Genetic drift at expanding frontiers promotes gene segregation

TL;DR

This study reveals that genetic drift during range expansions causes significant gene segregation in microbial colonies, driven by chance effects at the frontier, which may serve as a footprint of past expansions.

Contribution

It demonstrates that chance effects at the expanding frontiers can lead to gene segregation even in large populations, a phenomenon previously thought to be limited to small populations.

Findings

Formation of sector-like regions with fractal boundaries in colonies.

Genetic sectoring driven by stochastic fluctuations at the frontier.

Observed similar patterns in bacterial and yeast colonies.

Abstract

Competition between random genetic drift and natural selection plays a central role in evolution: Whereas non-beneficial mutations often prevail in small populations by chance, mutations that sweep through large populations typically confer a selective advantage. Here, however, we observe chance effects during range expansions that dramatically alter the gene pool even in large microbial populations. Initially well-mixed populations of two fluorescently labeled strains of Escherichia coli develop well-defined, sector-like regions with fractal boundaries in expanding colonies. The formation of these regions is driven by random fluctuations that originate in a thin band of pioneers at the expanding frontier. A comparison of bacterial and yeast colonies (Saccharomyces cerevisiae) suggests that this large-scale genetic sectoring is a generic phenomenon that may provide a detectable footprint of past range expansions.