A computer program for the statistical analysis of disease prevalence data from survival/sacrifice experiments

This paper presents a computer program for analyzing disease prevalence data from animal survival experiments in which there may also be some serial sacrifice. The method has been described in Biometrics 35 (1979) 221-234. The user is interrogated about the details of particular models he wishes to fit. Then a generalized EM algorithm is used to compute maximum likelihood estimates of various quantities of interest concerning the effects of treatment, time and presence of other diseases on the prevalences and lethalities of specific diseases of interest.     

Loss and resynthesis of a developmentally regulated membrane protein (gp80) during dedifferentiation and redifferentiation in Dictyostelium

When developing cultures of Dictyostelium discoideum are disaggregated and resuspended in nutrient medium, they lose the capacity to rapidly reaggregate after 90 min, in a rapid and synchronous step referred to as the "erasure event." They then proceed to lose remaining developmentally acquired functions in a program of dedifferentiation culuminating with the loss of EDTA-resistant cohesion roughly 5 hr later. Immediately following the erasure event, cells can be stimulated to reenter the developmental program even though they still possess a number of developmentally acquired functions. These cells therefore appear to undergo dedifferentiation and redifferentiation simultaneously (D. R. Soll and L. H. Mitchell, 1982, Dev. Biol. 91, 183-190). In this report, we have employed an antiserum made against a developmentally acquired membrane glycoprotein, gp80, to examine whether gp80 is lost during dedifferentiation and whether it is either reutilized or resynthesized during redifferentiation. Results are presented which demonstrate that (1) when 9-hr developing cells are disaggregated and resuspended in nutrient medium, gp80 continues to accumulate for several hours after the erasure event, then is lost at roughly the same time as EDTA-resistant cohesion; (2) when cells are stimulated to reenter the developmental program immediately after the erasure event, both gp80 and EDTA-resistant cohesion are still lost according to the program of dedifferentiation, but are then reacquired soon afterwards according to the program of redifferentiation; (3) during redifferentiation, cells do not reutilize gp80 which had been synthesized during initial development; rather they synthesize gp80 de novo; and (4) developing cells of a dedifferentiation-defective variant, HI4, when disaggregated and resuspended in nutrient medium, retain gp80, EDTA-resistant cohesion, and the capacity to rapidly reinitiate aggregation for at least 12 hr. This last result indicates that the loss of gp80 is regulated by the dedifferentiation process and is not an independent response to disaggregation or the reintroduction of nutrients. Together, these results reinforce the conclusion that dedifferentiation and redifferentiation can function independently and simultaneously in the same cells.     

An analysis of developmental timing in Dictyostelium discoideum

A new method has been developed to assess the minimum complexity and relationships of those pathways (developmental timers) which time the consecutive stages of a developing system (Soll, 1983). This method has been applied to the morphogenetic program of Dictyostelium discoideum and has resulted in (1) a minimum estimate of the number of components comprising the timers for the first seven stages of morphogenesis, (2) a characterization of the temperature sensitivities of these components including demonstration of a reversible timer component, (3) detailed temporal definition of a number of transition points between rate-limiting components including a major branch point for the onset of several independent timer components coincident with the onset of aggregation, and (4) a temporal model for the relationships between the timers of the seven consecutive morphogenetic stages, including several examples of parallel timers.     

Chemical Detection of Microbial Prey by Bacterial Predators

A motile, predacious bacterium which degraded Pythium debaryanum was strongly attracted to substances released into the medium by the fungus. A nonpredacious bacterium was not attracted to these substances. The predator bacterium was specifically attracted to cellulose and its oligomers which are known to be components of the cell wall of Pythium. Ethanol inhibited chemotaxis of the bacterium without affecting either its motility or its ability to degrade cellulose. A second predacious bacterium was isolated for the alga, Skeletonema costatum. The role of chemoreception in the detection of microbial prey by bacterial predators in natural habitats is discussed.     

van der Waals Forces between Cylinders: II. Retarded Interaction between Thin Isotropic Rods

We derive the retarded van der Waals interaction between two long thin parallel dielectric cylinders immersed in a solvent. The result shows that the ultraviolet interactions which may be responsible for the specificity of macromolecular interactions do not operate strongly over distances R ≥ 50 Å. For greater distances the contribution of these frequencies ξ is damped by a factor ∞ e^-ξR/c.