131 research outputs found
Complexity of evolutionary equilibria in static fitness landscapes
A fitness landscape is a genetic space -- with two genotypes adjacent if they
differ in a single locus -- and a fitness function. Evolutionary dynamics
produce a flow on this landscape from lower fitness to higher; reaching
equilibrium only if a local fitness peak is found. I use computational
complexity to question the common assumption that evolution on static fitness
landscapes can quickly reach a local fitness peak. I do this by showing that
the popular NK model of rugged fitness landscapes is PLS-complete for K >= 2;
the reduction from Weighted 2SAT is a bijection on adaptive walks, so there are
NK fitness landscapes where every adaptive path from some vertices is of
exponential length. Alternatively -- under the standard complexity theoretic
assumption that there are problems in PLS not solvable in polynomial time --
this means that there are no evolutionary dynamics (known, or to be discovered,
and not necessarily following adaptive paths) that can converge to a local
fitness peak on all NK landscapes with K = 2. Applying results from the
analysis of simplex algorithms, I show that there exist single-peaked
landscapes with no reciprocal sign epistasis where the expected length of an
adaptive path following strong selection weak mutation dynamics is
even though an adaptive path to the optimum of length less
than n is available from every vertex. The technical results are written to be
accessible to mathematical biologists without a computer science background,
and the biological literature is summarized for the convenience of
non-biologists with the aim to open a constructive dialogue between the two
disciplines.Comment: 14 pages, 3 figure
Edge effects in game theoretic dynamics of spatially structured tumours
Background: Analysing tumour architecture for metastatic potential usually
focuses on phenotypic differences due to cellular morphology or specific
genetic mutations, but often ignore the cell's position within the
heterogeneous substructure. Similar disregard for local neighborhood structure
is common in mathematical models.
Methods: We view the dynamics of disease progression as an evolutionary game
between cellular phenotypes. A typical assumption in this modeling paradigm is
that the probability of a given phenotypic strategy interacting with another
depends exclusively on the abundance of those strategies without regard local
heterogeneities. We address this limitation by using the Ohtsuki-Nowak
transform to introduce spatial structure to the go vs. grow game.
Results: We show that spatial structure can promote the invasive (go)
strategy. By considering the change in neighbourhood size at a static boundary
-- such as a blood-vessel, organ capsule, or basement membrane -- we show an
edge effect that allows a tumour without invasive phenotypes in the bulk to
have a polyclonal boundary with invasive cells. We present an example of this
promotion of invasive (EMT positive) cells in a metastatic colony of prostate
adenocarcinoma in bone marrow.
Interpretation: Pathologic analyses that do not distinguish between cells in
the bulk and cells at a static edge of a tumour can underestimate the number of
invasive cells. We expect our approach to extend to other evolutionary game
models where interaction neighborhoods change at fixed system boundaries.Comment: 14 pages, 3 figures; restructured abstract, added histology to fig.
1, added fig. 3, discussion of EMT introduced and cancer biology expande
Measuring as a new mode of inquiry that bridges evolutionary game theory and cancer biology
We show that as game theory was transferred from mathematical oncology to experimental cancer biology, a new mode of inquiry was created. Modelling was replaced by measuring. The game measured by a game assay can serve as a bridge that allows knowledge to flow backwards from target (cancer research) to source (game theory). Our finding suggests that the conformist and creative (Houkes & Zwart, 2019) types of transfer need to be augmented. We conclude by introducing the expansive and transformative types to get a four-tier typology of knowledge transfer
Dirac fermions and flat bands in the ideal kagome metal FeSn.
A kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice-Dirac fermions and flat bands-have not been simultaneously observed. Here, we use angle-resolved photoemission spectroscopy and de Haas-van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, which has spatially decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that the coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in a kagome lattice is a frontier of great promise at the confluence of topology, magnetism and strongly correlated physics
Vortex core switching by coherent excitation with single in-plane magnetic field pulses
The bistability of the core magnetization of nano-scaled magnets with a
magnetic vortex configuration has great potential for data storage
applications. To exploit this, reliable switching between the two possible
states is needed. Time resolved x-ray microscopy was used to study the response
of the vortex core to excitation pulses at sub-ns timescales and image the
vortex core switching. A reliable switching process by coherent excitation with
leading and trailing edges of in-plane magnetic field pulses was found and
compared with micromagnetic simulations.Comment: 4 pages, 3 figure
Charge density waves in cuprate superconductors beyond the critical doping
The unconventional normal-state properties of the cuprates are often
discussed in terms of emergent electronic order that onsets below a putative
critical doping of xc = 0.19. Charge-density wave (CDW) correlations represent
one such order; however, experimental evidence for such order generally spans a
limited range of doping that falls short of the critical value xc, leading to
questions regarding its essential relevance. Here, we use x-ray diffraction to
demonstrate that CDW correlations in La2-xSrxCuO4 persist up to a doping of at
least x = 0.21. The correlations show strong changes through the
superconducting transition, but no obvious discontinuity through xc = 0.19,
despite changes in Fermi surface topology and electronic transport at this
doping. These results demonstrate the interaction between CDWs and
superconductivity even in overdoped cuprates and prompt a reconsideration of
the role of CDW correlations in the high-temperature cuprate phase diagram.Comment: 8 pages + 5 pages of supplemental material; accepted in npj Quantum
Material
Molecular simulation studies of cyanine-based chromonic mesogens: spontaneous symmetry breaking to form chiral aggregates and the formation of a novel lamellar structure
Allâatom molecular dynamics simulations are performed on two chromonic mesogens in aqueous solution: 5,5âČâdimethoxyâbisâ(3,3âČâdiâsulphopropyl)âthiacyanine triethylammonium salt (Dye A) and 5,5âČâdichloroâbisâ(3,3âČâdiâsulphopropyl)âthiacyanine triethylammonuim salt (Dye B). Simulations demonstrate the formation of selfâassembled chromonic aggregates with an interlayer distance of â0.35 nm, with neighboring molecules showing a predominantly headâtoâtail antiparallel stacking arrangement to minimize electrostatic repulsion between hydrophilic groups. Strong overlap of the aromatic rings occurs within the selfâassembled columns, characteristic of Hâaggregation in aqueous solution. At low concentrations, aggregates of Dye A form chiral columns, despite the presence of strictly achiral species. Chirality arises out of the minimization of steric repulsion between methoxy groups, which would otherwise disrupt the stacking of aromatic molecular cores. At higher concentrations, simulations suggest the interaction of short columns leads to the formation of an achiralâlayered structure in which hydrophobic aromatic regions of the molecule are sandwiched between two layers of hydrophilic groups. This novel lamellar structure is suggested as a likely candidate for the structure of a Jâaggregate. The latter is known to exhibit intense redâshifted absorption peaks in solution but their structure has not yet been characterized. Selfâorganization of such structures provides a route to the formation of âsmecticâ chromonic mesophases
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