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| 2. |
- Boman, Jesper, et al.
(author)
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Evolution of hybrid inviability associated with chromosome fusions
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Other publication (other academic/artistic)abstract
- Chromosomal rearrangements, such as inversions, have received considerable attention in the speciation literature due to their hampering effects on recombination. However, less is known about how other rearrangements, such as chromosome fissions and fusions, can affect the evolution of reproductive isolation. Here, we used crosses between populations of the wood white butterfly (Leptidea sinapis) with different karyotypes to identify genomic regions associated with hybrid inviability. We mapped candidate loci for hybrid inviability by contrasting allele frequencies between F2 hybrids that survived until the adult stage with individuals of the same cohort that succumbed to hybrid incompatibilities. Hybrid inviability factors were predominantly found in fast-evolving regions with reduced recombination rates, especially in regions where chromosome fusions have occurred. By analyzing sequencing coverage, we excluded aneuploidies as a direct link between hybrid inviability and chromosome fusions. Instead, our results point to an indirect relationship between hybrid inviability and chromosome fusions, possibly related to reductions in recombination rate caused by fusions. These results highlight that the extensive variation in chromosome numbers observed across the tree of life does not only distinguish species but can also be involved in speciation by being hotspots for the early evolution of postzygotic reproductive isolation.
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| 3. |
- Boman, Jesper, et al.
(author)
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On the origin of an insular hybrid butterfly species
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Other publication (other academic/artistic)abstract
- Most species arise from the divergence of two populations within a species, but this is not only way speciation can occur. A new species can also evolve when diverging lineages hybridize and give rise to a persistent and ecologically differentiated species. Hybrid speciation in animals has been intensely debated, partly because hard evidence for the process have been difficult to gain. Recent access to large-scale, whole-genome sequencing data and development of novel analytical methods have made it more feasible to statistically test for hybrid origin of lineages. Here we report the discovery of a hybrid butterfly lineage. This lineage is mainly inhabiting an island in the Baltic Sea in Northern Europe and was previously described as a subspecies (horkei) of one of the parental species (Aricia artaxerxes). By analyzing whole-genome resequencing data, we conclude that horkei originated as a consequence of hybridization between A. artaxerxes and A. agestis. We show that this hybridization event occurred approximately 54,000 years ago, predating the last glaciation of the current distribution range. Horkei must therefore have persisted long enough to be able to colonize its current distribution range, despite that this range lies between the current ranges of the parental species. The hybrid origin, the maintenance of genomic integrity through time periods with dramatic climatic changes and the expression of a combination of parental traits - such as voltinism and host plant use - suggest that horkei can be considered a distinct species (Aricia horkei stat. nov.). Thus, we add to a growing list of hybrid speciation cases in animals.
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| 4. |
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| 5. |
- Höök, Lars, et al.
(author)
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Dualistic dosage compensation and rapid evolution of expression balance in response to W chromosome degeneration in Leptidea butterflies
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Other publication (other academic/artistic)abstract
- The evolution of dimorphic sex chromosomes from initially homologous autosomes is generally explained by sex-specific selection to maintain linkage between a sex determining locus and genes that are beneficial to the same sex. While initially beneficial, the strong linkage and reduced recombination causes differentiation and degeneration of many initially shared genes. Reduced copy numbers can have severe consequences for the balance of gene expression levels between sex-linked genes and the rest of the genome. Consequently, dosage compensation has evolved independently in different lineages to mitigate the detrimental effects of unbalanced expression of sex-linked genes in the heterogametic sex. However, the variation in sex chromosome regulation in different lineages, puts the need to restore expression to ancestral levels into question. In particular, a general difference has been observed between male- (XY) and female-heterogametic (ZW) systems. In contrast to the X chromosome upregulation in heterogametic males in the XY-systems, the Z chromosomes are rarely upregulated in the heterogametic females in organisms with ZW-systems. Instead, the Z chromosomes are often downregulated in the homogametic males to achieve inter-sexual balance. Although progress has been made to understand what causes this discrepancy, comparative approaches are limited by long divergence times and ancient sex chromosome systems. An attractive approach is therefore to study the evolution of gene regulation on recently derived neo-sex chromosomes, formed through fusions between ancestral sex chromosomes and autosomes. Here, we investigated dosage compensation of neo-sex chromosomes in three closely related butterflies in the cryptic wood white clade (Leptidea). Importantly, the species have acquired multiple sex chromosomes, and dosage compensation could therefore have evolved repeatedly in the clade. Our analyses reveal a mixture of gene expression patterns which suggests that distinct modes of dosage compensation have evolved on the different Z chromosomes. In addition, we detect evidence that dosage balancing mechanisms have been rapidly recruited to the youngest neo-Z chromosome, to counteract an ongoing degeneration of neo-W gametologs. The results add to a growing list of examples where diverse dosage compensation mechanisms can evolve within a single species, and suggests that various regulatory mechanisms are not restricted to specific sex chromosome systems.
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| 6. |
- Höök, Lars, 1980-, et al.
(author)
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Temporal dynamics of faster neo-Z evolution in butterflies
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Other publication (other academic/artistic)abstract
- The faster-Z/X hypothesis predicts that sex-linked genes should diverge faster than autosomal genes and may therefore play important roles in speciation. However, studies across different lineages have shown mixed support for this effect, a variation that has been explained by various evolutionary mechanisms. So far, most analyses have focused on systems with old and well differentiated sex chromosomes, but less is known about divergence of more recently acquired neo-sex chromosomes. In the female heterogametic order Lepidoptera (moths and butterflies), fusions between the ancestral Z chromosome and autosomes are relatively frequent, but the evolutionary dynamics of neo Z-linked genes have not been explored in detail. Here, we analysed the faster-Z effect in Leptidea sinapis, a butterfly with an exceptionally reorganized genome and three Z chromosomes. We show that the neo-Z chromosomes have been acquired in a stepwise fashion, resulting in distinct strata of differentiation and masculinization. While Z-linked divergence generally seems to have been driven by adaptive processes, the relative effects of selection and drift showed a temporal trend where selection has been more prevalent for genes located on older Z linked regions, causing increased divergence of Z-linked genes with female-biased expression. In contrast, the intensity of selection on genes located on the most recently acquired neo-Z chromosome (Z3) appears to have been hampered by the presence of gametologs on the largely intact, homologous neo-W chromosome. However, the intermediately aged neo-Z chromosome (Z2), which is completely differentiated from W gametologs, showed less evolutionary constraint than the ancestral Z, resulting in particularly fast evolution. Our results therefore support that neo-sex chromosomes can constitute temporary hot-spots of adaptation and divergence. The underlying dynamics are likely causally linked to shifts in selective constraints, evolution of gene expression and the degeneration of W-linked gametologs which gradually expose Z-linked genes to selection.
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| 7. |
- Künstner, Axel, et al.
(author)
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Gene content and patterns of gene expression in the flycatcher genome
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Other publication (other academic/artistic)abstract
- Phenotypic evolution may be driven by changes in the sequence of protein-coding genes or by the way (when, where, at what level) proteins are expressed. Generally, our knowledge about the evolution of gene expression is relatively limited, and this is particularly so for wild populations. Collared flycatcher (Ficedula albicollis) and pied flycatcher (F. hypoleuca) are two recently diverged passerine birds, which have been subject to extensive ecological research, including aspects of speciation. We obtained RNA-seq data with Illumina technology from 10 adult individuals per species (five females and five males) using brain, kidney, liver, lung, muscle, skin, ovary, and testis tissue (plus eight embryos of each species). A total of more than 1 billion sequencing reads were assembled into >15.000 gene models for each species. The proportion of differentially expressed genes between species ranged from 8% to 18% per adult tissue. Very few GO categories were found to be overrepresented among differentially expressed genes, which at least in part might reflect that orphan and not yet annotated genes are prone to evolve more rapidly in gene expression level. However, in testis, the category olfactory receptor activity was significantly overrepresented among differentially expressed genes and it is of interest to note that this category of genes is involved in sperm-egg communication and thereby potentially may contribute to reproductive incompatibility between the two species. Genes with a high degree of differentiation in gene expression between species tended to have high rates of sequence evolution (high dN/dS). Overall, this study illustrates both the feasibility and usefulness of deep transcriptome sequencing in non-model organisms.
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| 8. |
- McFarlane, S. Eryn, et al.
(author)
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RNA sequencing provides insight into metabolic dysfunction of hybrids between a recently diverged songbird species pair
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Other publication (other academic/artistic)abstract
- Hybrid dysfunction is thought to gradually build up through the accumulation of clashes between genes as they diverge between the parental species. However, analyses of genetic incompatibilities are generally biased towards long diverged species that are kept under laboratory conditions. Here, we used RNAseq to evaluate 1) whether there was differential gene expression between naturally occurring Ficedula flycatcher hybrids and parental species in energetically expensive alimentary organs, and 2) if such differential gene expression was, based on Gene Ontology (GO) terms, functionally related to Resting Metabolic Rate (RMR) and energy production. We found substantial differential gene expression in all pairwise contrasts, but fewer functional differences between the parental species than between hybrids and either parental species. Some of the differentially expressed genes underlay the OXPHOS pathway, and significantly more than expected GO terms associated with metabolic function were differentially expressed between hybrids and either parental species in the liver. Our results corroborate the idea that tightly co-evolved mitochondrial and nuclear genes underlying the Oxidative Phosphorylation (OXPHOS) pathway can become miss-matched in hybrids and cause malfunctioning phenotypes. Mitonuclear interactions affecting OXPHOS have the potential to both quickly diverge in allopatry as populations adapt to different climate regimes and to cause hybrid genetic dysfunction at secondary contact
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| 9. |
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| 10. |
- Näsvall, Karin, et al.
(author)
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Activity profiles of regulatory elements and associations with the oogenesis-flight syndrome in a long-distance butterfly migrant.
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Other publication (other academic/artistic)abstract
- The oogenesis-flight syndrome reflects the temporal allocation of energy resources between dispersal / migration and reproduction and is a key concept in research on migratory behaviour in animals. Here, we used an experimental set-up to assess how variation in host plant abundance affected the activity of regulatory elements in the painted lady butterfly (Vanessa cardui), a model species for insect migratory behaviour studies. The results indicate that recently eclosed females that had access to host plants invested in reproduction at an earlier stage and that variation in host plant abundance triggered significant differences in regulatory element activity via acetylation. By analysing functions of genes in the vicinity of significant differences in regulatory activity we pinpointed a set of categories that can be relevant for how females perceive the environment and allocate resources for either migration or reproduction. The functions of genes in the vicinity of differentially activated regions were associated with metabolism, egg shell formation, female receptivity, muscle activity, pheromone binding and mini-chromosome maintenance. Our results provide a first glimpse into the regulatory underpinnings of the oogenesis-flight syndrome and a starting point for more detailed understanding of the links between environmental variation, gene regulation and migratory behaviour in butterflies.
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