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

Abstract

AbstractSome definitions of the term ‘lichen’ have often emphasized the role of the mycobiont as exhabitant in the symbiosis.Mastodia tessellataandTurgidosculum ulvae, both forming lichen-like associations with foliose algae, have traditionally defied that definition. In this study, we delve into the poorly known association ofT. ulvaewithBlidingia minima. Using four molecular markers (nrLSU, nrSSU,RPB1, mtSSU) we show thatT. ulvaeis a member of the familyVerrucariaceae, closely related to the marine speciesVerrucaria ditmarsica. The presence of bitunicate asci and single-cell ascospores is confirmed. Our analysis of a fragment of therbcL marker demonstrates that the photosynthetic partner belongs toB. minima, although relationships within this taxon remain unclear. Transmission electron microscopy allowed us to illustrate howT. ulvaeinteracts withBlidingiacells, and how haustoria in that species differ from those previously investigated in other marine lichen-forming fungi.