Lichen speciation is sparked by a substrate requirement shift and reproduction mode differentiation

Abstract

We show that obligate lignicoles in lichenized Micarea are predominately asexual whereas most facultative lignicoles reproduce sexually. Our 3 loci phylogenetic analyses (ITS, mtSSU, Mcm7) together with ancestral state reconstruction show that the shift in reproduction mode has evolved independently several times within the group and that facultative and obligate lignicoles are sister species. The analyses support the assumption that the ancestor of these species was a facultative lignicole. We hypotezise that a shift in substrate requirement from bark to wood leads to differentiation in reproduction mode and becomes a driver of speciation. This is the first example of lichenized fungi where reproduction mode is linked to substrate requirement. This is also the first example where such a linkage is demonstrated to spark lichen speciation. Our main hypothesis is that obligate species on dead wood need to colonize new suitable substrata relatively fast and asexual reproduction is more effective a strategy for successful colonization. Our main hypothesis for explaining the discovered phenomenon involves the species life cycle: species on decaying wood face a significant challenge because the ecological properties of their substratum change relatively fast. When this happens, species need to colonize new suitable substrata. This may set a time limit, where asexual reproduction is a faster and more effective strategy for successful colonization.