TRADEOFF BETWEEN HORIZONTAL AND VERTICAL-MODES OF TRANSMISSION IN BACTERIAL PLASMIDS

It has been hypothesized that there is a fundamental conflict between horizontal (infectious) and vertical (intergenerational) modes of para site transmission. Activities of a parasite that increase its rate of infectious transmission are presumed to reduce its host's fitness. Thi s reduction in host fitness impedes vertical transmission of the paras ite and causes a tradeoff between horizontal and vertical transmission . Given this tradeoff, and assuming no multiple infections (no within- host competition among parasites), a simple model predicts that the de nsity of uninfected hosts in the environment should determine the opti mum balance between modes of parasite transmission. When susceptible h osts are abundant, selection should favor increased rates of horizonta l transfer, even at the expense of reduced vertical transmission. Conv ersely, when hosts are rare, selection should favor increased vertical transmission even at the expense of lower horizontal transfer. We tes ted the tradeoff hypothesis and these evolutionary predictions using c onjugative plasmids and the bacteria that they infect. Plasmids were a llowed to evolve for 500 generations in environments with different de nsities of susceptible hosts. The plasmid's rate of horizontal transfe r by conjugation increased at the expense of host fitness, indicating a tradeoff between horizontal and vertical transmission. Also, reducti ons in conjugation rate repeatedly coincided with the loss of a partic ular plasmid-encoded antibiotic resistance gene. However, susceptible host density had no significant effect on the evolution of horizontal versus vertical modes of plasmid transmission. We consider several pos sible explanations for the failure to observe such an effect.