Interplay Between Random Genetic Drift and Natural Selection Plays a Central Role

Abstract

Interplay between random genetic drift and natural selection plays a central role

in evolution. During a range expansion, genetic drift can prevail over natural selection

and enable gene surfing. This process is modeled by constructing a colony

of two differently labeled, but otherwise identical, strains of microorganisms. The

colony grows outwards and after leaving the inoculation spot, the already spatially

structured population demixes into sectors of the two different labels.

In a series of preliminary experiments, the morphology of the boundaries between

these sectors was seen to depend strongly on the type of microorganism used

in the experiment. Circular S. cerevisiaegrows in colonies characterised by a large

number of sectors separated with straight boundaries. rod-like E. colion the other

hand forms colonies with a lower number of sectors which have stronger wandering

sector boundaries. Here the difference in colony morphology is explored, with

the aim of understanding why this difference occurs. It is assumed that shape is an

important factor in the morphology of the growing colonies.

Colonies are grown from rod-like and spherically shaped microorganisms. The

meso scale morphology of the colonies is compared, as well as their growth on the

micro scale. The sectors, which form in colonies of rod-like S. pombe, are not seen

to resemble those which form in colonies of rod-like E. coli. Instead morphologically

they are seen to resemble the straight sectors, which form in colonies of S.

cerevisiae. Round E. colimutants were seen to grow in colonies similar to those of

S. pombeand S. cerevisiae. The findings suggest that the shape of the microbe is an

important factor in the resulting sector morphology. However, there are other factors,

which may be important and should be taken into account in further research.

Examples could be cell-cell interactions and cell substrate interactions.