Evolution of a selfish genetic element: the 2 micron plasmid of saccharomyces spp.

Abstract

The 2 Micron plasmid is a multicopy DNA circle inhabiting the genome of the budding yeasts, Sacchormyces spp. The plasmid confers no known benefits to the host, but imposes a small fitness cost. However the plasmid is able to drive, i.e. to transmit to >50% of sexual offspring, which allows the element to spread through an outcrossing host population. Therefore we can consider the plasmid a selfish genetic element of yeast. Here we draw on a number of approaches to improve our understanding of this element. Firstly, we examined the relationship between the cost of plasmid carriage and copy number by experimentally manipulating the number of plasmids in the host. We find that host fitness decreases at a rate of ~0.09% per additional plasmid. Secondly we use experimentally evolving yeast populations to test the hypothesis that sexual reproduction, which is fundamental to the evolution of selfish genetic elements, will drive increasing virulence in the plasmid. We find that 2 Micron copy number increased in outcrossing populations but remained constant in asexual populations. We also find that sex allowed the invasion of non-functional mitochondria in to the populations, showing that sex has the capacity to generate a driving selfish genetic element from one of the most fundamental endosymbionts of the eukaryotic cell. In addition, we have investigated plasmid variation from global populations of Saccharromyces spp. in order to better understand the population biology and evolution of this plasmid. Here we find evidence that the plasmid is able to move between species, recombine with other plasmids within the cell, and exist at a surprisingly wide range of copy numbers in different host populations. Understanding the population structure and evolution of this element allows us to view the plasmid as an autonomous unit evolving in its own right in the genomes of its hosts.