The effects of co-culture on the expression of selected PKS genes in the lichenized fungus Xanthoparmelia taractica

Abstract

Lichens produce diverse secondary metabolites. A diversity of these compounds is synthesized by fungal polyketide synthases (PKSs). In this study, we catalogued the PKS genes from Xanthoparmelia taractica, a lichen with global distribution. To accomplish this, we isolated the symbionts to sequence the whole genome of the mycobiont and established an in vitro co-culture system for this lichen. We also added an endolichenic fungus, Coniochaeta fibrosae, to this co-culture to evaluate its effect on lichen symbiosis. The genome of the mycobiont X. taractica was around 43.1 Mb with 10,730 ORFs. Twenty-eight PKS genes were identified in the genome. These included 27 Type I and one Type III gene. Except for three PKS genes, XTPKS12, XTPKS18, and XTPKS22, the function of the majority of PKS genes remained unknown. We selected these genes for the expression analyses using a co-culture system. The co-culture system that included the mycobiont and the photobiont showed an early stage of lichenization because the fungi produced a hyphal network connecting and penetrating the algal cells. Also, XTPKS12 was down-regulated and XTPKS18 and XTPKS22 were modestly up-regulated. As predicted, C. fibrosae did not participate in the symbiosis. This study reconfirms that Type I is the most dominant PKS gene in lichenized fungi and the function of these genes might be influenced by symbiosis.