Chris M. N. Tofts

Evolutionary ecology of vertically transmitted parasites: transovarial transmission of a microsporidian sex ratio distorter in Gammarus duebeni

Vertically transmitted parasites are transmitted from generation to generation of hosts usually via the hosts gametes. Owing to gamete size dimorphism, the major transmission route is transovarial and selection (on the parasite) favours strategies which increase the relative frequency of the transmitting (female) host sex. These strategies impose unusual selection pressures on the host, and coevolution between hosts and vertically transmitted parasites has been implicated in speciation, in the evolution of symbiosis, and in the evolution of novel systems of host reproduction and sex determination. We review the evolutionary implications of vertically transmitted parasites in arthropods before focusing on strategies of transmission of a parasitic sex ratio distorter in Gammarus duebeni . The efficiency of parasite transmission to new hosts is a key factor underlying the relationship between vertically transmitted parasites and their hosts. Vertically transmitted parasites must overcome 2 bottlenecks in order to ensure successful infection of future host generations: first, transmission from adult to gamete; and secondly, transmission to the germ-line of the infected host. We investigate these 2 processes with regard to transovarial transmission by a microsporidian parasite in Gammarus duebeni . Parasite transmission from adult to eggs is highly efficient, with 96% of eggs of infected mothers inheriting the infection, whereas transmission to germ-line within infected embryos is relatively inefficient (72%). We measure parasite distribution between cells of developing embryos, and use these distributions to infer possible mechanisms of parasite transmission to germ-line. Parasite distribution within the embryo is dependent on host cell lineage, and is not consistent with unbiased segregation between daughter cells. These results indicate that parasites segregate together at host cell division, and may reflect a strategy of differential segregation to the host germ-line. We consider alternative parasite strategies at the cell-level in terms of their evolutionary implications.

A denotational semantics for a process-based simulation language

In this article, we present semantic translations for the actions of μDemos, a proocess-based, discrete event simulation language. Our formal translation schema permits the automatic construction of a process algebraic representatioon of the underlying simulation model which can then be checked for freedom from deadlock and livelock, as well as system-specific safety and liveness properties. As simulation methodologies are increasingly being used to design and implement complex systems of interaction objects, the ability to perform such verifications in of increasing methodological importance. We also present a normal form for the syntactic construction of μDemos programs that allows for the direct comparison of such programs (two programs with the same normal form must execute in identical fashion), reduces model proof obligations by minimizing the number of language constructs, and permits an implementer to concentrate on the basic features of the language (since any program implementation that efficiently evaluates normal forms will be an efficient evaluator for the complete language).

Getting Demos models right. (I). Practice

Abstract We present a method for translating the synchronisation behaviour of a process oriented discrete event simulation language into a process algebra. Such translations serve two purposes. The first exploits the formal structure of the target process algebraic representations to enable proofs of such properties of the source system as deadlock freedom, safety, fairness and liveness which can be very difficult to establish by simulation experiment. The second exploits the denotational semantics to better understand the language constructs as abstract entities and to facilitate reasoning about simulation models. Here we give the intuition and the basic translation mechanisms using a variety of the Demos simulation language and the CCS and SCCS process algebras. The translations have been automated as SML programs and produce CWB compatible input allowing the automated checking of formal system properties.

Exact, analytic, and locally approximate solutions to discrete event-simulation problems

Abstract We demonstrate how discrete event simulation problems can be encoded within a probabilistic proc algebra. From this encoding we can exploit vavious solution techniques to derive exact numerical solutions, analytic solutions and piecewise polynomial approximations for system properties. Our approach allows us to combine the advantages of simulation, viz problem decomposition and an executable representation, with standard analytic and numerical computation methods for calculating system properties.