Colored bacteria reveal how different lineages compete with and disentangle from each other in expanding populations

The spread of invasive species leads to naturally occurring population expansions that have played a role in our evolutionary history, such as when humans migrated out of Africa.

HFSP Program Grant holders Andrew Murray and David Nelson and colleagues
authored on Thu, 01 March 2018

Our HFSP-funded project investigates how a variety of interactions between microbial cells (yeast and bacteria) influences their behavior in a direct, if distant, analogy to how the interactions between individuals and groups shape the behavior of human societies.

In this study, we asked how an expanding population, composed of many types of individuals, gradually segregates into different groups as the front of the population moves forward. To follow the genetically different members of the population, we mark them with different colored fluorescent proteins, mix them together, and place roughly a million as a circular drop at the center of an agar plate. As time goes on, the population expands outwards and what was initially a mixture of four colors sorts out into sectors of different colors, a process known as genetic demixing.

Figure: To quantitatively study the dynamics of competing genotypes in a population growing into new territory (a range expansion), four strains of E. coli, differing only by a heritable fluorescent marker, were mixed and inoculated onto a nutrient-rich agar plate. Over the course of eight days, cells at the front divided and formed a dense colony. The resulting image, which can be analyzed and understood quantitatively, depicts the formation of monoclonal sectors and provides insight into the evolutionary consequences of population expansion. Image Credit: Bryan T. Weinstein. Image first appeared in PLoS Comput Biol. 2017 Dec 1;13 (12).

We developed a theory to explain how the boundaries between the different colors drift and wiggle as the population expands. This process is random if the differently colored sectors that meet are composed of cells that grow and divide at equal speed, but if the cells of one color divide faster than those of another, the more slowly growing sector is squeezed out, and the speed at which it is extinguished allows us to compute how much slower its cells are dividing.

Our work is relevant to populations expanding outside the laboratory, such as the expansion of human populations out of Africa to colonize the rest of the globe.


Genetic drift and selection in many-allele range expansions. Weinstein BT, Lavrentovich MO, Möbius W, Murray AW, Nelson DR. PLoS Comput Biol. 2017 Dec 1;13 (12):e1005866. doi: 10.1371/journal.pcbi.1005866. eCollection 2017 Dec.

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