A century-long genetic record reveals that protist effective population sizes are comparable to those of macroscopic species

Effective population size (N_e) determines the rate of genetic drift and the relative influence of selection over random genetic changes. While free-living protist populations characteristically consist of huge numbers of cells (N), the absence of any estimates of contemporary N_e raises the question whether protist effective population sizes are comparably large. Using microsatellite genotype data of strains derived from revived cysts of the marine dinoflagellate Pentapharsodinium dalei from sections of a sediment record that spanned some 100 years, we present the first estimates of contemporary N_e for a local population in a free-living protist. The estimates of N_e are relatively small, of the order of a few 100 individuals, and thus are similar in magnitude to values of N_e reported for multicellular animals: the implications are that N_e of P. dalei is of many orders of magnitude lower than the number of cells present (N_e/N ∼ 10⁻¹²) and that stochastic genetic processes may be more prevalent in protist populations than previously anticipated.