Dynamical Heterogeneity and Its Adaptive role.

It is known for a long time that populations of isogenic single cell organisms show a remarkable heterogeneity in the levels of expression of proteins . The general cause of this quantitative diversity has been shown to be the stochastic effects due to low numbers of molecules present . Although most studies focus on prokaryote organisms, the same principle should hold true on eukaryotes, which have more layers of genetic regulation: low copy number of mRNA molecules and stochastic fluctuations of local concentration due to diffusion are impossible to eliminate, even to the more evolved eukaryotes. In fact, this randomness is as omnipresent as mutation. The impossibility of eliminating such randomness raises the hypothesis that organisms actually developed ways to exploit it. In fact, this heterogeneity has been shown to play an important role in several phenomena such as developmental decisions in phages , phenotypic switching and chemotaxis in bacteria. In all these cases there is a shift in focus from the individual cell to the population as a whole. In this perspective the population takes advantage of its naturally occurring diversity to solve problems that would be intractable for the single cell, such as ensuring survival of the population or search strategies in an heterogeneous environment. This leads to the interesting question of whether this strategy has been conserved from prokaryotes to multicellular organisms, in particular vertebrate, where interaction between different cell populations plays an important role in their physiology. To address the question of whether dynamical heterogeneity might play a significant role in population competition, we used the example of the imune system where competition between populations plays a significant role in its function. We set up an individual based simulation of competition between populations with different heterogeneoty characteristics and provide evidence that heterogeneity might be a selectable trait. Moreover, we show that lognormal dynamics, in particular, provide an advantage for the population by endowing the population with greater adaptability.

Mullers Ratchet revisited: the accumulation of deleterious mutations in random graphs and scale free networks.

Mullerâs ratchet is a well studied evolutionary process, referred as important in the extinction of asexual species, the evolution of mitochondria, the degeneration of the Y chromosome, the evolution of sex and recombination and the evolution of microbes. In this work we focused on the study of the speed of Mullerâs ratchet in a subdivided population. Particularly, we compare the speed of the ratchet in two distinct types of topologies: scale free networks and random graphs. Results and its interpretation will be discussed.

A common framework for relating multiple typing methods illustrated using macrolide- resistant Streptococcus pyogenes.

The studies that correlate the results obtained by different methodologies rely solely on a qualitative comparison of the groups defined by each methodology. We propose a framework of measures for the quantitative assessment of correspondence between different typing methods as a first step to the global mapping of type equivalences. A collection of 325 macrolide resistant Streptococcus pyogenes associated with pharyngitis cases in Portugal was used to benchmark the proposed measures. The application of Adjusted Rand and Wallace indices allowed the evaluation of the strength of the correspondences between the various typing methods and show that if PFGE or MLST data are available one can confidently predict the emm type (Wallace coefficients of 0.952 for both methods). In contrast, emm was a poor predictor of PFGE cluster and MLST sequence type (Wallace coefficients of 0.803 and 0.655 respectively). This was confirmed by the analysis of the larger data set available from spyogenes.mlst.net and underscores the necessity of performing PFGE or MLST to unambiguously define clones in S. pyogenes.

There will no seminar this week.