Introgression between highly divergent fungal sister species

To understand how species evolve and adapt to changing environments, it is
important to study gene flow and introgression due to their influence on
speciation and radiation events. Here, we apply a novel experimental
system for investigating these mechanisms using natural populations. The
system is based on two fungal sister species with morphological and
ecological similarities occurring in overlapping habitats. We examined
introgression between these species by conducting whole genome sequencing
of individuals from populations in North America and Europe. We assessed
genome-wide nucleotide divergence and performed crossing experiments to
study reproductive barriers. We further used ABBA-BABA statistics together
with a network analysis to investigate introgression, and conducted
demographic modelling to gain insight into divergence times and
introgression events. The results revealed that the species are highly
divergent and incompatible in vitro. Despite this, small regions of
introgression were scattered throughout the genomes and one introgression
event likely involves a ghost population (extant or extinct). This study
demonstrates that introgression can be found among divergent species and
that population histories can be studied without collections of all the
populations involved. Moreover, the experimental system is shown to be a
useful tool for research on reproductive isolation in natural populations.