Adaptive divergence under gene flow along an environmental gradient in two coexisting stickleback species

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Contrasting population genetic responses to migration barriers in two native and an invasive freshwater fish

Habitat fragmentation impacts the distribution of genetic diversity and population genetic structure. Therefore, protecting the evolutionary potential of species, especially in the context of the current rate of human-induced environmental change, is an important goal. In riverine ecosystems, migration barriers affect the genetic structure of native species, while also influencing the spread of invasive species. In this study, we compare genetic patterns of two native and one highly invasive riverine fish species in a Belgian river basin, namely the native three-spined stickleback (Gasterosteus aculeatus) and stone loach (Barbatula barbatula), and the non-native and invasive topmouth gudgeon (Pseudorasbora parva). We aimed to characterize both natural and anthropogenic determinants of genetic diversity and population genetic connectivity. Genetic diversity was highest in topmouth gudgeon, followed by stone loach and three-spined stickleback. The correlation between downstream distance and genetic diversity, a pattern often observed in riverine systems, was only marginally significant in stone loach and three-spined stickleback, while genetic diversity strongly declined with increasing number of barriers in topmouth gudgeon. An Isolation-By-Distance pattern characterizes the population genetic structure of each species. Population differentiation was only associated with migration barriers in the invasive topmouth gudgeon, while genetic composition of all species seemed at least partially determined by the presence of migration barriers. Among the six barrier types considered (watermills, sluices, tunnels, weirs, riverbed obstructions, and others), the presence of watermills was the strongest driver of genetic structure and composition. Our results indicate that conservation and restoration actions, focusing on conserving genetic patterns, cannot be generalized across species. Moreover, measures might target either on restoring connectivity, while risking a rapid spread of the invasive topmouth gudgeon, or not restoring connectivity, while risking native species extinction in upstream populations

Sliding contacts and the dynamics of magnetic tape transport

Lateral tape motion (LTM) is the motion of a tape perpendicular to the tape transport direction. It is a problem in magnetic tape recording technology that limits the track density on a tape. To reduce LTM, it is important to characterize the main sources of LTM in tape transports. In this dissertation, the effect of tape edge contact as well as tape tension transients on LTM is investigated. An optical non-contact tension sensor is developed and a correlation between LTM and tension transients is observed. Additionally, a method based on acoustic emission is established to measure tape edge contact. Tape edge contact is observed to cause high frequency LTM, and the magnitude of the impact is shown to be function of the tape pack size. The dynamics of a tape as it moves over a cylindrical guide are studied theoretically and validated experimentally. Good agreement between theory and experiments is observed. In the experimental analysis, the tape/guide friction coefficient is observed to be function of different operating and design parameters. A model for the friction coefficient between a tape and a cylindrical guide is presented and evaluated with experimental data. Finally, the use of laser surface texturing (LST) for improved tape guiding is proposed and investigated both experimentally and numerically. LST guides are observed to create an air bearing at low tape speeds and thus reduce the transition speed between boundary lubrication and full fluid lubrication. Additionally, the design of a dual stage actuator tape head for increased bandwidth track-following is introduced, as a means to enable increased track density on a tape for future high performance tape drive

Orientation of carbon nanotubes using bulk acoustic waves

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Differential modes of MHC class IIB gene evolution in cichlid fishes

Cichlid fishes are emblematic models for the study of adaptive radiation, driven by natural and sexual selection. Parasite mediated selection is an important component in these processes, and the evolution of their immune system therefore merits special attention. In this study, light is shed on the phylogeny of the b family of cichlid major histocompatibility complex (MHC) class IIB genes. Full-length coding sequences were used to reconstruct phylogenies using criteria of maximum parsimony, maximum likelihood and Bayesian inference. All analyses suggest monophyly of the b family of cichlid MHC class IIB genes, although sequences of the cichlid sister taxa are currently not available. Two evolutionary lineages of these genes, respectively encompassing the recently defined genomic regions DBB-DEB-DFB and DCB-DDB, show highly contrasting levels of differentiation. To explore putative causes for these differences, exon 2 sequences were screened for variation in recombination rate and strength of selection. The more diversified lineage of cichlid MHC class IIB b genes was found to have higher levels of both recombination and selection. This is consistent with the observation in other taxa that recombination facilitates the horizontal spread of positively selected sites across MHC loci and hence contributes to fast sequence evolution. In contrast, the lineage that showed low diversification might either be under stabilizing selection or is evolutionary constrained by its low recombination rate. We speculate whether this lineage might include MHC genes with non-classical functions.status: publishe