| 1. |
- Chase, Madeline A., et al.
(författare)
-
Positive selection plays a major role in shaping signatures of differentiation across the genomic landscape of two independent Ficedula flycatcher species pairs
- 2021
-
Ingår i: Evolution. - : John Wiley & Sons. - 0014-3820 .- 1558-5646. ; 75:9, s. 2179-2196
-
Tidskriftsartikel (refereegranskat)abstract
- A current debate within population genomics surrounds the relevance of patterns of genomic differentiation between closely related species for our understanding of adaptation and speciation. Mounting evidence across many taxa suggests that the same genomic regions repeatedly develop elevated differentiation in independent species pairs. These regions often coincide with high gene density and/or low recombination, leading to the hypothesis that the genomic differentiation landscape mostly reflects a history of background selection, and reveals little about adaptation or speciation. A comparative genomics approach with multiple independent species pairs at a timescale where gene flow and ILS are negligible permits investigating whether different evolutionary processes are responsible for generating lineage-specific versus shared patterns of species differentiation. We use whole-genome resequencing data of 195 individuals from four Ficedula flycatcher species comprising two independent species pairs: collared and pied flycatchers, and red-breasted and taiga flycatchers. We found that both shared and lineage-specific FST peaks could partially be explained by selective sweeps, with recurrent selection likely to underlie shared signatures of selection, whereas indirect evidence supports a role of recombination landscape evolution in driving lineage-specific signatures of selection. This work therefore provides evidence for an interplay of positive selection and recombination to genomic landscape evolution.
|
|
| 2. |
- Chase, Madeline, et al.
(författare)
-
Evidence that genetic drift not adaptation drives fast-Z and large-Z effects in Ficedula flycatchers
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The sex chromosomes have been hypothesized to play a key role in driving adaptation and speciation across many taxa. The reason for this is thought to be the hemizygosity of the heteromorphic part of sex chromosomes in the heterogametic sex, which exposes recessive mutations to natural and sexual selection. The exposure of recessive beneficial mutations increases their rate of fixation on the sex chromosomes, which results in a faster rate of evolution. In addition, genetic incompatibilities between sex-linked loci are exposed faster in the genomic background of hybrids of divergent species, which makes sex chromosomes contribute disproportionately to reproductive isolation. However, in birds, which show a Z/W sex determination system, the disproportionate role of the Z-chromosome in adaptation and reproductive isolation is still debated. Instead, genetic drift has been proposed as the main driver of the so-called fast-Z and large-Z effects in birds. Here, we address this question in Ficedula flycatchers based on population resequencing data of six flycatcher species. Our results provide evidence for both the fast-Z and large-Z effects in Ficedula flycatchers and that these two phenomena are driven by genetic drift rather than positive selection. Genomic scans of selective sweeps and fixed differences in fact suggest a reduced action of positive selection on the Z-chromosome. We propose that the observed reduction in the efficacy of purifying selection on the Z-chromosome helps to establish genetic incompatibilities between Z-linked and autosomal loci, which could result in pronounced selective sweep signatures for compensatory mutations on the autosomes.
|
|
| 3. |
- Chase, Madeline
(författare)
-
Speciation genomics in Ficedula flycatchers
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Understanding what evolutionary processes have shaped patterns of genomic differentiation between species is a major aim of speciation genomics. However, disentangling the role of different processes that generate similar patterns remains a substantial challenge. Within this thesis, I aimed to infer the action of different evolutionary processes through population-level genome re-sequencing of closely related species. I explored how processes such as recombination, natural selection, and genetic drift interact to shape the genomic differentiation landscape among multiple species of Ficedula flycatcher. Collared flycatcher and pied flycatcher are a pair of closely related species, which hybridize in regions of secondary contact. Reproductive isolation is strong and hybrids appear to be sterile. I compared the differentiation landscape between collared and pied flycatchers with a more distantly related species pair, the red-breasted and taiga flycatchers. This comparison revealed elevated regions of genomic differentiation shared between the two pairs, i.e. shared differentiation peaks, and those unique to a single pair, i.e. lineage-specific differentiation peaks. Since the two species pairs share a negligible portion of genetic variation, shared patterns in the differentiation landscape should be driven and maintained by conserved processes, while lineage-specific patterns should be driven by lineage-specific changes in relevant evolutionary processes. Selective sweep scans suggested that both shared and lineage-specific peaks can result from adaptive evolution and that lineage-specific adaptation is not a sufficient determinant of lineage-specific peaks. Instead, lineage-specific differentiation peaks appeared to be driven by evolutionary changes in the recombination landscape, the dynamics of which had strong impacts on the detection of signatures of linked selection. I also found that adaptation did not play a prominent role on Z-chromosome differentiation. Both the fast-Z and large-Z effects were apparent within the flycatchers but appeared to be primarily driven by the increased role of genetic drift on the Z-chromosome due to its reduced effective population size compared to the autosomes. I hypothesized that the increased impact of genetic drift could speed up the buildup of genetic incompatibilities of Z-linked and autosomal loci and contribute to reproductive isolation. Finally, using long-read and HiC sequencing data, I generated high-quality reference genomes for the collared flycatcher and pied flycatcher, and provided a first glimpse of the role of structural variation in speciation. I observed an increased prevalence of inversions and translocations on the sex chromosomes and in differentiation peaks. Structural rearrangements may therefore represent an important source of genomic variation contributing to species divergence.
|
|
| 4. |
- Chase, Madeline, et al.
(författare)
-
The combination of HiFi and HiC sequencing technologies enables the investigation of structural variants in speciation of Ficedula flycatchers
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Structural variants, typically defined as mutations affecting more than 50bp, have been shown to encompass a significant portion of the genome and can have large phenotypic effects. Additionally, increasing empirical evidence demonstrates that structural variants may play a substantial role in speciation, which could previously have been overlooked because of difficulties in identifying them with short-read data. However, with the increased availability of long-read sequencing technology we are now equipped better than ever to address this limitation and study the contribution of different types of structural variants to genetic variation within and genetic differentiation between closely related species. Here, we follow this approach and combine PacBio HiFi and HiC sequencing for two closely related passerine birds, the collared flycatcher and the pied flycatcher. This enables us to generate a chromosome-level genome assembly for both species, and identify structural variants between the two species. Based on population-level HiFi sequencing for both species, we then investigate patterns of single nucleotide diversity and differentiation within and between species and their association with different types of structural variation. We find widespread structural variation between the two species, where both the sex chromosomes show a disproportionate number of structural variants, which may help explain the suspected role of the Z-chromosome in contributing to genetic incompatibilities. We also find that genomic differentiation peaks are enriched in both translocations and inversions, which supports a mechanistic role of structural variation in population differentiation and speciation.
|
|
| 5. |
- Chase, Madeline, et al.
(författare)
-
The role of recombination dynamics in shaping signatures of direct and indirect selection across the Ficedula flycatcher genome
- 2024
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Recombination is a central evolutionary process that reshuffles combinations of alleles along chromosomes, and consequently is expected to influence the efficacy of direct selection via Hill-Robertson interference. Additionally, the indirect effects of selection on neutral genetic diversity are expected to show a negative relationship with recombination rate, as background selection and genetic hitchhiking are stronger when recombination rate is low. However, owing to the limited availability of recombination rate estimates across divergent species, less is known about the impact of evolutionary changes in recombination rate on genomic signatures of selection. To address this question, we estimate recombination rate in two Ficedula flycatcher species, the taiga flycatcher (F. albicilla) and collared flycatcher (F. albicollis). We show that recombination rate is strongly correlated with signatures of indirect selection, and that evolutionary changes in recombination rate between species have observable impacts on this relationship. Conversely, signatures of direct selection on coding sequences show little to no relationship with recombination rate, even when restricted to genes where recombination rate is conserved between species. Thus, using measures of indirect and direct selection that bridge micro- and macro-evolutionary timescales, we demonstrate that the role of recombination rate and its dynamics varies for different signatures of selection.
|
|
| 6. |
- Chase, Madeline, et al.
(författare)
-
The role of recombination dynamics in shaping signatures of direct and indirect selection across the Ficedula flycatcher genome
- 2024
-
Ingår i: Proceedings of the Royal Society of London. Biological Sciences. - : ROYAL SOC. - 0962-8452 .- 1471-2954. ; 291:2015
-
Tidskriftsartikel (refereegranskat)abstract
- Recombination is a central evolutionary process that reshuffles combinations of alleles along chromosomes, and consequently is expected to influence the efficacy of direct selection via Hill-Robertson interference. Additionally, the indirect effects of selection on neutral genetic diversity are expected to show a negative relationship with recombination rate, as background selection and genetic hitchhiking are stronger when recombination rate is low. However, owing to the limited availability of recombination rate estimates across divergent species, the impact of evolutionary changes in recombination rate on genomic signatures of selection remains largely unexplored. To address this question, we estimate recombination rate in two Ficedula flycatcher species, the taiga flycatcher (Ficedula albicilla) and collared flycatcher (Ficedula albicollis). We show that recombination rate is strongly correlated with signatures of indirect selection, and that evolutionary changes in recombination rate between species have observable impacts on this relationship. Conversely, signatures of direct selection on coding sequences show little to no relationship with recombination rate, even when restricted to genes where recombination rate is conserved between species. Thus, using measures of indirect and direct selection that bridge micro- and macro-evolutionary timescales, we demonstrate that the role of recombination rate and its dynamics varies for different signatures of selection.
|
|
