Barrier loci and progress towards evolutionary generalities

No abstract available

Genomics of adaptation revealed in threespine stickleback

Natural selection is the ultimate, but not only force underlying organismal diversity. Despite this general biological insight, our understanding of how selection targets and shapes the genome during adaptation remains incomplete and is the central quest of this thesis. My main model organism is the threespine stickleback fish (Gasterosteus aculeatus). Stickleback provide an outstanding opportunity to study adaptive evolutionary change, because marine ancestors have repeatedly colonized and adapted to different freshwater environments all over the northern hemisphere since the last glacial retreat about 12,000 years ago. Besides wild populations, I also make use of lab-raised stickleback hybrids from controlled crosses for this thesis work. Thousands of genome-wide genetic polymorphisms (i.e., genetic markers) called in marine, but predominantly in distinct lake and stream stickleback populations from different geographic locations allow me to decipher the number and position of genomic targets of selection in the early phase of adaptive divergence. I find that selection acts on many loci distributed widely across the genome. On a genomic scale, the recombination landscape along chromosomes proves to be - in concert with selection - an important factor in driving heterogeneous genetic differentiation among populations. To investigate the rate of recombination across the stickleback's genome in more detail, I use an artificially crossed second-generation (F2) population. This reveals constraints in the frequency and location of detectable recombination events (i.e., cross-overs) within the genome. For example, cross-overs prove to be more frequent in chromosome peripheries than centers. This, together with selection, results in decreased within-population genetic diversity and increased between-population differentiation in the centers of chromosomes as opposed to the peripheries. Furthermore, I show that the cessation of recombination between the heterogametic sex chromosomes occurred in independent bouts. As a consequence, I find extended genomic regions distinct in their degree of degeneration between the X and Y chromosome, so called evolutionary strata. Finally, recombination reveals to be an important determinant of other aspects of a genome, such as its nucleotide composition. Integrating theoretical modeling with targeted and genome-wide sequencing, my research further demonstrates that the inference and interpretation of genomic regions exhibiting particularly high and low population differentiation is not as straightforward as commonly believed. This is because the type of genetic variation available to selection (i.e., pre-existing vs. de novo variation) as well as the mode of adaptation (i.e., divergent vs. parallel adaptation) influence the way neutral variation is shaped by selection across the genome. I demonstrate that a genomic region of high differentiation may not necessarily be indicative of divergent selection when populations adapt in parallel to similar environments from a shared pool of genetic variation. Based on several hundreds of F2 specimens reared under standardized conditions in the laboratory, I also link variation in heritable phenotypic traits to genetic variation, a research program generally referred to as quantitative trait locus (QTL) mapping. Corroborating with the results from my genome scans within and between wild populations (indicating that adaptive divergence involves many loci widespread across the genome), QTL mapping reveals that most phenotypic traits are controlled by numerous genetic loci. In general, each of these loci explains a small fraction of the entire phenotypic trait variation. I also use high resolution SNP data to infer the demographic history of several lake and stream stickleback populations from the Lake Constance watershed (Central Europe) and demonstrate that the repeated occurrence of similar stream phenotypes are, in this particular system, better explained by an evolutionary scenario of 'ecological vicariance' rather than repeated parallel divergence. I then show how selection has shaped local and broad-scale linkage, diversity and differentiation across the genome in these populations. Interestingly, I find evidence for strong divergent selection acting on large chromosomal rearrangements I had previously detected to be important for marine vs. freshwater adaptation. This finding provides a strong case for the re-use of pre-existing genetic variation in stickleback and demonstrates that the same genomic regions can be involved in adaptive divergence between disparate ecotype pairs. Overall, I come to conclude that signatures of selection are - at various physical scales - frequent within the stickleback genome. Stickleback repeatedly use pre-existing genetic variation, shared across various geographic ranges, to adapt to similar or disparate environments. Yet, there is a substantial degree of genetic non-parallelism - at least at the level of neutral markers - that goes along with parallel phenotypic evolution. My thesis emphasizes that the reliable detection and interpretation of genomic signatures of selection requires integrating many replicate study populations within a clear-cut ecological and demographic framework, as well as complementary analytical approaches. Controlled crossing experiments and theoretical modeling are key to deriving predictions about the genomics of adaptation in the wild and to facilitate our understanding of complex biological processes and patterns

Genetic architecture of a pollinator shift and its fate in secondary hybrid zones of two Petunia species.

BACKGROUND Theory suggests that the genetic architecture of traits under divergent natural selection influences how easily reproductive barriers evolve and are maintained between species. Divergently selected traits with a simple genetic architecture (few loci with major phenotypic effects) should facilitate the establishment and maintenance of reproductive isolation between species that are still connected by some gene flow. While empirical support for this idea appears to be mixed, most studies test the influence of trait architectures on reproductive isolation only indirectly. Petunia plant species are, in part, reproductively isolated by their different pollinators. To investigate the genetic causes and consequences of this ecological isolation, we deciphered the genetic architecture of three floral pollination syndrome traits in naturally occurring hybrids between the widespread Petunia axillaris and the highly endemic and endangered P. exserta. RESULTS Using population genetics, Bayesian linear mixed modelling and genome-wide association studies, we found that the three pollination syndrome traits vary in genetic architecture. Few genome regions explain a majority of the variation in flavonol content (defining UV floral colour) and strongly predict the trait value in hybrids irrespective of interspecific admixture in the rest of their genomes. In contrast, variation in pistil exsertion and anthocyanin content (defining visible floral colour) is controlled by many genome-wide loci. Opposite to flavonol content, the genome-wide proportion of admixture between the two species predicts trait values in their hybrids. Finally, the genome regions strongly associated with the traits do not show extreme divergence between individuals representing the two species, suggesting that divergent selection on these genome regions is relatively weak within their contact zones. CONCLUSIONS Among the traits analysed, those with a more complex genetic architecture are best maintained in association with the species upon their secondary contact. We propose that this maintained genotype-phenotype association is a coincidental consequence of the complex genetic architectures of these traits: some of their many underlying small-effect loci are likely to be coincidentally linked with the actual barrier loci keeping these species partially isolated upon secondary contact. Hence, the genetic architecture of a trait seems to matter for the outcome of hybridization not only then when the trait itself is under selection

In search of the most mysterious orthopteran of Europe: the Reed cricket Natula averni (Orthoptera: Gryllidae).

In the last few years a lot of new information has become available on Natula averni. As the common name we propose Reed cricket, because the species was found almost exclusively in reed beds. Recent findings show that this species is more abundant than previously thought. The species can easily be found with knowledge of distribution, habitat and song, all described in this publication. Nevertheless a lot of questions remain about the taxonomy. We hope that information gathered after this publication will help us to reveal the proper identity of reed crickets in Europe

Genetic architecture of a pollinator shift and its fate in secondary hybrid zones of two Petunia species

Background: Theory suggests that the genetic architecture of traits under divergent natural selection influences how easily reproductive barriers evolve and are maintained between species. Divergently selected traits with a simple genetic architecture (few loci with major phenotypic effects) should facilitate the establishment and maintenance of reproductive isolation between species that are still connected by some gene flow. While empirical support for this idea appears to be mixed, most studies test the influence of trait architectures on reproductive isolation only indirectly. Petunia plant species are, in part, reproductively isolated by their different pollinators. To investigate the genetic causes and consequences of this ecological isolation, we deciphered the genetic architecture of three floral pollination syndrome traits in naturally occurring hybrids between the widespread Petunia axillaris and the highly endemic and endangered P. exserta. Results Using population genetics, Bayesian linear mixed modelling and genome-wide association studies, we found that the three pollination syndrome traits vary in genetic architecture. Few genome regions explain a majority of the variation in flavonol content (defining UV floral colour) and strongly predict the trait value in hybrids irrespective of interspecific admixture in the rest of their genomes. In contrast, variation in pistil exsertion and anthocyanin content (defining visible floral colour) is controlled by many genome-wide loci. Opposite to flavonol content, the genome-wide proportion of admixture between the two species predicts trait values in their hybrids. Finally, the genome regions strongly associated with the traits do not show extreme divergence between individuals representing the two species, suggesting that divergent selection on these genome regions is relatively weak within their contact zones. Conclusions: Among the traits analysed, those with a more complex genetic architecture are best maintained in association with the species upon their secondary contact. We propose that this maintained genotype–phenotype association is a coincidental consequence of the complex genetic architectures of these traits: some of their many underlying small-effect loci are likely to be coincidentally linked with the actual barrier loci keeping these species partially isolated upon secondary contact. Hence, the genetic architecture of a trait seems to matter for the outcome of hybridization not only then when the trait itself is under selection

One-Stage Coverage of Leg Region Defects with STSG Combined with VAC Dressing Improves Early Patient Mobilisation and Graft Take: A Comparative Study

Lower limb skin defects are very common and can result from a wide range of aetiologies. Split thickness skin graft (STSG) is a widely used method to address these problems. The role of postoperative dressing is primary as it permits one to apply a uniform pressure over the grafted area and promote adherence. Focusing on lower limb reconstruction, our clinical study compares the application of V.A.C. (Vacuum Assisted Closure) Therapy vs. conventional dressing in the immediate postoperative period following skin grafting. We included in the study all patients who received skin grafts on the leg region between January 2015 and December 2018, despite the aetiology of the defect. Only reconstructions with complete preoperative and postoperative follow-up data were included in the study. Patients were divided into two groups depending on if they received a traditional compressive dressing or a VAC dressing in the immediate postoperative period. We could retain 92 patients, 23 in the No VAC group and 69 in the VAC group. The patients included in the VAC group showed a statistically significant higher rate of graft take together with a lower immobilisation time (p < 0.05). Moreover, a lower rate of postoperative infection was recorded in the VAC group. This study represents the largest in the literature to report in detail surgical outcomes comparing the use of VAC therapy vs. conventional dressing after STSG in the postoperative management of lower limb reconstruction using skin grafts. VAC therapy was used to secure the grafts in the leg region, increasing the early graft take rate while at the same time improving patient mobilisation

Depth-dependent abundance of Midas Cichlid fish ( Amphilophus spp: ) in two Nicaraguan crater lakes

The Midas Cichlid species complex (Amphilophus spp.) in Central America serves as a prominent model system to study sympatric speciation and parallel adaptive radiation, since small arrays of equivalent ecotype morphs have evolved independently in different crater lakes. While the taxonomy and evolutionary history of the different species are well resolved, little is known about basic ecological parameters of Midas Cichlid assemblages. Here, we use a line transect survey to investigate the depth-dependent abundance of Amphilophus spp. along the shores of two Nicaraguan crater lakes, Apoyo and Xiloá. We find a considerable higher density of Midas cichlids in Lake Xiloá as compared to Lake Apoyo, especially at the shallowest depth level. This might be due to the higher eutrophication level of Lake Xiloá and associated differences in food availability, and/or the presence of a greater diversity of niches in that lake. In any case, convergent forms evolved despite noticeable differences in size, age, eutrophication level, and carrying capacity. Further, our data provide abundance and density estimates for Midas Cichlid fish, which serve as baseline for future surveys of these ecosystems and are also relevant to past and future modeling of ecological speciatio

Genomics of Divergence along a Continuum of Parapatric Population Differentiation

MM received funding from the Max Planck innovation funds for this project. PGDF was supported by a Marie Curie European Reintegration Grant (proposal nr 270891). CE was supported by German Science Foundation grants (DFG, EI 841/4-1 and EI 841/6-1)