A new paper from +Jill Gallaher and +Alexander Anderson is out on the arXiv.  I asked Jill for a PLoS style 'author summary' in non-technical language, and here it is:

Author Summary:

A tumor can be thought of as an ecosystem, which critically means that we cannot just consider it as a collection of mutated cells. A tumor is more of a complex system of many interacting cellular and microenvironmental elements. There is variation among cells within the tumor, and with an increased proliferation capacity, there is competition for space, so evolution and selection occurs.  Because our current understanding at the genetic scale gives little information on translating to actual changes in cell behavior, we bypass the translation of genetics to behavior by focussing on the functional end result of the cell’s traits (phenotype) combined with the environmental influence of limited space, which will ultimately dictate tumor aggressiveness and treatability. 

The evolution of the population depends on the way in which traits are passed on as cells divide. We investigate trait inheritance by building a cell based simulation in which individual cells with varied trait combinations compete for space over time. Specifically, we characterize cell behavior in terms of two traits: proliferation rate and migration speed. The mode in which these traits are inherited significantly affects the evolution, composition, and fitness of a tumor population. To investigate competition for space, we initiate the population as a tight cluster, representing a growing tumor mass, and as a dispersed population, representing a cell culture experiment. We find that the dispersed population has more space, less competition, and reduced selection.  With a growing cluster of cells, there is more competition and selection. But constraining the allowable trait combinations so that several phenotypes are equally fit reduces competition and leads to the coexistence of several phenotypes. In this case, local heterogeneity may be advantageous to maximize growth.

Evolution of intratumoral phenotypic heterogeneity: the role of trait inheritance

Jill Gallaher, Alexander R. A. Anderson

(Submitted on 2 May 2013)

A tumor can be thought of as an ecosystem, which critically means that we cannot just consider it as a collection of mutated cells but more as a complex system of many interacting cellular and microenvironmental elements. At its simplest, a growing tumor with increased proliferation capacity must compete for space as a limited resource. Hypercellularity leads to a contact-inhibited core with a competitive proliferating rim. Evolution and selection occurs, and an individual cell's capacity to survive and propagate is determined by its combination of traits and interaction with the environment. With heterogeneity in phenotypes, the clone that will dominate is not always obvious as there are both local interactions and global pressures. Several combinations of phenotypes can coexist, changing the fitness of the whole.
To understand some aspects of heterogeneity in a growing tumor we build an off-lattice agent based model consisting of individual cells with assigned trait values for proliferation and migration rates. We represent heterogeneity in these traits with frequency distributions and combinations of traits with density maps. How the distributions change over time is dependent on how traits are passed on to progeny cells, which is our main inquiry. We bypass the translation of genetics to behavior by focussing on the functional end result of inheritance of the phenotype combined with the environmental influence of limited space.