Spatially-induced nestedness in a neutral model of phage-bacteria networks

Ecological networks, both displaying mutualistic or antagonistic interactions, seem to share common structural traits: the presence of nestedness and modularity. A variety of model approaches and hypothesis have been formulated concerning the significance and implications of these properties. In phage-bacteria bipartite infection networks, nestedness seems to be the rule in many different contexts. Modeling the coevolution of a diverse virus-host ensemble is a difficult task, given the dimensionality and multi parametric nature of a standard continuous approximation. Here we show that this can actually be overcome by using a simple model of coevolving digital genomes on a spatial lattice and having exactly the same properties, i.e. a genome-independent fitness associated to fixed growth and death parameters. A matching allele model of phage-virus recognition rule is enough to generate a complex, diverse ecosystem with heterogeneous patterns of interaction and nestedness, provided that interactions take place under a spatially-constrained setting. It is found that nestedness seems to be an emergent property of the coevolutionary dynamics. Our results indicate that the enhanced diversity resulting from localized interactions strongly promotes the presence of nested infection matrices.