Modelling the evolution of holobionts: an incomplete review

A holobiont is a biological individual that consists of a host and its associated microbiome. As a biological individual, a holobiont participates in the evolutionary process, although the dynamics of how it does so are non-trivial. In particular, the constituent members of the holobiont have their own fitnesses, and holobiont evolution has to be examined in the light of the complementary or competing interests of its component parts. Additionally, evolution requires that (holobiont) offspring and parent share traits in common. This implies that there are mechanisms of inheritance, which raises the interesting question of how the degree of microbiome inheritance influences holobiont evolution. In this paper, I review the issues that modellers have to consider when they construct explicit models of holobiont evolution. I review three models, and show how different modelling assumptions can influence the results that models return. Finally, I discuss the future developments and innovations that these models might include.