Do lichen secondary compounds play a role in highly specific fungal parasitism?

Chemical interactions between highly host-specific lichenicolous fungi and their lichen hosts have been little studied. In an allometric study, we quantified carbon-based secondary compounds (CBSCs) in a mixed natural Lobarina scrobiculata population (N = 147) of the normal and the stictic acid-deficient chemotypes, both with and without galls of Plectocarpon scrobiculatae. We assessed the correlation between the presence/abundance of parasite galls and the lichen CBSCs contents, and quantified size-dependent contents of CBSCs. The parasite produced galls similarly in both chemotypes, indicating that the stictic acid complex does not deter Plectocarpon. Within both chemotypes, thalli with Plectocarpon had half the contents of all individual CBSCs than those without galls. There was a significant size-dependent increase in CBSC contents in thalli without galls, but not in those with. This study shows that lichen chemistry is involved in highly host-specific fungal parasitism, and widens our knowledge of specialized biotrophic fungal interactions.