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Abstract: In this study we consider a population subdivided in groups. Each group is connected to two neighboring groups by means of migration channels. The migration channels are used with different probability, according to a gradient that gives a preferential direction of colonization. Within groups we have a fast population dynamics modeled with a Moran process. Between groups we have a process of colonization selection at low migration rate, based on the propagule model. We consider resident and invading individuals, with the invading individuals having a relative fitness r. Groups composed only of invaders will also have a relative group fitness R. We show that, independently of the value of r, a strong gradient reduces the advantage of groups with large fitness and thus works against group selection. [Copyright &y& Elsevier]

Myoviruses, bacteriophages with T4-like architecture, must contract their tails prior to DNA release. However, quantitative kinetic data on myovirus particle opening are lacking, although they are promising tools in bacteriophage-based antimicrobial strategies directed against Gram-negative hosts. For the first time, we show time-resolved DNA ejection from a bacteriophage with a contractile tail, the multi-O-antigen-specific Salmonella myovirus Det7. DNA release from Det7 was triggered by lipopolysaccharide (LPS) O-antigen receptors and notably slower than in noncontractile-tailed siphoviruses. Det7 showed two individual kinetic steps for tail contraction and particle opening. Our in vitro studies showed that highly specialized tailspike proteins (TSPs) are necessary to attach the particle to LPS. A P22-like TSP confers specificity for the Salmonella Typhimurium O-antigen. Moreover, crystal structure analysis at 1.63 Å resolution confirmed that Det7 recognized the Salmonella Anatum O-antigen via an 15-like TSP, DettilonTSP. DNA ejection triggered by LPS from either host showed similar velocities, so particle opening is thus a process independent of O-antigen composition and the recognizing TSP. In Det7, at permissive temperatures TSPs mediate O-antigen cleavage and couple cell surface binding with DNA ejection, but no irreversible adsorption occurred at low temperatures. This finding was in contrast to short-tailed Salmonella podoviruses, illustrating that tailed phages usecommonparticle-opening mechanisms but have specialized into different infection niches. [ABSTRACT FROM AUTHOR]