Introgression between highly divergent fungal sister species

Abstract

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.

Reticulate evolution and rapid development of reproductive barriers upon secondary contact pose challenges for species delineation in a forest fungus

“…In addition, Treemix identified introgression into the East Asian lineages from an unsampled lineage related to the outgroup T. fuscoviolaceum . Kinneberg et al 64 found support for ancient introgression between T. abietinum and T. fuscoviolaceum , and it is conceivable that introgressed genes from the T. fuscoviolaceum clade have spread into North America A and B through populations in East Asia.…”

“…In addition, Treemix identified introgression into the East Asian lineages from an unsampled lineage related to the outgroup T. fuscoviolaceum. Kinneberg et al 64 found support for ancient introgression between T. abietinum and T. fuscoviolaceum, and it is conceivable that introgressed genes from the T. fuscoviolaceum clade have spread into North America A and B through populations in East Asia. We applied demographic modelling with Fastsimcoal2 65 to explicitly test for secondary contact between the reproductively isolated North America A and B, and to estimate divergence time between the major lineages (table1).…”