Biology subjects

Lu, D. S.

Publications and source records attributed to Lu, D. S..

3 recordsLinked to original sources

bioRxiv2023

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

Reproductive barriers within morphospecies of the mushroom-forming fungi tend to be stronger between sympatric lineages, leading to speculation on whether they are being reinforced by selection against hybrids. In this study we use population genomic analyses together with in vitro crosses of a global sample of the wood decay fungus Trichaptum abietinum to investigate reproductive barriers within this morphotaxon and the processes that have shaped them. Our phylogeographic analyses show that the morphospecies is delimited into six major lineages, one in Asia, two in Europe, and three in North America. The two lineages present in Europe are interfertile and admixed, whereas our crosses show that the North American lineages are reproductively isolated. In Asia a more complex pattern appears, with partial intersterility between multiple sub-lineages that likely originated independently and more recently than the reproductive barriers in North America. We found pre-mating barriers in T. abietinum to be moderately correlated with genomic divergence, whereas fitness of the mated hybrids showed a strong positive correlation with increasing genomic divergence. Genome wide association analyses identified candidate genes with programmed cell death annotations, which are known to be involved in intersterility in distantly related fungi. Our demographic modelling and phylogenetic network analyses suggest that reproductive barriers in Trichaptum abietinum could have been reinforced upon secondary contact between lineages that diverged in allopatry during the Pleistocene glacial cycles. Our combination of experimental and genomic approaches demonstrate how T. abietinum is a tractable system for studying speciation mechanims.

evolutionary biology↗

bioRxiv2022

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.

evolutionary biology↗

bioRxiv2021

Long-term balancing selection in highly dynamic mating loci generates trans-species polymorphisms in fungi

Balancing selection, an evolutionary force that retains genetic diversity, has been detected in multiple genes and organisms, such as the sexual mating loci in fungi. However, to quantify the strength of balancing selection and define the mating-related genes require a large number of specimens. In tetrapolar basidiomycete fungi, sexual type is determined by two unlinked loci, MATA and MATB. Genes in both loci defines mating type identity, control successful mating and completion of the life cycle. These loci are usually highly diverse. Previous studies have speculated, based on culture crosses, that species of the non-model genus Trichaptum (Hymenochaetales, Basidiomycota) possess a tetrapolar mating system, with multiple alleles. Here, we sequenced a hundred and eighty specimens of three Trichaptum species. We characterized the chromosomal location of MATA and MATB, the molecular structure of MAT regions and their allelic richness. Our sequencing effort was sufficient to molecularly characterize multiple MAT alleles segregating before the speciation event of Trichaptum species. Our analyses suggested that long-term balancing selection has generated trans-species polymorphisms. Mating sequences were classified in different allelic classes based on an amino acid identity (AAI) threshold supported by phylogenetics. The inferred allelic information mirrored the outcome of in vitro crosses, thus allowing us to support the degree of allelic divergence needed for successful mating. Even with the high amount of divergence, key amino acids in functional domains are conserved. The observed allelic classes could potentially generate 14,560 different mating types. We conclude that the genetic diversity of mating in Trichaptum loci is due to long-term balancing selection, with limited recombination and duplication activity. Our large number of sequenced specimens highlighted the importance of sequencing multiple individuals from different species to detect the mating-related genes, the mechanisms generating diversity and the evolutionary forces maintaining them. Author summaryFungi have complex mating systems, and basidiomycete fungi can encode thousands of mating types. Individuals with the same mating type cannot mate. This sexual system has evolved to facilitate sexual mating, increasing the chances to recombine into advantageous allelic combination and prune deleterious alleles. We explored the genomes of hundred and eighty specimens, combined with experimental mating studies of selected specimens, from a non-model organism (Trichaptum). We characterized the genomic regions controlling sex. The mating ability of the specimens confirmed the role of the mating alleles observed in the genomic data. The detailed analyses of many specimens allowed us to observe gene duplication and rearrangements within the mating loci, increasing the diversity within these loci. We supported previous suggestions of balancing selection in this region, an evolutionary force that maintains genomic diversity. These results supports that our fungal specimens are prone to outcross, which might facilitate the adaptation to new conditions.

genomics↗