Analysis of a stochastic chemical system close to a sniper bifurcation of its mean field model

A framework for the analysis of stochastic models of chemical systems for which the deterministic mean-field description is undergoing a saddle-node infinite period (SNIPER) bifurcation is presented. Such a bifurcation occurs for example in the modelling of cell-cycle regulation. It is shown that the stochastic system possesses oscillatory solutions even for parameter values for which the mean-field model does not oscillate. The dependence of the mean period of these oscillations on the parameters of the model (kinetic rate constants) and the size of the system (number of molecules present) is studied. Our approach is based on the chemical Fokker Planck equation. To get some insights into advantages and disadvantages of the method, a simple one-dimensional chemical switch is first analyzed, before the chemical SNIPER problem is studied in detail. First, results obtained by solving the Fokker-Planck equation numerically are presented. Then an asymptotic analysis of the Fokker-Planck equation is used to derive explicit formulae for the period of oscillation as a function of the rate constants and as a function of the system size

Letter from I. Chapman to John Muir, 1904 Jun 20.

[letterhead] [letterhead]302 California St.San Francisco,20/6/04Dear Mr. MuirI write a few lines on the off chance of your being home again. Since I wrote last my plans have been altered, & I am compelled to return to Aush[illegible] earlier than I expected. I go down to Honolulu next Saturday (25th), and shall be glad to have a line from you saying how you are. I fear I shall not be able to get time to visit your home, which I sincerely regret. Perhaps you will be here before I leave next Saturday? A line to this address will find me.I shall long remember our pleasant trip together, and will try & send you some fossils from our Coll[illegible] on return. With very kind regardsBelieve meyours sincerelyI. Chapman0338

Population Genetics of the Potato Psyllid, Bactericera Cockerelli

The potato psyllid (Bactericera cockerelli) is native to North America and occurs from Central America to Canada. A new disease of potatoes, Zebra Chip, has recently been associated with potato psyllid occurrence. Potato psyllids transmit a gram-negative α-proteobacter, Candidatus Liberibacter solanacearum, the putative causal agent of Zebra Chip in potatoes. Symptoms of Zebra Chip first appeared in potato plants grown outside of Saltillo, Mexico in 1994, and by 2000, this invasive disease complex was found in potatoes in south Texas. Since then, Zebra Chip has spread northward, throughout the central plains of the US and on the west coast. Potato psyllid population management is complicated by the presence of two biotypes, one of which may be a stronger vector of Ca. L. solanacearum. Improved detection of Ca. L. solanacearum is needed to maintain supply for potatoes in the United States. In this study, potato psyllid biotypes were delineated by melt temperature analysis following Sybr Green qrt-PCR, and then were grouped further into populations using ISSR-PCR. Potato psyllid populations were mapped, and showed a northward migration throughout the 2009 growing season. The putative causal agent of Zebra Chip, Ca. L. solanacearum, was detected using pyrosequencing at low levels. These molecular tools will increase efficiency in the management of potato crops in zones of high probability of interaction with inoculative populations of potato psyllids

Molecular Detection of Pathogenic Leptospira and Microbial Source Tracking of Fecal Pollution in San Juan, Puerto Rico

Leptospirosis, caused by pathogenic Leptospira, is endemic to tropical regions. Leptospira is released into the environment through the secretion of urine from animals, making it easily transmissible through water sources. The estuarian environment surrounding the area of San Juan, Puerto Rico and its high density of urban development creates ideal conditions for transmission of Leptospirosis. The goal of this study was to determine the presence of Leptospira in these surface waters and use Microbial Source Tracking (MST) to identify the possible source of pathogenic Leptospira. Eighty-seven water samples were collected during the dry (44) and wet (43) seasons. Phosphorus and nitrogen levels were determined using standard USEPA methods. The level of Leptospira interrogans was determined using quantitative polymerase chain reaction (qPCR) targeting the Lipl32 gene. Human (HF183), dog (BacCan-UCD), and horse (HoF597) MST assays were performed to determine the likely sources of fecal contamination at each site. Total phosphorous and total nitrogen exceeded USEPA safety guidelines in multiple locations. Leptospira interrogans was detected in 32% of samples collected in the dry season and was not detected in the wet season. There was a positive correlation (r =0.89) between the presence of L. interrogans and human fecal bacterial MST marker (HF183). The MST also indicated a positive correlation between horse fecal contamination and total phosphorus and total nitrogen. The correlation between L. interrogans gene copies and MST makers warrants further examination of the water quality in the estuaries of San Juan, Puerto Rico due to the possible public health implications