular waxes. Repeated interpenetration of suspended algal clusters by anastomosing mycobiont hyphae separated and distributed phycobiont cells within the expanding reticulum. Fungal ITS and LSU and algal rbcL sequences suggest closest proximity of mycobiont and phycobiont to Botryolepraria neotropica and Pseudostichococcus monallantoides, respectively, for the material studied.

CONCLUSIONS

Anastomosis of hyphae, in regions where algae are absent and at the surfaces of expanding phycobiont clusters, stabilizes the soma of Botrylopraria as a three-dimensional lattice. The dense covering of hydrophobic materials over an open aerial mycelium suggests adaptation to avoid surface condensation and optimize gas exchange.