Challenging the lichen concept: Turgidosculum ulvae (Verrucariaceae) represents an independent photobiont shift to a multicellular blade-like alga

Abstract

Some definitions of the term ‘lichen’ have often emphasized the role of the mycobiont as exhabitant in the symbiosis. Mastodia tessellata and Turgidosculum ulvae, both forming lichen-like associations with foliose algae, have traditionally defied that definition. In this study, we delve into the poorly known association of T. ulvae with Blidingia minima. Using four molecular markers (nrLSU, nrSSU, RPB1, mtSSU) we show that T. ulvae is a member of the family Verrucariaceae, closely related to the marine species Verrucaria ditmarsica. The presence of bitunicate asci and single-cell ascospores is confirmed. Our analysis of a fragment of the rbcL marker demonstrates that the photosynthetic partner belongs to B. minima, although relationships within this taxon remain unclear. Transmission electron microscopy allowed us to illustrate how T. ulvae interacts with Blidingia cells, and how haustoria in that species differ from those previously investigated in other marine lichen-forming fungi.