8,706 research outputs found
Evolved polygenic herbicide resistance in Lolium rigidum by low-dose herbicide selection within standing genetic variation
The interaction between environment and genetic traits under selection is the basis of evolution. In this study, we have investigated the genetic basis of herbicide resistance in a highly characterized initially herbicide-susceptible Lolium rigidum population recurrently selected with low (below recommended label) doses of the herbicide diclofop-methyl. We report the variability in herbicide resistance levels observed in F1 families and the segregation of resistance observed in F2 and back-cross (BC) families. The selected herbicide resistance phenotypic trait(s) appear to be under complex polygenic control. The estimation of the effective minimum number of genes (NE), depending on the herbicide dose used, reveals at least three resistance genes had been enriched. A joint scaling test indicates that an additive-dominance model best explains gene interactions in parental, F1, F2 and BC families. The Mendelian study of six F2 and two BC segregating families confirmed involvement of more than one resistance gene. Cross-pollinated L. rigidum under selection at low herbicide dose can rapidly evolve polygenic broad-spectrum herbicide resistance by quantitative accumulation of additive genes of small effect. This can be minimized by using herbicides at the recommended dose which causes high mortality acting outside the normal range of phenotypic variation for herbicide susceptibility
A cost function for similarity-based hierarchical clustering
The development of algorithms for hierarchical clustering has been hampered
by a shortage of precise objective functions. To help address this situation,
we introduce a simple cost function on hierarchies over a set of points, given
pairwise similarities between those points. We show that this criterion behaves
sensibly in canonical instances and that it admits a top-down construction
procedure with a provably good approximation ratio
Ferromagnetic phase transition for the spanning-forest model (q \to 0 limit of the Potts model) in three or more dimensions
We present Monte Carlo simulations of the spanning-forest model (q \to 0
limit of the ferromagnetic Potts model) in spatial dimensions d=3,4,5. We show
that, in contrast to the two-dimensional case, the model has a "ferromagnetic"
second-order phase transition at a finite positive value w_c. We present
numerical estimates of w_c and of the thermal and magnetic critical exponents.
We conjecture that the upper critical dimension is 6.Comment: LaTex2e, 4 pages; includes 6 Postscript figures; Version 2 has
expanded title as published in PR
Dynamic critical behavior of the Chayes-Machta-Swendsen-Wang algorithm
We study the dynamic critical behavior of the Chayes-Machta dynamics for the
Fortuin-Kasteleyn random-cluster model, which generalizes the Swendsen-Wang
dynamics for the q-state Potts model to noninteger q, in two and three spatial
dimensions, by Monte Carlo simulation. We show that the Li-Sokal bound z \ge
\alpha/\nu is close to but probably not sharp in d=2, and is far from sharp in
d=3, for all q. The conjecture z \ge \beta/\nu is false (for some values of q)
in both d=2 and d=3.Comment: Revtex4, 4 pages including 4 figure
New X-ray Detections of WNL Stars
Previous studies have demonstrated that putatively single nitrogen-type
Wolf-Rayet stars (WN stars) without known companions are X-ray sources.
However, almost all WN star X-ray detections so far have been of earlier WN2 -
WN6 spectral subtypes. Later WN7 - WN9 subtypes (also known as WNL stars) have
proved more difficult to detect, an important exception being WR 79a (WN9ha).
We present here new X-ray detections of the WNL stars WR 16 (WN8h) and WR 78
(WN7h). These new results, when combined with previous detections, demonstrate
that X-ray emission is present in WN stars across the full range of spectral
types, including later WNL stars. The two WN8 stars observed to date (WR 16 and
WR 40) show unusually low X-ray luminosities (Lx) compared to other WN stars,
and it is noteworthy that they also have the lowest terminal wind speeds
(v_infty). Existing X-ray detections of about a dozen WN stars reveal a trend
of increasing Lx with wind luminosity Lwind = (1/2) M_dot v_infty^2, suggesting
that wind kinetic energy may play a key role in establishing X-ray luminosity
levels in WN stars.Comment: 20 pages, 5 figure
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